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Welcome to our Structural Engineering Blog! I’m Paul McEntee, Engineering R&D Manager at Simpson Strong-Tie. We’ll cover a variety of structural engineering topics here that I hope interest you and help with your projects and work. Social media is “uncharted territory” for a lot of us (me included!), but we here at Simpson Strong-Tie think this is a good way to connect and even start useful discussions among our peers in a way that’s easy to use and doesn’t take up too much of your time. Continue reading

Top 5 LinkedIn Groups to Follow for Structural Engineers

MacBook Pro Retina with LinkedIn home page on the screen

A while back, we posted about how structural engineers can use social media like Facebook, Twitter and LinkedIn. We discussed how structural engineers can use LinkedIn as a tool to find out more about industry news. While that is one way to use LinkedIn, another way to get even closer to the pulse of your industry is to join industry-specific LinkedIn groups.

LinkedIn groups are places within LinkedIn that allow professionals to share content, post or view job openings, network, and help establish key opinion leaders in a particular industry.

If you are new to LinkedIn, it can be challenging to find all of the LinkedIn groups that you may want to join. We compiled a list of structural engineering LinkedIn groups that can help you get started:

American Society of Civil Engineers (ASCE): This group was initially formed to allow networking between engineers. It has now grown to over 200,000 members and includes other professionals who work in the industry. Since this is a large group, there are more focused sub-groups that you can also join. We recommend using the ASCE group for general information.

ASCE: Structural Engineering: This is a sub-group of ASCE. The members of this LinkedIn group are mainly structural engineers. This is a good place for discussion and asking for feedback on work-related topics.

American Concrete Institute: This is a great group for structural engineers who work with concrete. You can connect not just with engineers, but also with professionals in the concrete production, design and construction industries.

SEAOC-Structural Engineers Association of California: If you are a structural engineer in California, we highly recommend this group. If you are interested in structural and seismic engineering, this is the group to join.

NCSEA: The National Council of Structural Engineers Associations (NCSEA) is a great group to join to get industry information, find resources including webinars, and hear about local industry events and meetings.

While there are a lot more LinkedIn groups, we hope that the ones we have shared are useful for you. What LinkedIn groups do you recommend? Let us know in the comments below.

Firewalls for Wood Construction

What is a firewall?

A firewall is a term that is used in the construction industry to describe a fire-resistive-rated wall or fire-stop system, which is an element in a building that separates adjacent spaces to prevent the spread of fire and smoke within a building or between separate buildings. A firewall is actually one of three different types of walls that can be used to prevent the spread of fire and smoke.

 Types of fire-resistive-rated walls: 

 The three types of fire-resistive-rated walls are firewalls, fire barriers and fire partitions. They are listed in order from the most stringent requirements to the least. A firewall is a fire-resistive-rated wall having protected openings, which restricts the spread of fire and extends continuously from the foundation to or through the roof with sufficient structural stability under fire conditions to allow collapse of construction on either side without collapse of the wall. A fire barrier is a fire-resistive-rated wall assembly of materials designed to restrict the spread of fire which continuity is maintained.  A fire partition is a vertical assembly of materials designed to restrict the spread of fire in which openings are protected.  Each type has varying requirements and the table below displays some of the differences between them.

fire-resistive-rated-wallsWhat are some of the typical uses of each type of fire-resistive wall? 

As the requirements for each type of wall vary, so do the uses. Typical uses of each are as follows:

  • Firewalls – party walls, exterior walls, interior bearing walls
  • Fire barriers – shaft enclosures, exit passageways, atriums, occupancy separations
  • Fire partitions – corridor walls, tenant space walls, sleeping units within the same building

How do you determine whether your wood building design needs a firewall?

The 2012 International Building Code (the IBC, or “the Code” in what follows), which is adopted by most building departments in the United States, is the resource we are using in this discussion. (As a side note, it’s possible your city or county has supplemental requirements, and it is best to contact your local building department for this information up front.)

