SEAOSC Safer Cities Survey Results: How Are We Building Strength and Transparency in Our Communities?

Back in January, employees at Simpson were given the opportunity to learn more about the 401K retirement and investment plan. The big takeaways from my training session were a) save as much as you can as early as you can in life and b) use asset allocation to diversify your portfolio and avoid too much risk. Now, I’m not a big risk taker in general, so I dutifully picked a good blend of stocks and bonds with a range of low to high risk. It seems like a pretty sound strategy and it made me think of all the other ways I tend to minimize risk in my life. When I head to a restaurant, for example, I almost instinctively look for the county health grade sign in the window. When my husband and I went to go buy a new family car a couple years ago, I remember searching the National Highway Traffic Safety Administration (NHTSA) website for crash test ratings. Even when I’m doing something as mundane as having a snack, I will invariably flip over the Twinkie package to see just how many grams of fat are lurking inside (almost 5 per serving!). For all the rankings and information available to the general public for restaurants, cars and snacks, there isn’t much, if any, information to help us know if we’re minimizing our risk for one of the most common activities we do almost every day: walking into a building.

Risk level knob positioned on medium position, white background and orange light. 3D illustration concept for business security management.

 

Now before you accuse me of being overly dramatic about such a trivial activity, here’s some food for thought: research has shown that Americans spend approximately 90% of their day inside a building. That’s over 21 hours a day! Have you ever once thought to yourself, “I wonder if this building is safe? Would this building be able to withstand an earthquake or high wind event?” Or how about even taking a step back and asking, “Are there any buildings that are already known to be potentially vulnerable or unsafe, and has my city done anything to identify them?” Unfortunately, that kind of information about a city’s building stock is not usually readily available, but some in the community, including structural engineers, are working to change that.

Los Angeles skyline on a partly cloudy day with a row of palm trees in the foreground.

 

The charge is being led in California, a.k.a. Earthquake Country, where structural engineers are teaming up with cities to help identify buildings with known seismic vulnerabilities and provide input on seismic retrofit ordinances. Structural engineers have learned quite a bit about how buildings behave through observing building performance after major earthquakes, and building codes have been revised to address issues accordingly. However, according to the US Green Building Council, “…the annual replacement rate of buildings (the percent of the total building stock newly constructed or majorly renovated each year) has historically been about 2%, and during the economic recession and subsequent years, it’s been much lower.” This means that there are a lot of older buildings out there that have not been built to current building codes and were not designed with modern engineering knowledge.

Several cities in California have enacted mandatory seismic retrofit ordinances that require the strengthening of some types of known vulnerable buildings, but no state or nation-wide program currently exists. The Structural Engineers Association of Southern California (SEAOSC) recently decided to launch a study of which jurisdictions in the southern California region have started to take the steps necessary to enact critical building ordinances. According to SEAOSC President Jeff Ellis, S.E., “In order to develop an effective strategy to improve the safety and resilience of our communities, it is critical to benchmark building performance policies currently in place. For southern California, this benchmarking includes recognizing which building types are most vulnerable to collapse in earthquakes, and understanding whether or not there are programs in place to decrease risk and improve recovery time.” These results were presented in SEAOSC’s Safer Cities Survey, in partnership with the Dr. Lucy Jones Center for Science and Society and sponsored by Simpson Strong-Tie.

safer-cities-ca

This groundbreaking report is the first comprehensive look at what critical policies have been implemented in the region of the United States with the highest risk of earthquake damage. According to the Los Angeles Times, the survey “found that most local governments in the region have done nothing to mandate retrofits of important building types known to be at risk, such as concrete and wooden apartment buildings.”

The Safer Cities Survey highlights how the high population density of the SoCal region coupled with the numerous earthquake faults and aging buildings is an issue that needs to be addressed by all jurisdictions as soon as possible. An excerpt from the survey covers in detail why this issue is so important:

No building code is retroactive; a building is as strong as the building code that was in place when the building was built. When an earthquake in one location exposes a weakness in a type of building, the code is changed to prevent further construction of buildings with that weakness, but it does not make those buildings in other locations disappear. For example, in Los Angeles, the strongest earthquake shaking has only been experienced in the northern parts of the San Fernando Valley in 1971 and 1994 (Jones, 2015). In San Bernardino, a city near the intersection of the two most active faults in southern California where some of the strongest shaking is expected, the last time strong shaking was experienced was in 1899. Most buildings in southern California have only experienced relatively low levels of shaking and many hidden (and not so hidden) vulnerabilities await discovery in the next earthquake.

 The prevalence of the older, seismically vulnerable buildings varies across southern California. Some new communities, incorporated in the last twenty years, may have no vulnerable buildings at all. Much of Los Angeles County and the central areas of the other counties may have very old buildings in their original downtown that could be very dangerous in an earthquake, surrounded by other seismically vulnerable buildings constructed in the building booms of the 1950s and 1960s. Building codes do have provisions to require upgrading of the building structure when a building undergoes a significant alteration or when the use of it changes significantly (e.g., a warehouse gets converted to office or living space). Seismic upgrades can require changes to the fundamental structure of the building. Significantly for a city, many buildings never undergo a change that would trigger an upgrade. Consequently, known vulnerable buildings exist in many cities, waiting to kill or injure citizens, pose risks to neighboring buildings, and increase recovery time when a nearby earthquake strikes.

1994-northridge

The survey also serves as a valuable reference in being able to identify and understand what the known vulnerable buildings types are:

  1. Unreinforced masonry buildings: brick or masonry block buildings with no internal steel reinforcement — susceptible to collapse
  2. Wood-frame buildings with raised foundations: single-family homes not properly anchored to the foundation and/or built with a crawl space under the first floor — possible collapse of crawl space cripple walls or sliding off foundation
  3. Tilt-up concrete buildings: concrete walls connected to a wood roof — possible roof-to-wall connection failures leading to roof collapse
  4. Non-ductile reinforced concrete buildings: concrete buildings with insufficient steel reinforcement — susceptible to cracking and damage
  5. Soft first-story buildings: buildings with large openings in the first floor walls, typically for a garage — susceptible to collapse of the first story
  6. Pre-1994 steel moment frame buildings: steel frame buildings built before the 1994 Northridge earthquake with connections — susceptible to cracking leading to potential collapse

1933-earthquake-shot

Along with the comprehensive list of potentially dangerous buildings, the survey also offers key recommendations on how cities can directly address these hazards and reduce potential risks due to earthquakes. As a good starting point, the survey recommends having “…an active or planned program to assess the building inventory to gauge the number and locations of potentially vulnerable buildings…is one of the first steps in developing appropriate and prioritized risk mitigation and resilience strategies.

