Paul McEntee

About Paul McEntee

A couple of years back we hosted a “Take your daughter or son to work day,” which was a great opportunity for our children to find out what their parents did. We had different activities for the kids to learn about careers and the importance of education in opening up career opportunities. People often ask me what I do for Simpson Strong-Tie and I sometimes laugh about how my son Ryan responded to a questionnaire he filled out that day:

Q.   What is your mom/dad's job?
A.   Goes and gets coffee and sits at his desk

Q.   What does your mom/dad actually do at work?
A.   Walks in the test lab and checks things

When I am not checking things in the lab or sitting at my desk drinking coffee, I manage Engineering Research and Development for Simpson Strong-Tie, focusing on new product development for connectors and lateral systems.

I graduated from the University of California at Berkeley and I am a licensed Civil and Structural Engineer in California. Prior to joining Simpson Strong-Tie, I worked for 10 years as a consulting structural engineer designing commercial, industrial, multi-family, mixed-use and retail projects. I was fortunate in those years to work at a great engineering firm that did a lot of everything. This allowed me to gain experience designing with wood, structural steel, concrete, concrete block and cold-formed steel as well as working on many seismic retrofits of historic unreinforced masonry buildings.

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.

DoD-Compliant CFS Wall Framing Design

jeff-kreinkeThis week’s post comes from Jeff Kreinke, PE, SE, a structural Project Manager with Excel Engineering in Fond du Lac, WI. Jeff earned his Bachelor’s degree in Civil & Environmental Engineering from the University of Wisconsin-Madison. He has worked in Excel Engineering’s structural department for 15 years and specializes in cold-formed steel systems design. This specialization has included the design of dozens of “blast” resistant structures per the Unified Facilities Criteria standards. Jeff is licensed in 22 states as a Professional or Structural Engineer.

Back in the year 2000, the U.S. Department of Defense (DoD) was charged with incorporating antiterrorism protective features into the planning, design and execution of its facilities. The main document developed to meet this requirement is the Unified Facilities Criteria “DoD Minimum Antiterrorism Standards for Buildings” (UFC 4-010-01). The current version was published in October 2013. This document covers what is most commonly referred to as “blast” design.

All DoD inhabited buildings, billeting (military housing) and high-occupancy family housing projects are required to comply with this standard. The most common projects incorporating cold-formed steel framing are for the military branches (including National Guard and Reserve components). Notable exceptions are: low-occupancy buildings (11 occupants or fewer), transitional structures (with intended life cycles of five years or less), standalone retail establishments and parking structures. A complete list of excluded buildings types is located in chapter 1-9 of UFC 4-010-01.

General structural requirements for DoD projects are provided in the UFC 1-200-01 standard. The current model building code referenced is the 2012 IBC. Specific structural design criteria are also listed for all major military bases. UFC 4-010-01 specifically states that its requirements do not supersede the structural requirements of UFC 1-200-01. Basically, three lateral designs are required for all DoD projects: wind, seismic and blast.

The main design strategies employed by UFC 4-010-01 are to maximize standoff distance, prevent progressive collapse and minimize hazardous flying debris. Progressive collapse avoidance is addressed by UFC 4-023-03. The main criterion to note is that it is only required in structures of three or more stories in height.

standoff-distances-with-controlled-perimeter

UFC 4-010-01 tables B-1 and B-2 provide the conventional standoff distances and minimum standoff distances for a building based on its construction type and building category. The conventional standoff distance is where conventional construction may be used for building components other than windows and doors without a specific analysis of blast effects. The minimum standoff distance is the smallest permissible distance allowed regardless of any analysis or construction hardening.

standoff-distances-for-new-existing-buildings

conventional-construction-standoff-distances

To minimize hazardous flying debris, two basic components are addressed: typical wall framing and opening support framing. UFC 4-010-01 Table 2-3 provides the allowable height for cold-formed stud typical wall framing based on material, spacing, support conditions and supported weight.

conventional-construction-parameters

conventional-construction-parameters2

For both brick veneer and EIFS cladding, the maximum allowable height is 12″-0″.  Note that 50 ksi stud material is required to meet these requirements. There is no similar strength requirement for connections.

For framing that meets both the conventional standoff distance and allowable member height, NO further blast analysis is required. For framing that does not meet either the conventional standoff distance or allowable member height, additional blast analysis IS required. The support framing (head/sill/jambs) for windows and skylights ALWAYS requires additional blast analysis. Per section B-3.3.3 of UFC 4-010-01, the support framing for doors, glazed or solid, is not required to be analyzed for blast. Doors are designed to remain in their frames and not become hazards to building occupants. Sidelights and transoms that are included within the door assembly are also not required to be analyzed for blast. There are exclusions provided for unoccupied areas of buildings, such as exterior stairwells and exterior walkways. Personal experience with the U.S. Army Corps of Engineers Protective Design Center (USACoE PDC) has shown that unoccupied attic areas are also allowed to be excluded from the provisions.

Two “levels of protection” are provided for in the UFC 4-010-01. Only Low Level of Protection (LLOP) and Very Low Level of Protection (VLLOP) buildings are addressed. Low Level of Protection allows for moderate damage with collapse being unlikely and having the potential for serious but not fatal human injury. Very Low Level of Protection allows for heavy structural damage with progressive collapse unlikely and having serious human injury likely with potential fatalities. Any projects that require a higher Level of Protection designation must be referenced elsewhere. The project appropriate “Level of Protection” is provided in UFC 4-01-01 Tables B-1 and B-2.

UFC 4-010-01 allows for two basic analysis methods for performing blast design, static analysis and dynamic analysis. Static analysis can be performed with the aid of the Simpson Strong-Tie® CFS DesignerTM software, while dynamic analysis must be performed with the “Single-degree-of-freedom Blast Effects Design Spreadsheet” (SBEDS) obtained from the USACoE PDC. Generally, dynamic analysis will provide lighter members than static analysis.