To determine your fire-resistive wall requirements, review these chapters in the 2012 IBC:

  • Chapter 3, Identify Occupancy Group – typically Section 310 (“Residential Group”) for wood construction
  • Chapter 5, Select Construction Type – Section 504, Table 503
  • Chapter 6, Determine Fire-Resistive Rating Requirements – Table 601, typically Type III wood-constructed buildings require a two-hour fire separation for the exterior bearing walls

What are typical fire-resistive wall designs? 

 Information for one-hour, two-hour designs, etc. can be found in tables 721.1(2) and 721.1(3) of the Code provide information to obtain designs that meet the rating requirements (in hours) for your building, including the walls and floor/roof systems. The GA-600 is another reference that the Code allows if the design is not proprietary.

How do I know whether the structural attachments I specify for the wall and roof assemblies meet the Code requirement?

Once the wall or floor/roof assembly design is selected, the Designer must ensure that the components of the wall do not reduce the fire rating. The Code requires that products which pierce the membrane of the assemblies at a hollow location undergo a fire test to ensure they meet the requirements of the design. ASTM E814 and ASTM E119 are the standards governing the fire tests for materials and components of the fire-resistive wall. There are several criteria that the component in the assembly must meet: a flame-through criterion, a change-in-temperature criterion and a hose-stream test.

Simpson Strong-Tie has created the DHU hanger for use with typical two-hour fire-resistive walls for wood construction.The DHU hanger has passed the ASTM E814 testing and can be used on a fire-resistive wall of 2×4 or 2×6 constructions and up to two 5/8″ layers of gypsum board. The DHU and DHUTF have both an F (Fire) and a T (Temperature) rating.

dhutf-dhu-hangersThe DHU/DHUTF hanger has two options, a face-mount version (DHU) and a top-flange version (DHUTF).  The hanger doesn’t require any cuts or openings in the drywall, which ensures reliable performance; no special inspection is required.  To install the hanger, gypsum board must first be installed in a double or single layer, at least as deep as the hanger.  For installation, apply a two-layer strip of Type X drywall along the top of the wall, making the base layer a wider strip (bottom edge is 12″ or more below the face layer, depending on jurisdiction).  Then install ¼” x 3½” Simpson Strong-Tie Strong-Drive® SDS screws through the hanger and into top plates of the wall.  Since the hanger is more eccentric than typical, the top plates of the wall must be restrained from rotation. The SSP clip can be used for restraint, but the design may not require it if there is a sufficient amount of resistance already in place, such as sheathing, a bearing wall above, or a party wall as determined by the designer.  See the photos and installation illustration below for guidance or visit our website for further information.

typical-installation-over-2-layers-drywall

 

 

How to Select a Connector Series – Hurricane Tie

When it comes to wood-frame construction, hurricane ties are among the most commonly specified connectors. They play a critical role in a structure’s continuous load path and may be used in a variety of applications, like attaching roof framing members to the supporting wall top plate(s), or tying wall top or bottom plates to the studs. They are most commonly used to resist uplift forces, but depending on regional design and construction practices, hurricane ties may also resist lateral loads that act in- or out-of-plane in relation to the wall.

Simpson Strong-Tie manufactures approximately 20 different models of hurricane ties, not counting twist straps, other clips, or the new fully-threaded SDWC screws often used in the same applications. This assortment of models raises the question, “How do you select the right one?”

In this post, we’ll outline some of the key elements to consider when selecting a hurricane tie for your project.

Demand Load

Let’s start with the obvious one. If your building’s roof trusses have an uplift of 600 lb. at each end, don’t select a hurricane tie with a published capacity of less than 600 lb. It’s also important to consider combined loading if you plan to use the tie to resist both uplift and lateral loads. When the connector is resisting lateral loads, its capacity to resist uplift is reduced. I won’t go into too much detail on this topic since it was covered in a recent blog post, but in lieu of the traditional unity equation shown in Figure 1, certain Simpson Strong-Tie connectors (hurricane ties included) are permitted to use the alternative approach outlined in Figure 2.