Economic costs can be substantial for businesses whose buildings have been affected by an earthquake. After a major seismic event, a structure needs to be cleared by the building department as safe before it can be reoccupied, and it will generally receive a green (safe), yellow (moderately damaged) or red (dangerous) tag.  A typical yellow-tagged building could take up to two months to be inspected, repaired and then cleared, meaning an enormous absence of income for businesses. The survey offers a strategy for getting businesses up and running quickly after an earthquake, in order to minimize such losses. The Safer Cities Survey recommends that cities adopt a “Back-to-Business” or “Building Re-Occupancy” program, which would “create partnerships between private parties and the City to allow rapid review of buildings in concert with City safety assessments…Back-to-Business programs…[allow] private parties to activate pre-qualified assessment teams, who became familiar with specific buildings to shorten evaluation time [and] support city inspections.

oes-inspectors-program

Basically, a program like this would allow a property owner to work with a structural engineer before an earthquake occurs. This way, the engineer is familiar with the building’s layout and potential risks, and can plan for addressing any potential damage. Having a program like this in place can dramatically shorten the recovery time for a business, from two months down to perhaps two weeks. Several cities have already adopted these types of programs, including San Francisco and Glendale, and it showed up as a component of Los Angeles’ Resilience by Design report.

Ultimately, the survey found that only a handful of cities have adopted any retrofit ordinance, but many cities indicated they were interested in learning more about how they could get started on the process. As a result, SEAOSC has launched a Safer Cities Advisory Program, which offers expert technical advice for any city looking to enact building retrofit ordinances and programs. This collaboration will hopefully help increase the momentum of strengthening southern California so that it can rebound more quickly from the next “Big One.”

We all want to minimize the risk in our lives, so let’s support our local structural engineering associations and building departments in exploring and enacting seismic building ordinances that benefit the entire community.

For additional information or articles of interest, please visit:

Use Strong-Wall® Shearwall Selector to Design Shearwalls

This blog post was written by Travis Anderson.

Strong-Wall Shearwall Selector-Homepage

In time for spring and summer 2017 construction projects, Simpson Strong-Tie has launched the newest version of the Strong-Wall Shearwall Selector for use with engineered design. The latest release is an easy-to-use Web-based application (that’s right, no software to download) that has been updated to comply with the 2015 IBC and now provides solutions for all three Strong-Wall Shearwall types: the Steel Strong-Wall® shearwall (SSW), the Strong-Wall wood shearwall (WSW) and the wood Strong-wall shearwall (SW). If you are familiar with the Strong-Wall Shearwall Selector, you can begin using the web application immediately. For those of you who would like to know more about the web app, please read on.

The Strong-Wall Shearwall Selector was created to help the Designer select the appropriate shearwall solution for a given application in accordance with the latest building code requirements. By performing a technical analysis, the web app provides actual drift and uplift values for a wind or seismic design shear load.

The Strong-Wall analysis also considers simultaneous, vertically applied load. In cases of multiple walls in a line, the program performs a rigidity analysis and determines the actual distributed shear to each wall. When walls are stacked in a two-story configuration, the program evaluates cumulative overturning effects to ensure that the wall, anchor bolt and anchorage to the foundation are not overstressed.

The web app provides two modes for generating an engineered solution: Optimized In-Plane Shear or Manual In-Plane Shear. The Optimized mode lists several possible solutions for the selected criteria in the order of cost. The Manual mode evaluates any number or combination of walls for adequacy based on the selected criteria. The Designer has the option to generate an Anchorage Solution based on foundation type. Once a solution has been selected, the web app will generate a pdf output. Files can be saved and reused for future designs.

Input Variables Within the Two Solution Modes:

Job Name: Enables the Designer to provide a specific job name for a project.

Wall Name: Enables the Designer to provide a name for each wall line in a project.

Wall Type (Manual Only): Solutions are provided for the selected Strong-Wall panel type: SSW, WSW, SW

Application: Defines the proposed application (use) of the wall. The choices are for walls in a garage front, a standard wall on concrete, on a first-story wood-floor system, in a second-floor non-stacked application, in a two-story stacked application, or in a balloon-framed application. For the Steel Strong-Wall® (SSW) and Strong-Wall wood shearwall (WSW), garage front may be chosen with or without the portal kit. Higher shear capacities are available when the portal kit is used.

Cold-Formed Steel Construction (CFS): This option appears for “Garage Front,” “Standard Wall on Concrete,” “First-Story, Raised-Floor System” and “Two-Story Stacked” applications. If the check box is enabled, the program will provide the proper Steel Strong-Wall model for use in CFS construction.

1st Story Wall on Wood Floor (SW – Wood Strong-Wall Shearwall only): This check box only appears if a Two-Story Stacked application has been selected. If enabled, the program will then assume the lower story wall, in a stacked application, is installed on a wood floor.

Strong-Wall Shearwall Selector-Input Variables

Design Criteria:

The design criteria may now be selected. Drop-down menus provide options for Applicable Building Code, load type, concrete strength, wall height, wall geometry and floor depth (if applicable). Entry fields may be used to indicate shear- and axial-loading information. The following applies once the appropriate design criteria have been input: If Optimized In-Plane Shear has been selected, the possible solutions are displayed in the Strong-Wall Panel Solutions list. If Manual In-Plane Shear has been selected, a list of available walls will be displayed in the Strong-Wall Panel Solutions list, any of which may then be selected and added to the desired Solution.

Strong-Wall Shearwall Selector-Design Criteria

Code: Wall solutions are provided in accordance with the requirements of the 2015 and 2012 International Building Code (IBC). Code reports may be found here.

Load Type: This criterion defines whether the input shear load is due to wind or seismic forces. The Designer must input the controlling load. The appropriate seismic “R” values are provided for the selected code.

Concrete Strength: Concrete strength may be selected based on specific project conditions. Default concrete strengths of 2500 psi, 3000 psi, 3500 psi, 4000 psi and 4500 psi are provided in the drop-down menu. Note that for shearwall selection purposes, concrete strengths are only applicable to Steel Strong-Wall® (SSW) and Strong-Wall wood shearwall (WSW). In some cases, lower anchorage forces may be obtained with a higher concrete strength. The concrete strength is also used for determining the anchorage tension capacity.

Wall Height: Select the nominal wall height. Actual wall heights are shown under the “H” column of the Solution(s).

Shear Load: Input the total Allowable Stress Design (ASD) design (demand) shear load along the wall line. Include all appropriate load factors on the shear load prior to input for the load combination under consideration. For Two-Story Stacked applications, input the story shear at each level and the program will evaluate the first-story walls for the total shear.

Floor-Joist Depth: This option appears only with first-story raised-floor systems and two-story

stacked applications. Floor-joist depth affects the capacity of Steel Strong-Wall panels installed on wood floors. Floor-joist depth is also considered in the cumulative overturning evaluation of two-story stacked wood or steel walls.

Header Thickness: This option appears only when “Garage Front” applications and wall heights of 7′ or 8′ with a header on top are selected. This option is used to select the proper Wood Strong-Wall panel model (thickness) based on the nominal header thickness of 4″ or 6″.

Header Type: This option only appears when “Header Thickness” of 4″ is selected. It then provides an option to select a solid or double-ply header. Values for the wood Strong-Wall panels will slightly decrease if the double-ply header option is selected. Steel Strong-Wall panels with multi-ply headers are limited to wind designs and SDC A-C.  .

Maximum Number of Wall Segments per Wall Line (Optimized mode only): Here the maximum number of available wall segments along a particular wall line is specified. The program enables the Designer to select a maximum of four wall segments per wall line (3 segments maximum for garage fronts.) For more wall segments per wall line, use the Manual mode.