Static Analysis Method

Static analysis of “punched” openings (framed with head and sill members and supported by jamb studs) is only allowed to be performed if the conventional standoff distance is met. Ribbon windows, aluminum curtainwalls, storefronts, etc., are required to be designed by the dynamic analysis method.  The “punched” window supporting structure is to be designed to account for the increased tributary area representing the area of the window and the walls above and below it.  These supporting elements must have moment and shear capacities greater than the calculated conventional wall capacities multiplied by the applicable tributary area increase.

illustration-of-tributary-width-valuesFor example, if the tributary area of the “punched” opening is three times the typical full-height stud spacing, three members are required for the jamb stud assembly. An alternate member with moment and shear capacities greater than or equal to three times the typical full-height members’ capacity can also be used.  UFC 4-01-01 section B-3.1.4.1 states that connection loads shall be determined based on the increase in member shear capacity. It makes a difference which members are chosen for the jamb stud assembly, as the connection must be designed for the shear capacity of that specific assembly. Per UFC 4-010-01 section B-3.1, the connectors themselves are designed using LRFD methods with a Load Factor of 1.0. The resistance factor for bending is allowed to be 1.0, while the resistance factors for other failure modes remain per the AISI code (Shear = 0.95). Per UFC 3-340-02 section 5-47, when LRFD values are not published for connectors, a value of 1.7 times the allowable strength is permitted. Published LRFD strengths already have the appropriate resistance factors incorporated. Simpson Strong-Tie publishes LRFD values for all of its connectors.

Dynamic Analysis Method

As mentioned above, dynamic analysis is performed with the SBEDS spreadsheet tool provided free of charge by the USACoE PDC. There is a simple approval process to undergo in order to receive a software key for the program. The general inputs for the program are fairly straightforward. There is a list of standard preset stud shapes that can be selected. The list is not comprehensive, but shape property inputs are provided for “user defined” members.

SBEDS General Input

SBEDS General Input

The SBEDS spreadsheet is formatted to analyze single-span wall framing. For analyzing opening support framing, the stud spacing and supported weight will have to be adjusted. Per PDC TR-10-02, the stud spacing is to be figured the same way as in the static analysis method. The spreadsheet analyzes the entire wall at one supported weight, so this will also have to be adjusted to account for the differing weights of the building cladding and glazing, providing an average support weight. For EIFS cladding, all weights are typically assumed to be the same (6 psf actual). For brick veneer (46 psf actual), the supported weight must be reduced to account for the lower weight of the glazing (6 psf actual). The wall span will need to be adjusted to the actual length of the head and sill members.

The blast parameters are input as “Charge Weight & Standoff.”  The appropriate project explosive weight is referenced from the UFC 4-010-02 standard. This publication is authorized only to U.S. Government agencies and their contractors. Approval must be obtained from the USACoE PDC to obtain this document. The standoff distance is entered as the project-specific standoff distance. The Incidence angle can be calculated as the arc tangent (ATAN) of the height to the center of the opening divided by the actual standoff distance. The remaining typical blast parameter inputs are shown below.

SBEDS Blast Parameters Input

SBEDS Blast Parameters Input

The typical response criteria inputs are shown below. Typically stud framing is required to be “connected top and bottom.” There is an option to select “Top Slip Track,” but personal experience has shown that this option does not pass the analysis. There are two options provided with the “Level of Protection” type. “Primary” would be selected for load-bearing projects, while “Secondary-NS (Non-Structural)” would be selected for curtainwall-type projects.

SBEDS Response Criteria Input

SBEDS Response Criteria Input

For the Solution Control inputs, the main check is to verify that the inputted time step matches the calculated recommended time step. The calculated value will change when various other inputs are changed. The “% of Critical Damping” can be entered as 5% for CFS framing, per the spreadsheet cell comment. The “Initial Velocity” is entered as zero, also per the spreadsheet cell comment.

SBEDS Solution Control Input

SBEDS Solution Control Input

Connections are designed to meet or exceed the “Peak Reactions Based on Ultimate Flexural Resistance” value given in the SBEDS output. Per UFC 4-010-01 section B-3.1, design is done per LRFD methodology with Load Factors equal to 1.0. The Resistance Factor for bending is allowed to be 1.0, while the Resistance Factors for other failure modes are per the AISI code (Shear = 0.95). Published LRFD strengths already have the appropriate resistance factors incorporated. The conservative assumption would be that shear controls the failure, and an increase in the LRFD Resistance Factor is not appropriate.

SBEDS Reactions Output

SBEDS Reactions Output

Per UFC 3-340-02, there are additional increases in connection strength that can be taken for dynamic blast design. The Strength Increase Factor (SIF, a.k.a. Static Increase Factor or Average Strength Factor) considers that yield stresses for all CFS materials provided are typically higher than the minimum yield stresses required by ASTM A446 (replaced by ASTM A653) steel. Per section 5-12.1, the SIF is listed as 1.21 for all CFS framing but only applies to the yield stress (Fy). Any calculations involving the ultimate stress (Fu) value are not allowed to be increased. The SIF also does not apply to fasteners (screws, bolts, PAFs, welds, etc.). The Dynamic Increase Factor (DIF) considers the strain-rate effects from rapid blast loading. Per section 5-34.2, this increase is listed as 1.10 for all CFS framing and per UFC 4-0-010-01 section 4-7, 1.05 for welds. Conservatively, it is assumed that the DIF equals 1.0 for all other fasteners (screws, bolts, PAFs, etc.).