Figure 1. Traditional Linear Interaction Equation

Figure 1. Traditional Linear Interaction Equation

Figure 2. Alternative Approach for Simultaneous Loading

Figure 2. Alternative Approach for Simultaneous Loading

What if the tabulated loads in the catalog for a single connector just aren’t enough? Use multiple connectors! An important note on using multiple connectors, though: Using four hurricane ties doesn’t always mean you’ll get 4x the load. Check out the recently updated F-C-HWRCAG16 High Wind-Resistant Construction Application Guide for allowable loads using multiple connectors and for guidance on the proper placement of connectors so as to avoid potential overlap or fastener interference.

Figure 3. Allowable Load Comparison for Single and Multiple H2.5A Connectors

Figure 3. Allowable Load Comparison for Single and Multiple H2.5A Connectors

Figure 4. Proper Placement of (4) H2.5A’s to Avoid Fastener Interference

Figure 4. Proper Placement of (4) H2.5A’s to Avoid Fastener Interference

 

Dimensional Requirements

While the majority of the hurricane ties that Simpson Strong-Tie offers are one-sided (such as the H2.5A), some are designed so the truss or rafter fits inside a “U” shape design to allow for fastening from both sides (such as the H1). If using the latter, make sure the width of the truss or rafter is suitable for the width of the opening in the hurricane tie – don’t select an H1 for a 2-ply roof truss.

Figure 5. H2.5A and H1 Hurricane Ties

Figure 5. H2.5A and H1 Hurricane Ties

Additionally, the height of the hurricane tie and the wood members being attached should be compatible. For example, an H2.5A would not be compatible with a roof truss configured with only a nominal 2×4 bottom chord over the plate since the two upper nail holes in the H2.5A will miss the 2×4 bottom chord (see Figure 4). This is actually such a common mis-installation that we specifically tested this scenario and have developed an engineering letter on it (note the greatly reduced capacity). In this case the ideal choice would be the H2.5T, which has been specifically designed for a 2×4 truss bottom chord.

Figure 6. H2.5A Installed on 2x4 Truss Bottom Chord

Figure 6. H2.5A Installed on 2×4 Truss Bottom Chord

Figure 7. H2.5T Installed on 2x4 Truss Bottom Chord

Figure 7. H2.5T Installed on 2×4 Truss Bottom Chord

Fasteners

It’s also essential to pay close attention to the diameter and length of the fasteners specified in the Simpson Strong-Tie literature. While many hurricane ties have been evaluated with 8d x 1½” nails for compatibility with nominal 2x roof framing, some require the use of a longer, 8d common (2½” long) nail and others require a larger-diameter 10d nail.

When specifying products for a continuous load path, it’s a good idea to select connectors that all use the same size nail to avoid improper installations on the job. It’s much easier if the installer doesn’t need to worry about which size nail he currently has loaded in his pneumatic nailer.

Wall Framing

Do your roof and wall framing members line up? If so, creating a continuous load path can be made simpler by using a single hurricane tie to fasten the roof framing to studs. The H2A, H7Z, and H10S are some of the connectors designed to do just that. If your framing doesn’t align, though, you can use two connectors to complete the load path. For simplification and to reduce potential mix-ups in the field, consider selecting the same hurricane tie for your roof framing-to-top-plate and top plate-to-stud connections, like the H2.5A.

Figure 8. Roof-Framing-to-Stud Connection with Single Hurricane Tie

Figure 8. Roof-Framing-to-Stud Connection with Single Hurricane Tie

Besides the added benefit of fewer connectors to install, using a single hurricane tie from your roof framing to your wall studs can eliminate top-plate roll, a topic discussed at length in one of our technical bulletins.

Other Factors

Some additional factors that may influence your selection of a hurricane tie are:

  • Environmental factors and corrosion should be considered when selecting any product. Nearly every hurricane tie is available in ZMAX®, our heavier zinc galvanized coating, and several are available in Type 316 stainless steel. A full list of products available in ZMAX or stainless steel may be found on our website. On a related note, be sure to use a fastener with a finish similar to that of the hurricane tie in order to avoid galvanic corrosion caused by contact between dissimilar metals.
  • When retrofitting an existing structure, local jurisdiction requirements will also influence your decision on which hurricane tie to use. As an example, the state of Florida has very specific requirements for roof retrofitting, which we outline in a technical bulletin, and they specifically mention the roof-to-wall connection. Be sure to check with your local city, county or state for specific requirements before you decide to retrofit.
  • Availability of wind insurance discounts in your area could also affect your decision on which type of hurricane tie to use on your home. Your insurance company may provide a greater discount on your annual premium for ties that wrap over the top of your roof framing and are installed with a certain minimum quantity of nails. Check with your insurance provider for additional information and requirements.