Fill Each Segment (Optimized mode only): If this checkbox is disabled, then the minimum number of Strong-Wall shearwalls that can serve as solutions is provided up to the “Max # of Wall Segments” previously specified. If this checkbox is enabled, then the “Max # of Wall Segments” will always be used and filled with Strong-Wall shearwalls.

Segment Number, Maximum Width, Axial (lb.) (Optimized mode only): For each wall segment along a wall line, the maximum desired width of that segment and the axial load on that particular segment may be specified. The axial load is the total vertical upward or downward load assumed to act on the entire panel width. Include all appropriate load factors on the axial load prior to input for the load combination under consideration. A positive axial load reduces the actual uplift of the panel, while a negative axial load increases the actual uplift of the panel. The combined effect of the vertical axial load and overturning force is considered in the Steel Strong-Wall® (SSW) and Strong-Wall wood shearwall (WSW) solutions. The combined effect of the vertical axial load and overturning on the wood Strong-Wall (SW) shall be evaluated by the Designer so as not to exceed the “C4” and “T1” allowable vertical loads. Download an excerpt from our catalog for more information.

Axial Load 1st Story (Manual mode only): See discussion above on axial load. The axial load selected is initially applied on all Available Wall solutions. As walls are selected using the “Add” button, the axial load remains constant. If it is desired that each wall have a different axial load, then input the corresponding axial load value for the first wall and click on “Add Solution” to send it to the Selected Solution. Then enter the new axial load value for the next wall and continue this process until all the product selections are complete.

Maximum Wall Segment Width: This optional input limits the Available Strong-Wall Panels to the maximum width specified.

Available Wall(s) (Manual mode only): Based on the input Design Criteria, all Available Strong-Wall Panels and their allowable loads are listed as an option for selection. The Available Strong-Wall list is independent of the input shear load and instead represents a list whereby any quantity or combination of walls can be selected to resist the shear load.

Solution(s) and Output :

 Possible Solution(s) (Optimized mode only): Up to four possible solutions may be displayed and are designated as Sol # (solution number) in the order of relative cost (lowest to highest material cost).

Selected Solution (Manual mode only):

Add Another Solution: Click on the “Add” button to select wall from Available Wall(s) list, which enters it into the Selected Solution list. You may also double-click on an Available Wall to add it to the Selected Solution.

Clear: Click on the “Clear Selected Solutions” button to entirely remove all previously selected walls in the Selected Solution.

Generate PDF: This button creates a .pdf summary of the wall solution. Under Optimized mode, the output solution is created for the Sol# (solution number) that is highlighted. Under Manual mode, the Output is created for all walls shown in the selected solution list.

Design Anchorage: This option appears at the bottom of the page. If desired, enable the check box next to “Design Anchorage” and select Foundation Type. Anchorage design solutions will then be included in the PDF output.

Notes for Designer: Special notes related to the input variables are displayed in this window during the input process. When the Manual In-Plane Shear tab is selected, the Notes for Designer will indicate whether the Selected Solution is adequate to resist the applied design loads.

Strong-Wall Shearwall Selector-SolutionsStrong-Wall Shearwall Selector-Solution Output

Anchorage Solutions and Output:

 The Designer will have the option to generate an Anchorage Solution appended to the Strong-Wall shearwall solution. If desired, Select Foundation Type, then enable the check box next to Design Anchorage, and the .pdf file will be generated with the anchorage solution on subsequent pages. The designer can choose anchorage solutions based on foundation type for all shearwalls. The two foundation types are slab-on-grade and stemwall and are selected from a drop-down menu. Within each foundation type, the Designer can choose a specific footing type as follows:

Slab-on-Grade Footing Types: Garage curb, slab edge, brick ledge and interior.

Stemwall Footing Types: Garage front and perimeter.

Anchorage solutions are provided based on the shearwall solution(s) selected and the following design criteria: application, load type, actual uplift and concrete strength.

Anchor Bolt: Two anchor bolt solutions are available for the wood Strong-Wall®. They are the PAB7 and the SSTB, both of which are ASTM F1554 Gr. 36 material. The Steel Strong-Wall® uses a single anchor type, SSWAB, which may be either ASTM F1554 Gr. 36 or ASTM A449 (high-strength) material depending on the actual uplift. The Strong-Wall wood shearwall uses a single anchor type, WSW-AB, which may be either ASTM F1554 Gr. 36 or ASTM A449 (high-strength) material depending on the actual anchor tension.

Concrete Service Condition: This criterion refers to whether the concrete is determined to be cracked or uncracked based on analysis at service loads. See ACI 318 for the different reduction factors associated with cracked and uncracked concrete.

Strong-Wall Shearwall Selector-Anchorage Strong-Wall Shearwall Selector-Anchorage Output

The anchorage design .pdf output summarizes all applicable design details including the footing type, minimum footing dimensions, anchor bolt and shear anchorage. The Designer is responsible for foundation design (size and reinforcement) to resist overturning, soil pressure, etc.

Product Information:  Select for more product and application information.

Upload a Saved File: Designer can upload any previously used solution.

Report Applications Issues or Provide Feedback: If you are experiencing issues with the application or simply would like to provide feedback, please use this link. Simpson Strong-Tie values your feedback.

Strong-Wall Shearwall Selector-Info Save Issue

Get started on your next design project with the Strong-Wall® Shearwall Selector web application!

How do you Design Sole-Plate-To-Rim-Board Attachments?

For many years, builders have struggled with the awkward sole-plate-to-rim-board attachment. They often install a few nails and call it good, resulting in a connection with significantly less capacity than needed. This connection is critical to ensure that seismic and wind loads are adequately transferred to the lateral-force-resisting system. With screws becoming much more common in construction, we saw an opportunity to address this problem.

We offer a variety of structural wood screws that have shank diameters ranging from 0.135″ to 0.244″. They form our Strong-Drive® line of structural fasteners. The Simpson Strong-Tie® Strong-Drive SDWC Truss, SDWH Timber-Hex, SDWS Timber, SDWV Sole-to-Rim and SDS Heavy-Duty Connector structural wood screws as shown in Figure 1 can be used to attach sole plates to a rim board as shown in Figure 2. These screws provide structural integrity in the wall-to-floor connection.

The sole-to-rim connection is considered a dry service location. When the sole plate and the rim are both clean wood (not treated), then any of the screws can be used as long as they meet the design loads. However, if one or both members of the connection are treated with fire retardants or preservatives, then you must use the SDWS Timber screw, SDWH Timber-Hex screw or SDS Heavy-Duty Connector screw. The SDWS, SDWH and SDS screws all have corrosion-resistance ratings in their evaluation reports.

Figure 1. Simpson Strong-Tie Strong-Drive screws for fastening the sole-to-rim connection: (a) SDWS Timber screw, (b) SDWV Sole-to-Rim screw, (c) SDWH Timber-Hex screw, (d) SDS Heavy-Duty Connector screw, (e) SDWC Truss screw.

Figure 1. Simpson Strong-Tie Strong-Drive screws for fastening the sole-to-rim connection: (a) SDWS Timber screw, (b) SDWV Sole-to-Rim screw, (c) SDWH Timber-Hex screw, (d) SDS Heavy-Duty Connector screw, (e) SDWC Truss screw.

Figure 2. The load rating for the sole-to-rim connection is for transfer of loads parallel to the sole plate to the rim. This is a dry service condition.