Dynamic Connection Strength = (LRFD Strength)*(SIF)*(DIF)

For example, per Table 1 given below (reference Simpson Strong-Tie® engineering letter L-CF-CWCLRFD15), the SCB45.5 Bypass Slide Clip (3 screws to 16 ga. material) has a Dynamic Connection Strength = (2,025)*(1.0)*(1.10) = 2,227.5 lb., meeting the required reaction shown above.

scs-bypass-framing-slide-clip-connector

Again per UFC 3-340-02 section 5-47, when LRFD values are not published for connectors, a value of 1.7 times the allowable strength is permitted.

Dynamic Connection Strength = 1.7*(Allowable Strength)*(DIF)

Simpson Strong-Tie publishes LRFD values for all of its connectors.

References:

UFC 4-010-01:  February 9, 2012 (Change 1 – October 1, 2013)

UFC 3-340-02:  December 5, 2008 (Change 2 – September 1, 2014)

PDC-TR 06-01:  December 2012

PDC TR-10-02:  April 2012

https://pdc.usace.army.mil/

 

Building with Habitat for Humanity in Portugal

portugal-habitat-for-humanity-group

Five Simpson Strong-Tie employees had the opportunity to participate in a week-long Habitat for Humanity build in the small town of Amarante, Portugal, in late April. The group was originally scheduled to work on a Habitat project in Nepal late last year as part of Habitat’s Jimmy and Rosalynn Carter Work Project (CWP), but following the signing of a new constitution and civil unrest in the country, the project was canceled.

The company decided to allocate the funds for the CWP to Habitat’s Global Village program, allowing these employees to help renovate and remodel the older home of a widowed mother (Doña Margarida Ribiero) and daughter (Sonia) living in the Portuguese countryside.

The group, along with five other volunteers from the U.S., ranging from 29 to 76 years in age, was the first to start work on the 30-plus-year-old home. Alan Hanson, one of the Simpson Strong-Tie participants, was asked to share his thoughts about the experience.

My journey to Portugal began with a one-week vacation in the country with my wife, Holli. We traveled from Lisbon to Sintra, and then to Porto, the capital of port wine. It was a wonderful way to get to know the country. We met a number of friendly, unreserved people throughout the area. Language wasn’t a real barrier, since many locals spoke English. We toured cities, beaches, castles, palaces and other points of interest.

Holli flew out Saturday morning, so fellow Simpson Strong-Tie employee Rick Reid and I explored Porto for the rest of the day. We took a tour of the city, tasted some port wine, and had great meals. We met the other employees from Simpson Strong-Tie (Desiree Aquino, Phil Taylor, and Doug Melcolm) that night and had a seafood dinner near the water.

Rick Reid (l) and Alan Hanson at the jobsite.

Rick Reid (l) and Alan Hanson at the jobsite.

On Sunday morning, we met the rest of the volunteers from the U.S. as well as Florbela, our Habitat for Humanity representative. We took the 45-minute trip to Amarante, the city where the build would take place and had the afternoon free to explore. We were all excited about getting started on the build!

On Monday morning, we were taken to a rural part of Amarante where we met Doña Margarida, the homeowner, and Rogerio, the Habitat for Humanity superintendent. The house was very old and in need of many renovations. It had been added onto several times and was not very functional. Our work would entail remodeling rooms (a bedroom would become a living room), creating a hallway where none existed, and creating more space throughout the home.

L to R: Phil Taylor, Doug Melcolm and Alan Hanson hard at work.

L to R: Phil Taylor, Doug Melcolm and Alan Hanson hard at work.

We hit the ground running, cutting two new doorways into the granite and block, leveling out the irregular floors, filling in doorways that could no longer be used, patching various holes and openings, digging a ditch for the waste lines, removing paint and concrete from the granite interiors, and making many other improvements to the home throughout the build week. As a thank you at the end of each day, Doña Margarida served  us homemade red and “green” wine (the vino verde is a lightly carbonated white wine — delicious) with smoked ham and sausage. Despite the language barrier (she didn’t speak English), we could see that she was very grateful for our hard work, and she many times worked alongside us.

Alan Hanson fills in a former doorway.

Alan Hanson fills in a former doorway.

On Thursday we had our R&R day. We traveled to Guimaraes, about 45 minutes away. We had the opportunity to tour the “birthplace of Portugal” castle and palace and learn a lot of early Portuguese history. Friday came very early and we were off to Doña Margarida’s house again. We tore out another wall, finished fixing a few more openings, patched various holes in walls and leveled another floor.

Our last day at Doña Margarida’s house was actually only half a day on Saturday. We laid block in the door we removed earlier, filled in the floor where we tore out the wall, and made finishing touches to the patching on the other doors we filled in, as well as the hole from the wood stove. We accomplished a TON of work in 4½ days! We were told that we had finished ahead of schedule and completed more projects than were expected. We all had lunch together, including the family. In true Simpson Strong-Tie fashion, we had gifts for the family and our superintendent. We gave Rogerio a Simpson Strong-Tie-branded knife and sweatshirt and Doña Margarida a comforter and a wooden bowl that Phil made. He is quite a craftsman and did a wonderful job on it! Tears were shed, and we loaded into the van for the last time in Amarante. I took a nap when we got in because I was exhausted!

Doug Melcolm (l) and Rick Reid mixing cement for floor leveling.

Doug Melcolm (l) and Rick Reid mixing cement for floor leveling.

On Sunday, we left Amarante, heading to Porto. We attended a port wine tasting and took in a tour of the city. April 25 is “Freedom Day” in Portugal and marks the Carnation Revolution, when the military dictatorship was overthrown in 1974 with very little bloodshed, so there were fireworks at midnight and we had an incredible view. What a great way to end our trip to Portugal!

P.S. The complete renovation of the house is expected to be done in July, and we all can’t wait to see the home finished.