Although this is a lot to take in, hopefully it makes choosing the right hurricane tie easier for you on your next project. Are there any other items you consider in your design that weren’t mentioned above? 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/

 

Construction Referees: Evaluation Processes for Alternative Building Products

construction-refereeThere are products used in every building not referenced by the codes or standards.
These products can impact safety, public health and general welfare through their effect on structural strength, stability, fire resistance and other building performance attributes. I-code Section 104.11 (Alternative materials, design, and methods of construction and equipment) provides guidance on how these products are approved for use in the built environment and identifies the Building Official as the decision-maker. This is similar to a referee determining a player’s compliance with the rules.

Building Officials see submittals for a wide variety of alternative building products ranging from the simple to the very complex. The amount of data included in these submittals and their relevance and completeness varies significantly from insufficient and minimal to complete and very thorough. In the absence of publicly developed and majority-approved provisions, the Building Official is tasked to ensure the data provided is appropriate and adequately proves the alternative product meets code intent to protect public safety, no matter the product type or complexity. This is compared to the robust code and standards process in which committees with balanced representation publicly develop and deliberate on provisions in order to protect public safety. The question arises whether the 104.11 requirement implies that a similar robust process be used in the development of test and evaluation requirements for alternative products as is used for the development of code and standard provisions where there is public debate, resolution of negative opinions and a majority approval of the requirements. Requiring a similar code development process for alternative products would seems to make sense. Otherwise, a less rigorous process might be employed by those seeking to avoid a more robust code and standard process so as to achieve quicker and less stringent approval for their alternative products.

public-safetySome may argue that having to use a “code-like” evaluation process for alternative products would add too much of a burden in time and cost, and that it’s not necessary since individual registered design professionals and building officials have enough time, resources and expertise to determine acceptability. But this begs the question of why a similar public majority-approval process should not be required for new products as it is required for code-referenced products. Another question that comes up is ongoing acceptance of an alternative product, as their manufacture may have changed since their approval. Additionally, different jurisdictions have different expertise and resources and this can lead to different standards for approval for alternative products, leading to inconsistency.

Is there a solution which balances providing innovative and cost-effective alternative building product solutions to the industry in a timely manner with providing a thorough product assessment using a process similar to the codes and standards to better ensure consistency and public safety? Accredited building product certification companies, or evaluation service companies, that use a publicly developed and majority-approved acceptance or evaluation criteria and publish an evaluation report with the product’s description, design and installation requirements and limitations provide such a solution. These evaluation service companies are a third-party resource for building officials to assist in their determination of whether an alternative product meets code intent and should be approved for use in their jurisdiction.

The number of evaluation service companies has been increasing. The ICC Evaluation Service and the IAPMO Uniform Evaluation Service, two of the better-known such companies, are both ANSI accredited to ISO/IEC 17065 (Conformity Assessment – Requirements for bodies certifying products, processes, and services) to provide building-code product certifications (ICC-ES, IAPMO UES). However, accreditation by itself mainly verifies a certain process is implemented to ensure consistency and confidentiality. Both companies also have a public acceptance or evaluation criteria process. This process includes an evaluation committee made up of building enforcement officials. These officials evaluate the proposed criteria, listen to expert and industry input and only approve the criteria by a majority vote if products evaluated to those criteria will meet code intent. This is similar to how the codes and standards are developed — a transparent public process and a majority approval of requirements and not just an opinion of one or a couple of individuals.

The alternative building product review process for ICC-ES and IAPMO UES is similar and has the following important components.