Figure 2. The load rating for the sole-to-rim connection is for transfer of loads parallel to the sole plate to the rim. This is a dry service condition.

The Strong-Drive SDWV structural wood screw has the smallest diameter among these screws. The SDWV is 4″ long and has a 0.135″- diameter shank, and a large 0.400″-diameter ribbed-head with a deep six-lobe recess to provide clean countersinking. It is designed to be fast driving with very low torque. The Strong-Drive SDWS offers one of the larger diameters. It has a 0.220″-diameter shank and is offered in lengths of 4″, 5″ and 6″. It has a large 0.750″-diameter washer head which provides maximum bearing area. Longer screws allow designers to meet the minimum penetration requirement into a rim board, when the sole plate is a 3x or a double 2x member.

We have tested various combinations of sole plates, floor sheathing, and rim boards. Typical test assemblies were built and tested with two (2) Strong-Drive® screws spaced at either 3″ or 6″. Results were analyzed per ICC-ES AC233, “Acceptance Criteria for Alternate Dowel-type Threaded Fasteners.” The allowable loads listed in Table 1 are based on the average ultimate test load of at least 10 tests, divided by a safety factor of 5.0, and are rated per single fastener. The results of these tests can be found in the engineering letter L-F-SOLRMSCRW16.

The evaluated sole plates include southern pine (SP), Douglas fir-larch (DF), hem-fir (HF), and spruce-pine-fir (SPF) in single 2x, 3x or double 2x configurations. Floor sheathing thicknesses are allowed up to 1 1/8″ thick. Rim boards can be LVL or LSL structural composite lumber or DF, SP, HF or SPF sawn lumber. The load rating also assumes that the floor sheathing is fastened separately and per code.

sdwc-load-tables

See strongtie.com for evaluation report information if it is needed.

As a Designer, you can specify any of these Strong-Drive screws that fit your design requirements. Please visit our website and download L-F-SOLRMSCRW16 for more details.

Good luck!

Pile Construction Fasteners – New and Expanded Applications

The majority of Simpson Strong-Tie fasteners are used to secure small, solid-sawn lumber and engineered wood members. However, there is a segment in the construction world where large piles are the norm. Pile framing is common in piers along the coast, elevated houses along the beach, and docks and boardwalks.

While the term “pile” is generic, the piles themselves are not generic. They come in both square and round shapes, as well as an array of sizes, and they vary greatly based on region. The most common pile sizes are 8 inches, 10 inches, and 12 inches, square and round, but they can be found in other sizes. The 8-inch and 10-inch round piles are usually supplied in their natural shape, while 12-inch round piles are often shaped to ensure a consistent diameter and straightness. All piles are preservative-treated.

Historically, the attachment of framing to piles has been done with bolts. This is a very labor-intensive method of construction, but for many years there was no viable fastener alternative. Two years ago, however, Simpson Strong-Tie introduced a new screw, the Strong-Drive® SDWH Timber-Hex HDG screw (SDWH27G), specifically designed for pile- framing construction needs. It can be installed without predrilling and is hot-dip galvanized (ASTM A153, Class C) for exterior applications.

Figure 1 – SDWH27G Lengths

Figure 1 – SDWH27G Lengths

Simpson Strong-Tie tested a number of different pile-framing connections that can be made with the SDWH27G screw. This blog post will highlight some of the tested connections. More information can be found in the following three documents on our website:

  • The flier for the SDWH Timber-Hex HDG screw: F-FSDWHHDG14 found here.
  • The engineering letter for Square Piles found here.
  • The engineering letter for Round Piles found here.

The flier provides product information, and the engineering letters include dimensional details for common pile-framing connections that were tested.

Piles are typically notched or coped to receive a horizontal framing member called a “stringer.” The coped shoulder provides bearing for the stringer and serves as a means of transferring gravity load to the pile. The SDWH27G can be used to fasten framing to coped and non-coped round and square piles.

The connections that we tested can be put into four general groups that include both round and square piles:

  • Two-side framing on coped and non-coped piles
  • One–side framing on coped and non-coped piles
  • Corner framing on coped piles
  • Bracing connections

Additionally, the testing program included four different framing materials in several thicknesses and depths:

  • Glulam
  • Parallam
  • Sawn lumber
  • LSL/LVL

The total testing program included more than 50 connection conditions that represented pile shape and size, framing material and thickness and framing orientation and details. We assigned allowable uplift and lateral properties to the tested connections using the analysis methods of ICC-ES AC13. Figures 2 and 3 show some of the tested assemblies.

Figure 2 – Uplift Test of a 10" Coped Round Pile with a 3-2x10 SYP Stringer

Figure 2 – Uplift Test of a 10″ Coped Round Pile with a 3-2×10 SYP Stringer

Figure 3 – Lateral Test of an 8" Coped Square Pile with a 3.125" Glulam Stringer

Figure 3 – Lateral Test of an 8″ Coped Square Pile with a 3.125″ Glulam Stringer

Figures 4 through 9 illustrate some of the connections and details that are presented in the flier and engineering letters.

Some elements of practice are important to the design of pile-framing connections. Some of the basic practices include:

  • For coped connections, the coped section shall not be more than 50% of the cross-section.
  • For coped connections, the coped shoulder should be as wide as the framing member(s).
  • Fastener spacing is critical to the capacity of the connection.
  • When installing fasteners from two directions, lay out the fasteners so that they do not intersect.
Figure 4 – Square and Round Two-Sided Stringers

Figure 4 – Square and Round Two-Sided Stringers

Figure 5 – Single-Side Stringer with Notched Pile

Figure 5 – Single-Side Stringer with Notched Pile

Figure 6 – Single-Side Stringer with Unnotched Pile

Figure 6 – Single-Side Stringer with Unnotched Pile

Figure 7 – Round Pile Corner Condition

Figure 7 – Round Pile Corner Condition

Figure 8 – Square Pile Corner Condition

Figure 8 – Square Pile Corner Condition

In many cases, pile-framing connections use angled braces for extra lateral support. The SDWH27G can be used in these cases too.

Figure 9 – Braced Condition

Figure 9 – Braced Condition

In the flier and engineering letters previously referenced, you will find allowable loads and specific fastener specifications for many combinations of stringer and pile types and sizes.

What have you seen in your area? Let us know – perhaps we can add your conditions to our list.

 

FRP Concrete Strengthening – Five Case Studies

Fiber-reinforced polymer (FRP) composite systems can be used to strengthen walls, slabs and other concrete or masonry members in buildings and other structures. The case studies below show ways in which Composite Strengthening Systems™ (CSS) provide valuable solutions for strengthening buildings and other structures for our customers.

Residential Project in San Francisco

The homeowner for this project wanted to repair some spalling concrete on his concrete piers and also wrap the piers with FRP. We worked with the contractor and homeowner to design a cost-effective solution. This was a successful project for all involved, since the alternative was to jacket the piers with costly and unsightly steel jackets.

residential-project-san-francisco

Materials: CSS-CUCF Carbon Fabric, CSS-ES Epoxy Saturant & Primer

School Project in Argentina

The goal of the project was to analyze a standard design of approximately 400 schools in Argentina that were built in the 1980s and to make recommendations to retrofit the structures to meet current seismic code requirements.  On analysis, it was found that columns were in need of shear reinforcement for the schools to meet the new seismic requirements.