Simpson Strong-Tie® Strong-Wall® Wood Shearwall – The Latest in Our Prefabricated Shearwall Panel Line Part 2

In last week’s blog post, we introduced the Simpson Strong-Tie® Strong-Wall® Wood Shearwall. Let’s now take a step back and understand how we evaluate a prefabricated shear panel to begin with.

First, we start with the International Building Code (IBC) or applicable state or regional building code. We would be directed to ASCE7 to determine wind and seismic design requirements as applicable. In particular, this would entail determination of the seismic design coefficients, including the response modification factor, R, overstrength factor, Ωo, and deflection amplification factor, Cd, for the applicable seismic-force-resisting system. Then back to the IBC for the applicable building material: Chapter 23 covers Wood. Here, we would be referred to AWC’s Special Design Provisions for Wind and Seismic (SDPWS) if we’re designing a lateral-force-resisting system to resist wind and seismic forces using traditional site-built methods.

Design Documents: IBC, ASCE7 and SDPWS

Design Documents: IBC, ASCE7 and SDPWS

These methods are tried and true and have been shown to perform very well in light-frame construction during wind or seismic events. But over the years, many people have come to enjoy things like lots of natural light in our homes, great rooms with tall ceilings and off-street secure parking.

prefab2Due to Shearwall aspect ratio limitations defined in SDPWS as well as the strength and stiffness limitations of these traditional materials – including wood structural panel sheathing, plywood siding and structural fiberboard sheathing, to name a few – we’re left looking for alternative solutions. Thankfully, the IBC has left room for the use of innovative solutions beyond what’s explicitly stated in the code. Section 104.11 of the 2015 IBC provides the following provision:

104.11 Alternative material, design and methods of construction and equipment

The provisions of this code are not intended to prevent the installation of any material or prohibit any design or method of construction not specifically prescribed by this code, provided that any such alternative has been approved. An alternative material, design or method of construction shall be approved where the building official finds that the proposed design is satisfactory and complies with the intent of the provisions of this code, and that the material, method, or work offered is, for the purpose intended, not less than the equivalent of that prescribed in this code in quality, strength, effectiveness, fire resistance, durability and safety…

104.11.1 Research Reports. Supporting data, where necessary to assist in the approval of materials or assemblies not specifically provided for in this code, shall consist of valid research reports from approved sources.

104.11.2 Tests. Whenever there is insufficient evidence of compliance with the provisions of this code […] the building official shall have the authority to require tests as evidence of compliance…

Research Reports

The route we at Simpson Strong-Tie typically take is to obtain a research report from an approved source, i.e., the ICC Evaluation Service or the IAPMO Uniform Evaluation Service. Each of these evaluation service agencies publishes acceptance criteria that have gone through a public review process and contain evaluation procedures. The evaluation procedures might contain referenced codes and test methods, analysis procedures and requirements for compatibility with code-prescribed systems.

Prefabricated Panel Evaluation

Let’s once again take a step back and consider the function of our Strong-Wall® shearwalls. They’re prefabricated panels intended to provide lateral and vertical load-carrying capacity to a light-framed wood structure where traditional methods are not applicable or are insufficient. We need to provide a complete lateral load path, which ensures that the load continues through the top connection into the panel and then into the foundation through the bottom connection. To evaluate the panel’s ability to do what we’re asking of it, we use a combination of testing and calculations with considerations for concrete bearing, fastener shear, combined member loading, tension and shear anchorage, panel strength and stiffness, etc.

I could write a five-thousand-word feature story for the New York Times discussing the calculations in great detail, but let’s focus on the more exciting part – testing! Simpson Strong-Tie has several accredited facilities across the country where all of this testing takes place; click here for more info.

Testing Acceptance Criteria

Now to pull back the curtain a bit on the criteria we follow in our testing: We test our panels in accordance with the criteria provided in ICC-ES AC130 – Acceptance Criteria for Prefabricated Wood Shear Panels or ICC-ES AC322 – Acceptance Criteria for Prefabricated, Cold-Formed, Steel Lateral-Force-Resisting Vertical Assemblies, as applicable. These criteria reference the applicable ASTM Standard, ASTM E2126-11, which illustrates test set-up requirements and defines the loading protocol among other things. If you’re interested, the work done by the folks involved with the CUREE-Caltech Woodframe Project, which is the basis for the testing protocol we use today, makes for an excellent read. The CUREE protocol, as it’s known, is a displacement-controlled cyclic loading history that defines how to load a panel. A reference displacement, Δ, is determined from monotonic testing, and the cyclic loading protocol, which is a series of increasing displacements whose amplitudes are functions of Δ, is developed. I’ve provided a graphic depicting the protocol below.

CUREE Loading Protocol (Excerpt from ASTM E2126-11)

CUREE Loading Protocol (Excerpt from ASTM E2126-11)

When prefabricated shear panels are subjected to the loading protocol shown above, a load-displacement response is generated; we call this a hysteresis loop or curve.

Hysteresis Curve (Excerpt from ASTM E2126-11)

Hysteresis Curve (Excerpt from ASTM E2126-11)

We then use this curve to generate an average envelope (backbone) curve that will be used for analysis in accordance with the procedures defined in AC130 or AC322 as applicable.

Average Envelope Curve (Except from ASTM E2126-11)

Average Envelope Curve (Except from ASTM E2126-11)

Returning to the acceptance criteria, there are different points of interest on the average envelope curve depending upon whether we’re establishing allowable test-based values for wind-governed designs or for seismic-governed designs. I should also note that both wind and seismic designs consider both drift and strength limits when determining allowable design values.

Wind is fairly straightforward, so let’s start there. While the building code does not explicitly define a story drift limit for wind design, the acceptance criteria do. The allowable wind drift, Δwind, shall be taken as H/180, where H is the story height. The allowable ASD in-plane shear value, Vwind, is taken as the load corresponding to Δwind. I mentioned a strength limit as well; this is simply taken as the ultimate test load divided by a safety factor of 2.0.