  • CRITERIA: The accredited product evaluation service develops an acceptance or evaluation criteria, with the manufacturer’s and public’s input, that is publicly debated, revised and ultimately approved by a majority vote of a committee of building enforcement officials.
  • TESTING: The manufacturer contracts out to an accredited independent third-party test laboratory to either perform or witness the product testing in accordance with the criteria.
  • REVIEW: Registered design professionals with the accredited product evaluation service evaluate the testing and analyses performed and sealed by registered design professionals with the manufacturers or their representatives. The product evaluation service then publishes the evaluation report to their website, and the report typically contains the product description, design and installation requirements andsupervising-supervisorlimitations.
  • CONTINUOUS COMPLIANCE: The manufacturer’s quality system is inspected at least annually by the product evaluation service or an accredited third-party
    inspection agency to ensure that the product currently being manufactured is the same as that which was evaluated.

 

ICC-ES-ESR-2320-UES-AT-XP

While the term “product evaluation” is sometimes used, it is often “product certification” or “product conformity assessment.” ISO/IEC Guide 2:2004 defines “conformity assessment” as “Any activity concerned with determining directly or indirectly that relevant requirements are fulfilled. Some “product certification” companies also provide “product listing” services for when testing and evaluation requirements for the product are already in code-referenced consensus standards, making the development of acceptance criteria unnecessary, thus simplifying the process.

A couple of previous blog posts on evaluation or code reports that you may find informative discuss steps to obtain an evaluation or code report and provide a checklist to determine adequacy of a report.

A mechanism is available to the building industry to provide innovative and cost-effective alternative building products in a timely manner that implements a public and majority product acceptance criteria process, similar to the codes and standards development process. This solution involves the Building Official referencing building product evaluation service reports, based on acceptance criteria, offering a robust evaluation better ensuring that an alternative product meets code intent, thus protecting the public. In fact, several jurisdictions do require evaluation service reports for alternative products.

Should there be an easier path to approval for alternative products than for code-referenced products? What is a reasonable path to product approval? What basis do you use in reviewing evaluation or code reports to determine whether an alternative product is “in or out of -bounds”? We’d love to hear your thoughts.

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.

Treated Lumber and Trusses (and the One Condition Under Which MPC Wood Trusses Shouldn’t Be Used)

What do a chicken house, a water treatment plant and a raised wood floor system all have in common?  Very likely, they all involve preservative-treated lumber.  They’re also all examples of common environments in which preservative-treated, metal-plate- connected (MPC) wood trusses may be specified.

Although trusses are successfully used in a variety of environments that require treated lumber, the first mention of “treated lumber” usually sends up a red flag in a truss design office. While the corrosion protection of truss plates is no different from the corrosion protection of any other steel fastener or hanger that comes in contact with treated lumber, there are a few more considerations that come into play whenever treated lumber is going to be used in a truss application.

Raised Wood Truss Floor System

Raised Wood Truss Floor System

When fire-retardant-treated lumber or preservative-treated lumber is specified, the first (and easiest) step is to determine whether standard G60 truss plates are acceptable for use with the treated lumber, or whether the chemical treatment requires additional protection of the plates. Recent blog posts have discussed how fasteners are evaluated for corrosion resistance and how the Corrosion Resistance Classifications in our catalog help facilitate selection of hardware and fasteners for different types of treated wood and environmental conditions.  Similar guidelines are also available for determining the proper metal connector plate for different wood treatments. For example, when using the sodium borate–based preservatives and fire retardants, standard G60 galvanized metal connector plates are acceptable. However, ammoniacal/alkaline/amine copper quaternary preservative types require more protection, such as G185, ASTM A153 galvanized- or stainless-steel truss plates. The complete guidelines – Quick Guide for Alternative Preservative Treatments with Metal Connector Plates – are available from the SBCA website.

Truss Plate Corrosion from Treated Lumber

Truss Plate Corrosion from Treated Lumber

When trusses are used in particularly corrosive environments such as coastal environments or salt storage buildings, the ANSI/TPI 1 standard lists coatings that will provide increased corrosion protection for the plates (see insert, below).