Materials: CSS-UCF Carbon Fabric, CSS-CA Carbon FRP Anchors, CSS-ES Epoxy Saturant & Primer

Materials: CSS-UCF Carbon Fabric, CSS-CA Carbon FRP Anchors, CSS-ES Epoxy Saturant & Primer

Hospital Project in Butler, PA

The Engineer of Record on this project wanted to provide continuity across the slab construction joints, something which the existing rebar did not provide. We provided a design of Near-Surface-Mounted (NSM) laminates, which are installed in saw-cut grooves in the top of the concrete slab. This installation allows a flush finished surface, important for allowing the floor finishes to be installed on the slab.

Materials:CSS-CUCL Carbon Precured Laminate, CSS-EP Epoxy Paste & Filler

Materials: CSS-CUCL Carbon Precured Laminate, CSS-EP Epoxy Paste & Filler

Silo Project in Garden City, IA

The concrete silos on this project had spalling at the top portion, which caused a hazard at this site. After repairing the concrete, we provided a ring of carbon fabric to assist in keeping the top concrete of the silos solid for years to come.

Materials:CSS-CUCF Carbon Fabric, CSS-ES Epoxy Saturant & Primer

Materials: CSS-CUCF Carbon Fabric, CSS-ES Epoxy Saturant & Primer

Bridge Project in MN

MNDOT wanted to gain experience working with our CSS products on one of their bridges. We worked with their staff to design several types of strengthening solutions for bridge pier caps and columns. We then provided onsite installation training for the MNDOT maintenance staff to install the FRP products on the bridge.

Materials:CSS-CUCF Carbon Fabric CSS-CUGF E-glass Fabric CSS-ES Epoxy Saturant & Primer CSS-EP Epoxy Paste & Primer frp concrete strengthening

Materials: CSS-CUCF Carbon Fabric, CSS-CUGF E-glass Fabric, CSS-ES Epoxy Saturant & Primer, CSS-EP Epoxy Paste & Primer

We recognize that specifying Simpson Strong-Tie® Composite Strengthening Systems™ is unlike choosing any other product we offer. Leverage our expertise to help with your FRP strengthening designs. Our experienced technical representatives and licensed professional engineers provide complimentary design services and support – serving as your partner throughout the entire project cycle. Since no two buildings are alike, each project is optimally designed to the Designer’s individual specifications. Our pledge is to address your specific condition with a complete strengthening plan tailored to your needs, while minimizing downtime or loss of use, at the lowest possible installed cost.

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Your Partner During the Project Design Phase 

During the Designer’s initial evaluation or preparation of the construction documents, Simpson Strong-Tie can be contacted to help create the most cost-effective customized solution. These plans include detailed design calculations for each strengthening requirement and design drawings with all the necessary details to install the CSS system. Simpson Strong-Tie Engineering Services will work closely with the Design Engineer to provide all the necessary information required to design the system.

Why Use Our Design Services?

  • Assess feasibility studies to ensure suitable solutions to your application
  • Receive customized FRP strengthening solutions
  • Work with our trained contractor partners to provide rough-order-of-magnitude (ROM) budget estimates
  • Collaborate during the project design phase
  • Receive a full set of drawings and calculations to add to your submittal
  • Maintain the flexibility to provide the most cost-effective solution for your project
  • Gain trusted technical expertise in critical FRP design considerations

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Advanced FRP Design Principles

In this free webinar we will dive into some very important considerations including the latest industry standards, material properties and key governing limits when designing with FRP.


For complete information regarding specific products suitable to your unique situation or condition, please visit strongtie.com/css or call your local Simpson Strong-Tie RPS specialist at (800) 999-5099.

 

Cold-Formed Steel Curtain-Wall Systems

In August 2012, Simpson Strong-Tie launched a comprehensive, innovative solution for curtain-wall framing. Our lead engineer for developing our line of connectors for curtain-wall construction explains the purpose of the curtain wall with the illustrations below.

steel-stud-framingFirst, curtain walls are not what you put up if you shared a room with your brother and sister when you were growing up. When I first learned about the use of cold-formed steel curtain walls, I laughed and thought: “Gosh, how useful this would be for someone growing up with 5 siblings in one bedroom!” I have always enjoyed the sense of humor that our engineers use to help explain technical topics.

Curtain walls can be described as exterior building walls with the primary purpose of protecting the interior building against the exterior weather and natural phenomena such as sun exposure, temperature changes, earthquakes, rain and wind.

To put it in structural terms, a curtain-wall system consists of non-load-bearing exterior walls that must still carry their own weight. Curtain walls are not part of the primary structural framing for the building, but they typically rely on the primary structural framing for support. Additionally, curtain walls receive wind and seismic loads and transfer these forces to the primary building structure.

Types of Curtain Walls

Glass and cladding curtain walls make up two basic types of curtain-wall systems. Glass curtain-wall systems are usually designed using aluminum-framed walls with in-fills of glass. The cladding curtain wall is a system with back-up framing that is covered in some type of cladding material. The cladding curtain-wall system is the type in which Simpson Strong-Tie products can be used.

mid-rise-buildings-1The back-up framing is the structural element of the curtain-wall system. It is typically constructed with cold-formed steel studs ranging from 31/2″ to 8″ deep, in 33 mil (20 ga.) to 97 mil (12 ga.) steel thicknesses. The framing studs are typically spaced at 16″or 24″ on center. There are many different types of cladding materials. They include, but are not limited to, exterior insulation finish systems (EIFS), glass-fiber-reinforced concrete (GFRC), bricks, metal panels and stone panels.

building-material-examplesDeflection

One essential function of the curtain wall is to allow for relative movement between the curtain-wall system and the main building structure. At first, it was not obvious to me why making this allowance was necessary, but our product development team creatively explained some of the reasons why this is an important must-have feature for curtain walls.

deflection-examplesFirst, the primary building will move up and down as it is loaded and unloaded by the live-load occupancy, similar to beam live-load deflections.

Second, the structure sways and has torsional displacement due to movement from lateral wind or seismic loads.

Third, concrete structures typically encounter creep and shrinkage, and there may be foundation differential settlement or soil compression from high-gravity loads.

Lastly, the temperature differential may cause the building elements to expand and contract, which, again, can result in relative movement between structural elements. This is similar to a bridge’s steel plate expansion joint system.

And if you are a curious designer like me, you probably wonder why the relative vertical moment is so significant in engineering design.

One key reason is to ensure that the curtain walls do not collect gravity loads from the building, so as to prevent overloading and possible failure of the stud framing. In addition, a well-designed curtain-wall system needs to retain the primary structural load path as intended by the building designer.

The other reason is to protect the cladding of the building. If you remember earlier, the cladding material may be marble, granite or natural stones that are often very expensive and heavy. In some cases, the cladding can be one of the most expensive systems in a building. And there are times when it’s much more cost-effective to design for relative movement than it is to over-design structural framing to address the stringent deflection requirements.

Construction Type

Bypass framing is a term that is often used in curtain-wall construction. In this system, the metal studs bypass the floor and hang off the outside edges of the building. You can see from the illustration how the studs run past, or bypass, the edge of the slab. In this case, the studs are supported vertically on the foundation at the bottom, and then run continuously past multiple floor levels.