Contrary to wind design, the building code does define a story drift limit for seismic design. ASCE7 Table 12.12-1 defines the allowable story drift, δx, as 0.025H for our purposes, where H is the story height. The strength design level response displacement, δxe, is now determined using ASCE7 Equation 12.8-15 as referenced in AC130 and AC322 as follows:

formula1

Where:

  • δxe = LRFD strength design level response displacement
  • δx = Allowable story drift = 0.025H for Risk Category I/II Buildings (ASCE7 Table 12.12-1)
  • Ie = Seismic importance factor = 1.0 for Risk Category I Buildings (ASCE7 Table 1.5-2)
  • Cd = Deflection amplification factor = 4.0 for bearing wall systems consisting of light-frame wood walls sheathed with wood structural panels rated for shear resistance (ASCE7 Table 12.2-1)

We then consider the shear load corresponding to the strength level response displacement, VLRFD, and multiply this value by 0.7 to determine the allowable ASD shear based on the seismic drift limit, VASD. Lastly, the seismic strength limit is taken as the ultimate test load divided by a safety factor of 2.5.

Compatibility with Code-Prescribed Methods

We’ve gone through the steps to evaluate the allowable design values for our panels, but we’re not done yet. AC130 and AC322 define a series of criteria to ensure that the seismic response is compatible with code-defined methods with respect to strength, ductility and deformation capacity. Once we verify that these compatibility parameters have been satisfied, we may then apply the response modification factor, R, overstrength factor, Ωo, and deflection amplification factor, Cd, defined in ASCE7 for bearing wall systems consisting of light-frame wood or cold-formed steel walls sheathed with wood structural panels or steel sheets. This enables the prefabricated shearwalls to be used in light-frame wood or cold-formed steel construction. I’ve very briefly covered an important topic in seismic compatibility, but there has been plenty published on the issue; I recommend perusing the article here for more details.

We’ve now followed the path from building code to acceptance criteria to evaluation report. More importantly, we understand why Strong-Wall® shearwall panels are required and the basics of how they’re evaluated. If there are items that you’d like to see covered in more detail or if you have questions, let us know in the comments below.

 

Simpson Strong-Tie® Strong-Wall® Wood Shearwall – The Latest in Our Prefabricated Shearwall Panel Line Part 1

calebphoto1This week’s post comes from Caleb Knudson, an R&D Engineer at our home office. Since joining Simpson Strong-Tie in 2005, he has been involved with engineered wood products and has more recently focused his efforts on our line of prefabricated Strong-Wall Shearwall panels. Caleb earned both his Bachelor’s and Master’s degrees in Civil Engineering with an emphasis on Structures from Washington State University. Upon completion of his graduate work, which focused on the performance of bolted timber connections, Caleb began his career at Simpson and is a licensed professional engineer in the state of California.

Some contractors and framers have large hands, which can pose a challenge for them when they’re trying to install the holdown nuts used to attach our Strong-Wall® SB (SWSB) Shearwall product to the foundation. Couple that challenge with the fact that anchorage attachment can only be achieved from the edges of the SWSB panel, and variable site-built framing conditions can limit access depending upon the installation sequence. To alleviate anchorage accessibility issues, we’ve required a gap between the existing adjacent framing and SWSB panel equal to the width of a 2x stud to provide access so the holdown nut can be tightened. Even so, try telling a framer an inch and a half is plenty of room in which to install the nut!

SWSB Edge Access

SWSB Edge Access

2x Gap for SWSB Installation

2x Gap for SWSB Installation

 

 

 

 

 

 

 

 

 

While the SWSB is a fantastic product with many great features and benefits from its field adjustability to its versatility with different applications and some of the highest allowable values in the industry, the installation challenges were real.

Back to the Drawing Board

Our goal was to develop a new holdown for the SWSB that would allow for face access of the anchor bolts, making the panel compatible with any framing condition, while maintaining equivalent performance. All we needed to do is cut a large hole in each face of the holdown without compromising strength or stiffness — piece of cake, right? Well, that’s exactly what we did. In the process, we addressed the needs of the architect, the engineer and the builder — and for bonus points, anchorage inspection is now much easier, which should make the building official happy too.

Introducing the Simpson Strong-Tie® Strong-Wall® Wood Shearwall

Simpson Strong-Tie® has just launched the Strong-Wall® Wood Shearwall (WSW) panel, which replaces the SWSB. The new panel provides the same features and benefits, and addresses the same applications as the SWSB; however, now it also features face-access holdowns distinguished by their Simpson Strong-Tie orange color.

Strong-Wall Wood Shearwall

Strong-Wall Wood Shearwall

We’ve also updated the top connection, which now provides two options based on installer preference. The standard installation uses the two shear plates shipped with the panel which are installed on each side of the panel by means of nails. As an alternative, the builder can install a single shear plate from either side of the panel using a combination of Strong-Drive® SD Connector screws and Strong-Drive® SDS Heavy-Duty Connector screws.

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Allowable In-Plane Lateral Shear Loads

I mentioned that one of our primary development requirements was to meet the existing allowable design values of the SWSB. Not only did we meet our target values, but we exceeded them by as much as 25% for standard and balloon framing application panels and up to 50% for portal application panels. I’ve included a table below showing the most commonly specified standard and portal application SWSB models and how the allowable wind and seismic shear values compare to those of the corresponding WSW model.