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The paint coating systems listed in (a) and (b) have been specified in the TPI standard since 1985. These paint coatings, which are applied to the truss plates after the trusses are manufactured, provide alternatives to the double-dipped galvanized or stainless-steel plates used in coastal high hazard areas. In fact, the ANSI/TPI 1 Commentary states that one study – SSPC Report 87-08, Evaluation of Coatings for Metal Connector Plates – concluded that the paint coating systems over standard galvanized plates would be expected to outperform the double-galvanized metal connector plates in field use.

Coal Tar Epoxy-Coated Metal Connector Plate

Coal Tar Epoxy-Coated Metal Connector Plate

Once the necessary corrosion protection of the plates has been addressed, the next consideration is the effect of certain lumber treatments on the truss plates’ lateral resistance, or tooth-holding capacity. Fire-retardant treatments generally require strength reductions to be applied to both the lumber and metal connector plate design values. The proprietary treatment manufacturer specifies these design reductions. As soon as the specific treatment is known, the appropriate design reductions can be easily applied by the truss design software and noted on the truss design drawing accordingly.

Besides lumber treatment, there may be other reasons for plate design reductions whenever extra galvanization or special coatings are required. While extra galvanization itself does not necessarily require a reduction in plate values, if the treated lumber’s moisture content (MC) exceeds 19% at the time of truss fabrication, then a 20% reduction to the tooth-holding values is required. The same 20% reduction applies if the environment for the intended end use of the trusses is expected to result in wood moisture content exceeding 19%.

Special Considerations and Red Flags

One corrosive environment that requires special consideration is an enclosed swimming pool. ANSI/TPI 1 requires that trusses be separated from the pool environment by a vapor barrier and be separately ventilated from the pool environment. The exception to this requirement is if the truss plates are made with a stainless steel that is not susceptible to stress corrosion cracking (SCC), i.e., not Types 304 and 316.  Since truss plates made with SCC-resistant stainless steel are not readily available (if at all), a vapor barrier is basically required anytime trusses are used over enclosed swimming pools.

Another important consideration in roof truss applications involving treated lumber is the effect of elevated temperatures. For example, when FRT lumber is going to be used in an environment where high moisture content will exist, an FRT formulated for exterior use may be specified. However, if the exterior FRT has not been tested with elevated temperatures as specified in TPI 1 Section 6.4.9.1, it should not be used in a roof application.

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But the biggest concern when treated lumber is specified for use in metal-plate-connected wood trusses has nothing to do with corrosion at all.  When a truss Designer gets a job that calls for a preservative treatment for exterior use or an exterior FRT, the very first question will be why is an exterior treatment required/what is the application? Although trusses can be adequately designed for many types of environments, there is one environment that does not mix well with metal-plate- connected wood trusses – exposed exterior applications. The TPI/WTCA Guidelines for Use of Alternative Preservative Treatments with Metal Connector Plates concludes with the following statement:

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When trusses are exposed to repeated wetting and drying, the corresponding swelling/shrinkage of the wood causes what is commonly referred to as truss plate “back out”.  Since the ability of a truss plate to provide lateral resistance depends on the teeth having adequate embedment into the wood members, any plate “back out” or withdrawal from the lumber due to weathering has an adverse effect on the load capacity of the truss plate.

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Example of a truss plate that has “backed out”

For this reason, MPC wood trusses must be protected from the elements, from the time they are built and stored through the extent of their life in service. High moisture content that is consistently high can be accounted for; but if the trusses will be exposed to moisture cycling, then it is time to consider something other than a metal-plate-connected wood truss.

What are your experiences with treated lumber and/or corrosive environments and wood trusses? Let us know in the comments section below.

Simpson Strong-Tie® Research and Testing Lab Grand Opening, WSU Pullman

On Thursday, May 5, 2016, Washington State University at Pullman, state dignitaries, construction leaders, WSU construction alumni, PACCAR management, Simpson Strong-Tie management and the press celebrated the grand opening and dedication of the PACCAR Environmental Technology Building (PETB) and the Simpson Strong-Tie Research and Testing Laboratory.