Picture by Don Allen of Super Stud Building Products.

Picture by Don Allen of Super Stud Building Products.

In steel construction, concrete fill over metal deck is typically constructed with a heavy-gauge bent plate or structural angle. Connectors can attach directly to the steel angle or the web of an edge beam.

Simpson Strong-Tie SCB Bypass slide clip connections.

Simpson Strong-Tie SCB Bypass slide clip connections.

SSB Bypass Framing Slide-Clip Strut connections.

SSB Bypass Framing Slide-Clip Strut connections.

 

 

 

 

 

 

 

 

It may seem that this type of construction is too complex and requires great efforts to detail the many connections needed to hang the curtain wall off the outside of the building. So what are the compelling reasons to choose bypass framing construction?

Bypass framing can accommodate flexibility for the architect. In another words, the bypass configuration easily allows architects to create reveals, set-backs and other architectural features.  Plus, there are fewer joints to detail for movement when stud length can run continuously for several floors.  Another benefit is that the exterior finish can also be installed on a curtain-wall system with a tighter tolerance than the edge of the structure.

One other special bypass framing type is known as ribbon window or spandrel framing. Ribbon windows are a series of windows set side by side to form a continuous band horizontally across a façade. The vertical deflection for this type of bypass framing is typically accommodated at the window head. This type of bypass usually works well for panelized construction.

Another common curtain-wall system is infill framing, where the studs run from the top of one floor to the underside of the floor above. Sometimes it’s a challenge to attach bypass framing to the edge of thin concrete slabs. In the following illustration, deflection is designed at the top track of wall panels.

bypass-framing-in-actionIn Part 2 of this blog post series, I will provide more details about how we have innovated products to be used for this application, plus a more comprehensive post about the products we offer and how they are typically used.

In the meantime, you can check out our product offering. Our recent SC slide-clip and FC fixed-clip connectors are designed for high-seismic areas.

I would like to invite you to comment and provide feedback on this topic and tell us whether you’ve had any experience working with a Designer on a CFS curtain-wall project. If you are a Designer who specializes in this discipline, how are you designing curtain-wall systems for seismic forces?

 

Soft-Story Retrofits Using the New Simpson Strong-Tie Retrofit Design Guide

Thousands of soft-story buildings up and down the West Coast require retrofits to prevent collapse in the event of a major earthquake. Whether the retrofits are mandated by a city ordinance (as in San Francisco, Berkeley and Los Angeles) or are undertaken as voluntary upgrades, the benefits of adding necessary bracing to strengthen the ground story are immense. Simpson Strong-Tie has taken the lead, with our new Soft-Story Retrofit Guide, to provide information that helps engineers find solutions to reinforce soft-story buildings against collapse. We are also providing information on the two methods that can be used for the analysis and design of these soft story retrofits.

soft-story-retrofit-guideAfter the initial information section of the guide, a two-page illustrated spread (pp. 14–15) shows various retrofit products that could be used to retrofit the soft-story structure with reference to the following pages. Three main lateral-force-resisting systems highlighted in this graphic are the Strong Frame® special moment frame (SMF), the new Strong-Wall® wood shearwall, and conventional plywood shearwalls. Individual retrofit components are also shown, such as connection plates and straps for lateral-load transfer, anchors for attachment to the foundation, fasteners and additional products such as the RPBZ retrofit post base and AC post caps for providing a positive connection.

soft-story-product-illustrationTurning the page, you come to the section describing in detail the many benefits of the Strong-Frame special moment frame (SMF) in a retrofit situation. The engineered performance of the SMF provides the additional strength and ductility that the building requires and can be fine-tuned by selecting various combinations of beams, columns, and Yield-Link® structural fuse sizes. A typical retrofit Strong Frame® SMF comes in three complete pieces allowing for the frame to be installed on the interior of the structure in tight quarters. The frame is simply installed using a 100% snug-tight field-bolted installation with no on-site welding or lateral-beam bracing required.

field-installation-beam-to-columnThe next lateral system we focus on is the Strong-Wall® shearwall and the new grade beam solutions offered to reduce the concrete footprint. The new Strong-Wall wood shearwall includes an improved front-access holdown and top-of-wall connection plates for easier installation. Both the Strong Frame SMF and the Strong-Wall wood shearwall have load-drift curves available for use with FEMA P-807. Site-built shearwalls can be installed using retrofit anchor bolts at the mudsill and new holdowns at the shearwall end posts.

strong-wall-wood-shearwall-pushover-curveIn the pages following the lateral systems, various products are shown with tabulated LRFD capacities, whereas ASD capacities are typically provided in the order literature for these products. Both ASD and LRFD capacities have been provided for products with new testing values such as the A35 and L90 angles installed with ⅝”-long SPAX screws into three different common floor sheathing materials, as well as for the new HSLQ heavy-shear transfer angle designed to transfer higher lateral forces directly from 4x blocking to the 4x nailer on the Strong-Frame SMF, even when a shim is used between the floor system and the frame. LRFD capacities are provided in this new Soft-Story Retrofit Guide specifically for use with the FEMA P-807 design methodology. This methodology specifies in section 6.5.1 that:

Load path elements should be designed to develop the full strength and the intended mechanism of the principal wall or frame elements. Therefore, to ensure reliability, appropriate strength reduction factors should be applied to the ultimate strengths of load path elements. Specific criteria may be derived from principles of capacity design or from other codes or standards, such as ASCE/SEI 41 or building code provisions involving the overstrength factor, Ωo.

FEMA P-807 bases the capacity of the retrofit elements on the peak strength. LRFD capacities are provided for various load-path connector products, which can be used to develop the full strength of the lateral-force-resisting element to satisfy this requirement.

typical-a35-hslq412-installationWrapping up, the guide focuses on the various free design tools and resources available for the evaluation, design and detailing of the soft-story structure retrofit. These tools include the Weak Story Tool with Simpson Strong-Tie® Strong Frame® Moment Frames, Design Tutorials for the WST for both San Francisco– and Los Angeles–style buildings, our Soft-Story Retrofit Training Course offering CEUs, Strong Frame Moment Frame Selector Software, Anchor Designer™ Software for ACI 318, ETAG and CSA, and tailored frame solutions using our free engineering services.

soft-story-documentsFor other information regarding soft-story retrofits, refer to previous blogs in “Soft-Story Retrofits,”  “City of San Francisco Implements Soft-Story Retrofit Ordinance,” and “Applying new FEMA P-807 Weak Story Tool to Soft-Story Retrofit.”

 

 

 

How to Pick a Connector Series – Truss Hangers

In our second blog in the “How to Pick a Connector Series,” Randy Shackelford discussed the various considerations involved in selecting a joist hanger. So why is this blog post about truss hangers? A hanger is a hanger, right? Before I moved into the Engineering Department at Simpson Strong-Tie, I was the product manager for our Plated Truss product line. I can assure you that there is a bit more that goes into the selection (and design) of a truss hanger than does into selecting a joist hanger!

Of course, all of the considerations that were covered in the joist hanger blog apply to truss hangers as well. This blog post is going to discuss some additional considerations that come into play in selecting a hanger for a truss rather than a joist, and how some hangers have features designed especially for trusses.