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Grade-Beam Anchorage Solutions

I’d be remiss if I didn’t point out the grade-beam anchorage solutions we’ve developed for use with the Strong-Wall Wood Shearwall. The solutions have been calculated to conform to ACI 318-14, and testing at the Simpson Strong-Tie Tyrell Gilb Research Laboratory confirmed the need to comply with ACI 318 requirements to prevent plastic hinging at anchor locations for seismic loading. The testing consisted of 1) control specimens without anchor reinforcement, 2) specimens with closed-tie anchor reinforcement, and 3) specimens with non-closed u-stirrups. Flexural and shear reinforcement were designed to resist amplified anchorage forces and compared to test beams designed for non-amplified strength-level forces.

Significant Findings from Testing

We found that grade-beam flexural and shear capacity is critical to anchor performance and must be designed to exceed the demands created by the attached structure. In wind load applications, this includes the factored demand from the WSW. In seismic applications, testing and analysis have shown that in order to achieve the anchor performance expected by ACI 318 Anchorage design methodologies, the concrete member design strength needs to resist the amplified anchor design demand from ACI 318-14 Section 17.2.3.4. To help Designers achieve this, Simpson Strong-Tie recommends applying the seismic design moment listed below at the WSW location.

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We also found that closed-tie anchor reinforcement is critical to maintain the integrity of the reinforced core where the anchor is located. Testing with u-stirrups that did not include complete closed ties showed premature splitting failure of the grade beam. In a previous blog post, we discussed our grade-beam test program in much greater detail as it applies to our Steel Strong-Wall panels.

Strong-Wall® Wood Shearwall

To support the Strong-Wall Wood Shearwall, Simpson Strong-Tie has published a 52-page catalog with design information and installation details. We’ve also received code listing from ICC-ES; the evaluation report may be found here. Now that you’re all familiar with the WSW, be sure to check out next week’s blog post where we’ll cover the basics of prefabricated shear panel testing and evaluation. In addition, to help Designers understand all of the development and testing as well as design examples using prefabricated shearwalls, Simpson Strong-Tie will be offering a Prefabricated Wood Shearwall Webinar on June 21, 2016, covering:

  • The different types of prefabricated shearwalls and why they were developed.
  • The engineering and testing behind prefabricated shearwalls.
  • Best practices and design examples for designing to withstand seismic and wind events.
  • Code reports on shearwall applications.
  • Introduction of the latest Simpson Strong-Tie prefabricated shearwall.

You can register for the webinar here.

Last but not least, we always appreciate hearing from you, whether you’re an engineer specifying our panels or in the field handling the installation. If there are applications that we haven’t addressed or additional resources that would be beneficial, please let us know in the comments below.

Habitat STRONG Blog

This week’s post was written by Kevin Gobble of Habitat for Humanity. Kevin is the Program Manager for Habitat for Humanity’s new Habitat Strong initiative. Kevin has spent over 22 years in residential construction building energy-efficient, high-performing home, and has consulted with several sustainable building programs on ways to develop their own best practices. As a third-generation builder, he has knowledge in the field of residential building science and has furthered his education to include many industry certifications — NARI Certified Remodeler, NAHB Certified Green Professional, RESNET Certified Green Rater, BPI Building Analyst, FORTIFIED evaluator, and Level 1 Infrared Thermography — while working directly with industry partners to focus on cost-effective construction solutions. Kevin has built and remodeled numerous homes to high-performance standards as certified by various building programs, including his latest project for himself: converting a condemned historic property in Atlanta to EarthCraft House Platinum.

In a previous blog post, we discussed the background of the Habitat Strong program. Habitat Strong promotes the building of resilient homes that are better equipped to withstand natural disasters in every region of the country. This program uses IBHS FORTIFIED Home™ standards and works well within Habitat’s model of building affordable, volunteer-friendly homes.

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Project Spotlight – Habitat for Humanity New Haven, CT

Habitat for Humanity New Haven’s innovative approach to building a traditional New England– style home with modern improvements began with a design from an historic home they rehabilitated years ago. The original was an old Winchester factory worker home, but the style was adapted to fit the narrow lots and surviving character of New Haven. Along with the design shift, the plans were standardized to incorporate FORTIFIED Gold techniques and practices for hurricanes.

New Haven has fully embraced FORTIFIED building practices following Superstorm Sandy. They have completed eight FORTIFIED Gold homes to date with three more under construction, perfecting their techniques as they go. An example to other Habitat affiliates, they have provided a model for using affordable construction methods and volunteers. They have also created a positive impact on their community by sharing their knowledge with other builders in the area.

“Improving the roof is a no-brainer, and it makes sense to tape the plywood seams,” noted construction manager Antoine Claiborne. Habitat has gone one step further by using the ZIP system on the roof for safety and durability improvements. This eliminates the need to nail down the underlayment every six inches o.c. along the edge and in the field, which can prove difficult for volunteers.

In addition to employing these roof techniques, New Haven uses Simpson Strong-Tie connectors (after re-engineering plans) to meet these new guidelines and to create a continuous load path. To promote ease of use for new volunteers, an advanced framing center is set up onsite, using diagrams and videos to demonstrate how the process works and what to expect. Documentation is another key to the FORTIFIED process – in New Haven’s case, the onsite construction manager documents all the FORTIFIED elements.

For opening protection, New Haven uses pressure-rated doors with Hurricane Fabric for impact protection as well as impact-rated windows. It was recently discovered that impact windows can shatter in a small area if hit there while  otherwise remaining intact. Thus there’s a need to use caution when mowing grass where there are small rocks near the home.

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How can you help?  Contact Habitat for Humanity if you would like to donate or volunteer. If you have engineering expertise you can lend, I would love to hear from you at HabitatSTRONG@habitat.org.

Multi-Ply Beam Load Transfer

Larger beams are often built up out of smaller 2x or 1¾” members. This can be done for several different reasons: for the convenience of handling smaller members on the jobsite, or because solid 4x, 6x or glulam material is not readily available, or for reasons of cost. Engineered wood such as laminated veneer lumber (LVL) is often used for its high load capacity and multiple 1¾” plies are built up to get the required capacity for the application.