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The Simpson Strong-Tie team comprised senior leadership, engineering and marketing representatives, led by our CEO, Karen Colonias. In her speech at the opening ceremony, Karen Colonias highlighted the leadership of Simpson Strong-Tie in the engineering and construction materials industry in the U.S. and the world. She emphasized the longstanding partnership between WSU and Simpson Strong-Tie, which spans over twenty years of collaboration in various testing and code development programs, and communicated our excitement at the opportunity to collaborate more closely with WSU’s highly respected engineering department on testing and engineering programs.

Karen Colonias speaking at the Grand Opening

Karen Colonias speaking at the Grand Opening

The Paccar Environmental Technology Building (PETB) is 96,000 square feet and houses the Composite Materials and Engineering Center (CMEC) – a highly integrated hub of interdisciplinary research and education in the areas of renewable materials, sustainable design, water quality, and atmospheric research. The shared space in this new building will foster the synergy needed to find new solutions to complex industry problems, such as creating human environments that are at once safe, economical and resilient.

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The Simpson Strong-Tie® Research and Testing Lab at Washington State University (WSU) is a versatile laboratory designed specifically for the structural testing and prototyping of tall timber buildings, post frame buildings, concrete durability, building repair and retrofit and deck safety, as well as seismic and wind mitigation.

The lab includes a high-capacity reaction 28′ x 46′ concrete floor area with tie-downs, 75-kip capacity at two foot centers through the floor area; a high-capacity wall 28′ long by 2’thick by 18′ tall strong wall that is capable of withstanding a 200-kip reaction in any direction; a central 90-gallon-per-minute hydraulic pump, overhead crate and concrete mixing station. The laboratory is a dynamic space to test new material and design concepts developed in the PETB. This is one of the most visible spaces in the PETB and includes capabilities for mock-ups of new building systems, structural testing and advanced digital manufacturing. Adjoining the lab is an outdoor 32′ by 52′ reaction slab that allows for project display (e.g., Solar Decathlon competition), for developing taller and or larger structures than would be possible on the interior strong floor and for natural weather exposure testing.

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The lab is part of the Composite Materials and Engineering Center (CMEC), which has been a leader in the development of wood composite materials for more than 65 years. It is an International Code Council–accredited testing facility. The laboratory highlights engineered wood composites and is constructed of cross-laminated timber, glulam, Parallam and, of course, Simpson Strong-Tie® No- Equal connectors.

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Simpson Strong-Tie and WSU, as Karen Colonias mentioned in her speech, have a longstanding and productive partnership going back over 20 years. The two institutions have worked together in a number of areas, including new product testing, deck safety and seismic risk mitigation.

This year, Simpson Strong-Tie made a significant commitment and established the Simpson Strong-Tie Excellence Fund at the Voiland College of Engineering and Architecture at Washington State University (WSU). The fund provides an annual gift of $100,000 per year over the next eight years to support the new Simpson Strong-Tie® Research and Testing Lab in the PACCAR Environmental Technology Building (PETB). In addition to the lab, the Excellence Fund will support fellowships for professors and graduate students to present research findings, brainstorm about future research and conduct continuing education training.

The faculty of the Composite Materials and Engineering Center is committed to addressing the challenge of restoring and improving the U.S. civil infrastructure and offering an integrated approach linking material discovery, manufacturing innovation, product development, and customized design methodologies that will lead to high-performing, cost-effective solutions for the built environment. The core faculty possess diverse expertise that spans materials science (polymers, wood, cement, steel), durability and corrosion protection, manufacturing and sustainable design. The faculty also has a long history of involvement in developing building codes, standards and product acceptance criteria.

This year, the WSU Voiland College of Engineering and Architecture has more than 1,050 students enrolled in civil engineering, architecture and construction management programs. The alumni from these programs are founders of and senior executives in America’s top construction and design firms. The Wall Street Journal ranked WSU among the 25 universities whose graduates are top-rated by industry recruiters, and the Civil Engineering program is the 13th largest in the nation.