The first (and most obvious) truss-specific consideration is the presence of webs. Because of truss webs, top-flange hangers are not as conducive to truss applications as they are to joist applications. A better alternative for trusses is an adjustable-strap hanger that can be installed as a top-flange hanger or face-mount hanger. Take the THA29, for example, Simpson’s first hanger developed specifically for the truss industry (circa 1984). It can accommodate different girder bottom chord depths, which eliminates the need for multiple SKUs, and the straps can be field-formed over the top of the girder bottom chord to reduce the number of fasteners (just like top-flange hangers). When a web member is in the way of the top-flange installation method, the straps can be attached vertically to the web in a face-mount installation instead.

Typical THA29 Installation

What if the web at that location isn’t vertical? You can still install the strap onto the web, but if any nails land in the joint lines formed by the intersection of the wood members, they cannot be considered effective. Therefore, the hanger allowable load may need to be reduced to account for ineffective header nails. This alternative installation is acceptable for any face-mount hanger located at a panel point as shown in our catalog (see detail below).

hgus2102-installed

Although very versatile, not all adjustable-strap hangers can be installed on all sizes of bottom chords. Our catalog specifies a C-dimension for these hangers, which corresponds to the height of the side-nailing flanges. If that dimension exceeds the height of the bottom chord, then the straps cannot be field-formed as needed for the top-flange installation. And if the hanger isn’t located at a panel point, nailing the straps to any diagonal web that the straps can reach (see photo below) is not an acceptable option!

The wrong hanger selection for the application

The wrong hanger selection for the application

Another unique consideration that goes into the selection of a truss hanger is the heel height of the carried truss. A truss with a short heel height installed into a tall hanger will likely leave air (or “daylight,” as I call it) behind a lot of the nail holes running up the side flanges. When nail holes in a hanger have air behind them instead of wood, this equates to a reduction in hanger capacity. So when the carried truss has a heel height that is much less than the depth of the carrying member (and the hanger), it is important to use the appropriate hanger capacity for that condition and not overestimate the hanger’s capacity. Refer to our technical bulletin T-REDHEEL for allowable loads for reduced heel height conditions.

Example of a short heel installed in a tall hanger.

Because trusses are capable of carrying a lot of load –  and producing large reactions –  hangers for truss applications often require larger capacities than joist hangers. Unfortunately, there is only so much capacity that can be achieved from a hanger that fits entirely onto a girder truss bottom chord. Therefore, in order to use our highest load-rated truss hangers, a properly located vertical web is required, and the web must be wide enough for the hanger’s required face fasteners and minimum edge distances. The more capacity that is required, the more fasteners it takes, and the wider the vertical web must be. Our highest-load-rated truss hanger that installs with screws is the HTHGQ. It has a maximum download capacity of 20,735 lb., but it requires a minimum 2×10 vertical web. The THGQ/THGQH series can be installed onto as small as a 2×6 web, but the maximum possible capacity on a 2×6 web is 9,140 lb.

hthgq-installation

In addition to high-capacity hangers, truss applications often require high-capacity skewed hangers. When selecting skewed hangers, it’s important to realize that hangers with custom skew options usually have a reduction that must be applied to the hanger’s 90-degree capacity.  Another important factor that is sometimes overlooked in the selection of skewed hangers is whether the carried member is square-cut or bevel-cut. When the member is square cut – as in the case of trusses – not only does this typically result in a greater reduction in capacity, but some skewed hangers cannot be used at all with square-cut members. For example, the fastener holes on the side flange may not be located far enough away from the header to accommodate square-cut members. See the photo below for an example of what can happen if a skewed hanger that is intended for a bevel-cut member is used for a truss.

Incorrect hanger selection – this skewed hanger requires the carried member to be bevel-cut whereas the truss is square-cut.

Incorrect hanger selection – this skewed hanger requires the carried member to be bevel-cut whereas the truss is square-cut.

Not all skewed hangers can be used with square-cut members (trusses).

Not all skewed hangers can be used with square-cut members (trusses).

As discussed in the previous hanger blog, face-mount hangers offer the advantage of being installed after the joist (or truss) is installed. What if the truss is installed prior to the hanger and a gap exists between the truss and the carrying member? In that case, the best option may be to select a truss hanger that was designed with this type of installation tolerance in mind, the HTU hanger. Other face-mount truss hangers that use double-shear nailing are great when gaps are limited to ⅛” or less, but their capacities take a pretty large hit when the gap exceeds ⅛” (see our previous blog Minding the Gap in Hangers for more information). The HTU was designed to give an allowable load for up to a ½” gap between the end of the truss and the carrying member. In addition, it has built-in nailing options to accommodate short heel heights even in the taller models – definitely a truss hanger!

HTU Hanger

HTU Hanger

Finally, there is one more thing to consider when selecting a face-mount hanger for a truss application, which relates to how tall the carrying member is compared to the hanger. Assuming the bottom of the hanger will be installed flush with the bottom of the girder bottom chord, a hanger that is much shorter than the bottom chord will induce tension perpendicular to the grain in the chord. Due to wood’s inherent weakness in perpendicular-to-grain tension, a hanger that is too short may limit the amount of load that can be transferred– to something less than the hanger’s published allowable load. Therefore, it isn’t enough to check whether the hanger fits on the bottom chord; the hanger must also cover enough depth of the chord to effectively transfer the load (or else the allowable hanger load may need to be reduced to the member’s allowable cross-grain tension limit).

Cross-grain tension is not a truss-specific issue, but because it is an explicit design provision in the truss design standard (TPI 1), it is a necessary consideration to mention in a discussion about truss hanger selection. In fact, proper detailing for cross-grain tension in different wood applications could be a future topic in and of itself.

Add to all this the specialty truss hangers that can carry two, three, four, and even five trusses framing into one location, and it is no wonder that there is an entire section in our catalog that is dedicated to truss hangers. Are there any other truss hanger needs that you would like to discuss? Please let us know in the comments below!

 

Concrete Anchorage for ASD Designs

One of the first things I learned in school about using load combinations was that you had to pick either Load and Resistance Factor Design (LRFD)/Strength Design (SD) or Allowable Stress Design (ASD) for a building and stick with it, no mixing allowed! This worked for the most part since many material design standards were available in a dual format. So even though I may prefer to use LRFD for steel and ASD for wood, when a steel beam was needed at the bottom of a wood-framed building that was designed using ASD load combinations, the steel beam could easily be designed using the ASD loads that were already calculated for the wood framing above since AISC 360 is a dual- format material standard. And when the wood-framed building had to anchor to concrete, ASD anchor values were available in the IBC for cast-in-place anchors and from manufacturers for post-installed anchors in easy-to-use tables, even though ACI 318 was not a dual-format material standard. (Those were good times!)

Then along came ACI 318-02 and its introduction of Appendix D – Anchoring to Concrete, which requires the use of Strength Design. The 2003 IBC referenced Appendix D for Strength Design anchorage, but it also provided a table of ASD values for some cast-in-place headed anchors that did not resist earthquake loads or effects. This option to use ASD anchors for limited cases remained in the 2006, 2009 and 2012 codes. In the 2015 IBC, all references to the ASD anchor values have been removed, closing the book on the old way of designing anchors.