8-Ply LVL Beam in HHGU14 Test

8-Ply LVL Beam in HHGU14 Test

When a built-up beam is loaded concentrically as in the test setup shown, fastening the members is not critical since that giant steel plate will load each ply of the beam. In the field, built-up beams or girders commonly support joists or beams framing into their side. The built-up members must be connected to transfer load from the loaded ply into the other plies.

SDW - Uniform Allowable Loads

Allowable Uniform Loads and Spacing Requirements

SDW - Assembly Types and Spacing Requirements

Page 303 of our Fastening Systems catalog, C-F-14 provides allowable uniform load tables for side-loaded multi-ply assemblies using LVL, PSL or LSL material. The calculation for the allowable load applied to the outside ply of a multi-ply beam is:

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While uniform loads are very common, Designers often request additional information to design multi-ply beam connections to transfer concentrated loads. Simpson Strong-Tie has created a new engineering letter, L-F-SDWMLTPLY16, which complements the information in the Fastening Systems catalog by providing allowable loads in a single fastener format. Designers can use the information to calculate the number of fasteners required for a given point load.

Simpson Strong-Tie® Strong-Drive® SDW EWP-Ply Screw – Allowable Loads for Side-Loaded Multi-Ply Assemblies per Screw

Simpson Strong-Tie® Strong-Drive® SDW EWP-Ply Screw – Allowable Loads for Side-Loaded Multi-Ply Assemblies per Screw

In order to ensure load transfer, the SDW screws need to be located relatively close to the connection. At first glance, it may appear challenging to fit enough fasteners while meeting the non-staggered row-spacing requirements. However, we have found that most loads can be managed by taking advantage of the ⅝” stagger allowance.

SDW – Maximum Fastener Spacing from Point Load

SDW – Maximum Fastener Spacing from Point Load

If you are curious what happened in that HHGU14 test, the screws pulled out of the header with a load slightly exceeding 101,000 pounds. Failure photo 2 shows a close-up of the pullout failure. The tested load was very close to the maximum calculated capacity for the SDS screws in the connector, so it was a great test result. What are your thoughts? Let us know in the comments below.

HHGU14 Test Failure 1 HHGU14 Test Failure 2

Fine Homebuilding Video Series: How to Build a Deck

We’re partnering with folks at Fine Homebuilding on a video series on how to build a deck that is code compliant and that highlights the critical connections of a deck. This series is called Ultimate Deck Build 2016. The video series comprises five videos that walk professionals through the recent code changes for the key connections of a deck.

The series features David Finkenbinder, P.E., a branch engineer for Simpson Strong-Tie who is passionate about deck codes and safety. He offers information on load resistance and the hardware that professionals can use at the crucial connections to make a deck code compliant. “This was a great opportunity to collaborate with the team at Fine Homebuilding, to communicate the connections on a typical residential deck and the role that they serve to develop a strong deck structure,” said David. “These same connections would also likely be common in similar details created by an Engineer, when designing a deck per the International Building Code (IBC).”

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The videos are being released every Wednesday during the month of March and feature the following deck connections:

  • Ledger Connection: This is the primary connection between a deck and a house. David tells the Fine Homebuilding team about various code- compliant options for attaching a deck ledger to a home.
  • Beam and Support Posts: David explains how connectors at this critical point can prevent uplift and resist lateral and downward forces. He also discusses footing sizes and post-installation anchor solutions.
  • Joists: This video reviews proper joist hanger installation and the benefits of installing hurricane ties between the joists and the beams. David goes into common joist hanger misinstallations, such as using the wrong fasteners or using a joist hanger at the end of a ledger.
  • Guardrail Posts: David reviews the different ways that you can attach a guardrail post so as to resist an outward horizontal load.
  • Stairs: David explains code-compliant options for attaching stringers to a deck frame.

Make sure to watch the series and let us know what you think. For more information, Fine Homebuilding has created an article titled “Critical Deck Connections.”

(Please note: this article is member-only/subscription content, so to read it you’ll need to either subscribe online or pick up the April/May issue of Fine Homebuilding.)

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Installation Errors – They Happen

A few years ago, we did a post on creative uses of our products. Most of the uses shown were artistic, or functional do-it-yourself projects, with one odd car spoiler modification. This week, I was reviewing some slides in a presentation that I give a few times a year regarding product installation errors. I call them misinstallations, but I’m not sure that’s a word. I thought I’d share a few of the more instructional ones. Most of the photos were curated by our northwestern region training manager, Olga Psomostithis – thanks Olga!

Double Shear Hangers

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Double shear hangers require joist fasteners that are long enough to penetrate through the hanger, through the joist and into the header. The joist nails help transfer load from the joist into the header, resulting in higher allowable loads.

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The installation shown has had the double shear tabs bent back, and nails installed straight into the joist. Since the joist nails do not penetrate the header, this would result in a reduced capacity.

Holdowns

I’m including the trailer hitch installation because it makes me laugh no matter how many times I see it.

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A very common question we get about holdowns is related to posts being offset too far from the anchor bolt (or is the anchor too far from the post?). In the installation shown below, the holdown is not flush with the post as the anchor bolt is offset about 1 inch. For small offsets up to about 1½”, a common solution is to raise the holdown off the sill plate and extend the anchor bolt with a coupler and bend it so there is a small (1:12) slope to it.

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The holdown test standard, ICC-ES AC155, which is discussed in this post, requires that holdowns are tested raised off the test bed, which you can see in the photo below. Holdowns may be raised up to 18” above the top of concrete without a reduction in load provided that the additional elongation of the anchor rod is accounted for.

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I like this photo because the installer put on the nail stops to protect the pipes. It is good to remember that plumbing happens when laying out a structural system.