On October 29, 2016, and in line with this partnership, Simpson Strong-Tie is conducting its first annual engineering symposium at Washington State University Pullman. In this symposium, Simpson Strong-Tie engineers will share with the engineering and construction management students the various career opportunities that are available in the industry upon their graduation and introduce them to the exciting history of research and innovation at Simpson Strong-Tie. The Symposium will also include testing in the new lab of our No-Equal structural connectors and solutions.

At Simpson Strong-Tie, we are excited to be strengthening the partnership and increasing the collaboration with WSU faculty and students. We are looking forward to an extended and outstanding relationship that drives research and innovations and introduces new methods to design and construct safer, more resilient, sustainable and economical structures.

Onward and Upward!

Louay Shamroukh, P.E., S.E.

Engineering Manager, Northwestern U.S.

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.

 

Hurricane Strong

This week will see the ultimate combination of events intended to raise public awareness of the necessity for disaster-resistant construction: It is week three of ICC’s Building Safety Month; National Hurricane Preparedness Week, as proclaimed by the U.S. president; the  NOAA Hurricane Awareness Tour of the Gulf Coast; and the kickoff of the new HurricaneStrong program.

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The ICC says that “Building Safety Month is a public awareness campaign to help individuals, families and businesses understand what it takes to create safe and sustainable structures. The campaign reinforces the need for adoption of modern, model building codes, a strong and efficient system of code enforcement and a well-trained, professional workforce to maintain the system.” Building Safety Month has a different focus each week for four weeks.  Week One is “Building Solutions for All Ages.”  Week Two is “The Science Behind the Codes.”  Week Three is “Learn from the Past, Build for Tomorrow.” Finally, Week Four is “Building Codes, A Smart Investment.” Simpson Strong-Tie is proud to be a major sponsor of Week Three of Building Safety Month.

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National Hurricane Preparedness Week is recognized each year to raise awareness of the threat posed to Americans by hurricanes. A Presidential Proclamation urged Americans to visit www.Ready.gov and www.Hurricanes.gov/prepare to learn ways to prepare for dangerous hurricanes before they strike. Each day of the week has a different theme. The themes are:
⦁ Determine your risk; develop an evacuation plan
⦁ Secure an insurance check-up; assemble disaster supplies
⦁ Strengthen your home
⦁ Identify your trusted sources of information for a hurricane event
⦁ Complete your written hurricane plan.

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This week also marks the NOAA Hurricane Awareness Tour, where NOAA hurricane experts will fly with two of their hurricane research aircraft to five Gulf Coast Cities. Members of the public are invited to come tour the planes and meet the Hurricane Center staff along with representatives of partner agencies. The goal of the tour is to raise awareness about the importance of preparing for the upcoming hurricane season. The aircraft on the tour are an Air Force WC-130J and a NOAA G-IV. These “hurricane hunters” are flown in and around hurricanes to gather data that aids in forecasting the future of the storm. As with Hurricane Preparedness Week, each day of the tour features a different theme.  Simpson Strong-Tie is pleased to be a sponsor for Thursday, when the theme is Strengthen Your Home.  Representatives from Simpson Strong-Tie will be attending the event on Thursday to help educate homeowners on ways to make their homes safer.

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Finally, this week is the official kickoff of a new hurricane resilience initiative, HurricaneStrong. Organized by FLASH, the Federal Alliance for Safe Homes and in partnership with FEMA, NOAA and other partners, the program aims to increase safety and reduce economic losses through collaboration with the most recognized public and private organizations in the disaster safety movement. HurricaneStrong is intended to become an annual effort, with activities starting prior to hurricane season and continuing through the end of the hurricane season on November 30. To learn more, visit www.hurricanestrong.org.


Experts consider these public education efforts to be more important every year, as it becomes longer since landfall of a major hurricane and as more and more people move to coastal areas. The public complacency bred from a lull in major storms has even been given a name: Hurricane Amnesia.


All these efforts may be coming at a good time, assuming one of the hurricane season forecasts is correct. A forecast from North Carolina State predicts an above-average Atlantic Basin hurricane season. On the other hand, forecasters at the Department of Atmospheric Science at Colorado State University are predicting an approximately average year.


Are you prepared for the natural hazards to which your geographic area is vulnerable? If not, do you know where to get the information you need?