ICC-ES-equation-tensionSo what do you do now? Well, there is some guidance provided by ICC-ES for manufacturers to convert calculated SD capacities to ASD allowable load values. Since there is no conversion procedure stated in the IBC or referenced standards, designers may want to use this generally accepted method for converting anchor capacities designed using ACI 318. ICC-ES acceptance criteria for post-installed mechanical and adhesive anchors (AC193 and AC308) and cast-in-place steel connectors and proprietary bolts (AC398 and AC399) outline a procedure to convert LRFD capacities to ASD using a weighted average for the governing LRFD/SD load combination. So if the governing load combination for this anchor was 1.2D + 1.6L and the dead load was 1,000 pounds and the live load was 4,000, then the conversion factor would be (1.2)(0.2) + (1.6)(0.8) = 1.52 (keep in mind that the LRFD/SD capacity is divided by the conversion factor in the ICC-ES equation shown here for tension).

Right away, there are a few things that you may be thinking:

  1. What about load factors that may exist in ASD load combinations?
  2. It may just be easier to just recalculate my design loads using LRFD/SD combinations!
  3. The resulting allowable loads will vary based on the load type, or combination thereof.
  4. If the ACI 318 design strength is limited by the steel anchor, then the conversion will result in an allowable load that is different from the allowable load listed for the steel element in AISC 360.

Let’s take a look at these objections one by one.

Item 1: Since unfactored earthquake loads are determined at the ultimate level in the IBC, they have an LRFD/SD load factor of 1.0 and an ASD load factor less than 1.0, which is also true for wind loads in the 2012 and 2015 IBC (see graphic below). Using the LRFD/SD load factor of 1.0 obviously does not convert the capacity from LRFD to ASD so you must also account for ASD load factors when calculating the conversion factor. To do so, instead of just using the LRFD load factor, use the ratio of LRFD Factor over ASD Factor. So if the governing load combination for an anchor was 0.9D + 1.0E and the dead load was 1,000 pounds and the seismic load was 4,000, then the conversion factor would be (0.9)(0.2) + (1.0/0.7)(0.8) = 1.32.

ICC-ES-equations

Item 2: Even though the weighted average conversion requires you to go back and dissect the demand load into its various load types, often this can be simplified. ICC-ES acceptance criteria permit you to conservatively use the largest load factor. The most common application I run into is working with ASD-level tension loads for wood shearwall overturning that must be evaluated using SD-level capacities for the concrete anchorage. Since these loads almost always consist of wind or seismic loads, using the largest factor is not overly conservative. Depending on the direction in which you are converting the demand loads or resistance capacities, the adjustment factors are as shown in the figure below. Affected Simpson Strong-Tie products now have different allowable load tables for each load type. (For examples, see pp. 33-36 of our Wood Construction Connectors catalog for wind/seismic tables and pp. 28-30 of our Anchoring and Fastening Systems catalog for static/wind/seismic tables.)

IBC-ealier-later

Item 3: I am unsure whether there is any sound rationale for having allowable loads for an anchor resisting 10% dead load and 90% live load differ from those of an anchor that resists 20% dead load and 80% live load. Perhaps a reader could share some insight, but I just accept it as an expedience for constructing an ASD conversion method for a material design standard that was developed for SD methodology only.

Item 4: We have differing opinions within our engineering department on how to handle the steel strength component of the various SD failure modes listed in ACI 318. Some believe all SD failure modes in ACI 318 should be converted using the load factor conversion method. I side with others who believe that the ASD capacity of a steel element should be determined using AISC 360. So when converting SD anchor tension values for a headed anchor, I would apply the conversion factor to the concrete breakout and pullout failure modes from ACI 318, but use the ASD steel strength from AISC 360.

Finally, I wanted to point out that the seismic provisions in ACI 318, such as ductility and stretch length, must be considered when designing anchors and are not always apparent when simply converting to ASD. For this reason, I usually suggest converting ASD demand loads to SD levels so you can use our Anchor Designer™ software to check all of the ACI 318 provisions. But for some quick references, we now publish tabulated ASD values for our code-listed mechanical and adhesive anchors in our C-A-2016 catalog —  just be sure to read all of the footnotes!

How to Select a Connector Series – Holdowns

Keith Cullum started off our “How to Select a Connector” series with Hurricane Ties. This week we will discuss how to select holdowns and tension ties, which are key components in a continuous load path. They are used to resist uplift due to shearwall overturning or wind uplift forces in light-frame construction. In panelized roof construction, holdowns are used to anchor concrete or masonry walls to the roof framing.

shearwall-segment

Holdowns can be separated in two basic categories – post-installed and cast-in-place. Cast-in-place holdowns like the STHD holdowns or PA purlin anchors are straps that are installed at the time of concrete placement. They are attached with nails to wood framing or with screws to CFS framing. After the concrete has been placed, post-installed holdowns are attached to anchor bolts at the time of wall framing. The attachment to wood framing depends on the type of holdowns selected, with different models using nails, Simpson Strong-Tie® Strong-Drive® SDS Heavy-Duty Connector screws or bolts.

A third type of overturning restraint is our anchor tiedown system (ATS), which is common in multistory construction with large uplift forces. I discussed the system in this blog post.

methods-of-overturning-restraintGiven the variety of different holdown types, a common question is, how do you choose one?

For prescriptive designs, such as the IRC portal frame method, the IRC or IBC may require a cast-in-place strap-style holdown. Randy Shackelford did a great write-up on the PFH method in this post.

For engineered designs, a review of the design loads may eliminate some options and help narrow down the selection.

Holdown TypeMaximum Load (lb.)
Cast-in-Place5,300
Nailed5,090
SDS Screws14,445
Bolted19,070

sthd-installation

htt-installation

hdb-installation

hdu-installation

I like flipping through catalog pages, but our Holdown Selector App is another great tool for selecting a holdown to meet your demand loads. Select cast-in-place or post-installed, enter your demand load and wood species, and the application will list the holdown solutions that work for your application.

holdown-selector-app

The application lists screwed, nailed and bolted solutions that meet the demand load in order of lowest installed cost, allowing the user to select the least expensive option.

Adjustability should be considered when choosing between a cast-in-place and a post-installed holdown. Embedded strap holdowns are economical uplift solutions, but they must be located accurately to align with the wood framing. If the anchor bolt is located incorrectly for a post-installed holdown, raising the holdown up the post can solve many problems. And anchors can be epoxied in place for missing anchor bolts.

offset-holdown-raised-off-sillWe are often asked if you can double the load if you install holdowns on both sides of the post or beam. The answer is yes, and this is addressed in our holdown general notes.

notes-on-doubling-loads

Nailed or screwed holdowns need to be installed such that the fasteners do not interfere with each other. Bolted holdowns do not need to be offset for double-sided applications. Regardless of fastener type, the capacity of the anchorage and the post or beam must be evaluated for the design load.

double-sided-bolted-purlin-cross-tie

double-sided-hdu-offset-installation

Once you have selected a holdown for your design, it is critical to select the correct anchor for the demand loads. Luckily, I wrote a blog about Holdown Anchorage Solutions last year. What connector would you like to see covered next in our series? Let us know in the comments below.