Oh boy, does it happen.

Oh boy, does it happen.

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The photo above is not a misinstallation, but something that can happen. Embedded strap-style holdowns are cost-effective solutions for shearwall overturning or wind uplift. It is permitted to bend the straps to horizontal and back to vertical one cycle. If spalls form, they should be evaluated for reduced loads. Any portion of the strap left exposed should be protected against corrosion.

Hanger Gaps

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Gaps can occur between trusses and supporting girders for a variety of reasons. For standard hanger tests, a 1/8″ gap is required between the joist and header per ASTM D7147. A resource for evaluating conditions with larger gaps is our technical bulletin Allowable Loads for Joist Hangers with Gaps. The technical bulletin has load data for a variety of hangers with gaps up to 3/8″, as well as recommended repairs for larger gaps. Our HTU product series comprises truss hangers specifically engineered to allow gaps up to ½”.

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After going through a design project and carefully selecting the members and details of construction, it can be frustrating as an engineer to get that phone call from the general contractor or building inspector informing you that something is not right with the construction. Understanding some of the resources available to address installation errors can help solve these problems more quickly, and get you back to designing the next project.

Mass Timber Construction – Building for the Future

fredtai-2This week’s post comes from Fred Tai. Fred Tai is the Engineering Manager for Simpson Strong-Tie Canada where he has worked for the last 13 years. He has an extensive background in structural engineering and worked as a structural consultant in building designs prior to joining Simpson Strong-Tie. During his career, he was involved in ASTM D7 and is active in regional wood truss associations, regional building code committee and regional building research committee. He has a bachelor of applied science degree in Civil Engineering from the University of British Columbia, Canada and is a licensed engineer in Canada.

The future is here.

It is common knowledge that wood is a renewable and environmentally friendly building material. There are two types of wood-framing methods in North America. The most common method for residential construction is light-frame construction using either balloon-framing or platform-framing methods. Standardized dimensional lumber has become the dominant building material in light-frame construction because of its economy. The other method is heavy-timber construction, which often uses large solid-wood sections for nonresidential construction, such as for storage, mercantile and industrial buildings.

In Europe, there is a trend to create larger “laminated” wood sections using the more traditional standardized dimensional lumber of the 1990s. This trend culminated in what is now classified as cross-laminated timber, or CLT. CLT can be used to create floor panels and roof panels. In North America, this is classified either as cross-laminated timber (CLT) or generically as mass timber.

CLT is essentially multiple layers of wood panels. Each layer of wooden panels is laid crosswise on the one before at approximately a 90° angle and glued using a polyurethane adhesive to increase the stability of the entire panel. Typical thickness of the individual boards can vary from 3/8″ to 2″ thick. Typical board width can vary from 2-3/8” to 9-1/2” wide. CLT panels are fabricated and marketed from 3-ply CLT up to 7-ply CLT. CLT SE Blog 1manufacturers normally publish characteristic properties for their panels – such as bending strength, shear strength, modulus of elasticity and panel stiffness – to assist Designers in specifying these products.

A Cross Laminated Timber Handbook has been published by FPInnovations in Canada as an introduction to CLT. This handbook can be downloaded for free here. The American Wood Council has a self-study guide on CLT that can be downloaded here.

As in all wood buildings, connection designs are critical to the success of this new type of building material. Simpson Strong-Tie offices in Europe have been instrumental in developing and supplying connectors and fasteners in the CLT market. Simpson Strong-Tie has developed many connectors specifically for the CLT market in Europe (Figure 3).

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Those connectors are used to join the CLT floor panels to CLT wall panels and CLT wall panels to the concrete foundation (Figures 1 and 2).

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SE Blog 2Specialized ring-shank nails and long metal screws have been developed as well. In mid-2014, Simpson Strong-Tie North America (Pleasanton, California Testing Facility) embarked on an initial test program to assess those connectors and fasteners developed for the CLT market by Simpson Strong-Tie Europe, using North American CLT panels to verify and quantify the performance characteristics according to North American testing protocols (American Society for Testing and Materials and Canadian Construction Materials Centre).

The initial test program used CLT panels fabricated in Western Canada using Canadian Spruce-Pine-Fir (S-P-F) lumber. The connectors and ring-shank nails were imported from the Simpson Strong-Tie European manufacturing facilities. Testing of the connectors also included the Simpson Strong-Tie Strong-Drive® SD screws, which as expected, provided higher load capacity than the ring-shank nails. A summary of the test program and the load rating developed for both the Canadian and the U.S. market can be downloaded here.

Other types of long countersunk screws such as the Strong-Drive® SDWS Timber screw (countersunk) or Strong-Drive SDWH Timber-Hex (hex head) screw (shown) are used either to splice the floor panels together or to drag the diaphragm loads back to the column or post as necessary.

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As CLT continues to gain acceptance in North America, other connection details will also become more popular. Simpson Strong-Tie intends to continue developing and improving connection details to support this type of construction.

Building code acceptance is another important requirement and development that is in progress in both Canada and the U.S. In Canada, the 2014 edition of CSA O86 “Engineering Design in Wood” has reserved a section for CLT.

The 2015 edition of the International Building Code (IBC) recognized CLT when it is manufactured to the product standard. CLT walls and floors will be permitted in all types of combustible construction. The 2015 National Design Specification (NDS) for Wood Construction was recently published and approved as an ANSI American National Standard. The 2015 National Design Specification is also referenced in the 2015 IBC.

The future is here. Environmentally friendly mass timber (including CLT) is poised to grow in use, especially with the recognition of CLT in the building codes. North American manufacturing of CLT has been established and can only grow to support the expanding use of this new building material.

References:

www.cwc.ca

www.awc.org

https://fpinnovations.ca

*Images with permission from FPInnovations