What Makes Strong Frame® Special Moment Frames So Special

In a Structural Engineering Blog post I wrote last October, “Soft-Story Retrofits Using the New Simpson Strong-Tie Retrofit Design Guide,” one item I barely touched on at the time was the benefit of using Simpson Strong-Tie® Strong Frame special moment frames to retrofit vulnerable soft-story wood-framed buildings commonly found on the West Coast. In this post, I will be diving into more detail on a few features that make the Strong Frame special moment frame truly special.

In the recent release of the ANSI/AISC 358-16 (AISC 358-16), the Simpson Strong-Tie Strong Frame moment connection has been included as a prequalified special moment frame (SMF) connection.  Prequalified moment connections are structural-steel moment connection configurations and details that have been reviewed by the AISC Connection Prequalification Review Panel (CPRP) and incorporated into the AISC 358 standard. What’s unique about this newly prequalified connection is that it’s the first moment connection to be prequalified in AISC as a partially restrained (PR-Type) moment connection.

prequalified-connections

With this recent inclusion into AISC 358-16, we’ve also developed our newly released Strong Frame Design Guide  to help designers understand the differences in design and detailing between the Strong Frame connection and traditional SMF connections. The following are just a few of the key differences discussed in this guide.

SMF Yielding Elements

Traditional prequalified moment frames most often require a welded connection with either a weakened beam or a stiffened connection. SMF connections are designed so that the beam will yield as necessary under large displacements that may occur during a seismic event. The yielding of the beam section provides energy dissipation and is designed to ensure that the fully restrained beam-to-column connection isn’t compromised. The current design philosophy is the product of extensive testing of SMF connections based on studying the effects of the 1994 Northridge and 1989 Loma Prieta earthquakes in California. Figures 1, 2 and 3 below depict test specimens that demonstrate yielding at the designated areas of the beam.

special-moment-frame-development

The Strong Frame SMF has taken a different approach to the traditional connections by utilizing a Yield-Link® structural fuse designed to provide the energy dissipation for the beam-to-column moment connection. This is a modified T-Stub that has a reduced section in the stem. The yielding during a seismic event has been moved from the beams to the Yield-Link structural fuse. The fuse can be replaced after a major event, very much like an electrical fuse when overloaded. A traditional moment frame may require a much more invasive structural repair.

yielding-area-strong-frame

Beam Lateral Bracing

The traditional types of prequalified connections, as along with other proprietary connections included in AISC 358, all require the beam to yield so as to dissipate energy as discussed above. These types of connections require that the beam be braced to resist the lateral torsional buckling per code. However, it is difficult to meet the bracing requirements in the case of a steel SMF in a wood structure.

stiffness-model-beam-stability-wood-construction

With the Strong Frame SMF connection, the energy dissipation is moved from the beams to the Yield-Link structural fuses, with the connection following a capacity-based design approach. This allows the connection to remain elastic under factored load combinations. With the yielding confined to the structural fuses, inelastic deformation is not expected from the members and lateral beam buckling braces are not required. The beam can be designed to span the entire length without beam bracing. See also this blog post.

Column-Beam Relationship Requirements

Traditional SMF follow a strong column – weak beam requirement to ensure plastic hinging occurs in the beams and not the columns. If the energy dissipation takes place within such hinging in the beams, the column members will remain elastic so as to provide stability and strength for the above stories. If plastic hinges occur in the columns, there is a potential for the formation of a weak-story mechanism.

weak-story-mechanism

The Strong Frame special moment frame is unlike the traditional SMF, where the plastic hinges are formed by the buckling of the beam flange and web. In the Strong Frame SMF, the stretching and shortening of the links at the top and bottom of the Strong Frame beams are the yielding mechanisms. So instead of a strong column – weak beam check, the Strong Frame design procedure checks for a strong column – weak link condition where the ratio of the column moments to the moment created by the Yield-Link® couple is required to be greater than or equal to 1.0.

yielding-strong-frame-links

Installation

Traditional moment frame connections typically require welding in the field. Where bolted SMF connections are used, pretensioned bolts are necessary. Both welding and pretensioned bolts require third-party special inspection.

The Strong Frame SMF has been designed and tested as a 100% field-bolted connection. Unlike other bolted options, the Strong Frame’s field-bolted connections only need to be made snug tight. No onsite bolt pretensioning or special inspections are required with this system. This allows the beams and columns to be maneuvered into place, erected and installed in a fraction of the time needed for the welding, lateral-beam-bracing installation and additional inspections or repairs that traditional moment frames typically require.

T-Stub-link-installationv2

Design

One last item I’d like to discuss is the design service that Simpson Strong-Tie provides for the Strong Frame special moment frame. Whether you design moment frames only once in a while or on a regular basis, the Strong Frame design team will provide you with No-Equal design support at no additional cost. Designers receive a complete package that includes drawings and calculations, which are submittal-ready. This ensures that you’ll have a frame connection design meeting the latest codes and design requirements. Contact strongframe@strongtie.com for more information or to request design support.

To learn more about the special benefits and uses of Strong Frame moment frames, check out the following links:

Sneak Peek: Our New and Improved Deck Design Guide

One of the ways I get through winter every year is by looking forward to the weekend in March when we set our clocks ahead and “spring forward” into Daylight Savings time. Some people don’t like this change because of the lost hour of sleep, but to me it means the weather shouldn’t be cold for much longer.

The coming of spring means getting to walk to the car in daylight at the end of the workday. It also means getting the garden started for the year and spending more time outside in general.

Of course, I’m not alone in being happy to see winter go.

In the residential world, the phenomenon of “deck season” coincides with this time of year.  Homeowners with decks are getting ready for summer by giving their decks a cleaning and looking them over for any needed maintenance. Now’s the time that new or replacement decks are being planned and built to be enjoyed for the rest of the year.

deck-season

It’s no coincidence, then, that our deck-code guide has been updated again in time for warmer weather. The Deck Connection and Fastening Guide goes detail by detail (ledger connection, joist-to-beam connection, beam-to-post connection, etc.) through a typical deck and identifies the relevant building-code requirements (2012 and 2015 IRC/IBC) and connection options.

Our deck-code guide can be a helpful reference to an engineer who is just getting acquainted with decks, and can also bring you up to speed on revisions to the IRC that can necessitate engineering changes to even a relatively simple residential deck. Multilevel decks, guardrail details, ledger details and foundation challenges are all examples of things a deck builder could call you for assistance with.

For more information on resources available to engineers on deck design, feel free to consult my previous blog article, Wood-framed Deck Design Resources for Engineers.

The Deck Connection and Fastening Guide

F-DECKCODE17

This guide provides instructions on how to recognize defects and deficiencies in existing decks, and guidance for building a strong, safe, long-lasting new or renovated deck structures.


For more deck-related blog posts, check out the links below:

5 Steps to a Successful Soft-Story Retrofit

Last year, I gave a presentation at the annual National Council of Structural Engineers Associations (NCSEA) Summit in Orlando, Florida, titled “Becoming a Trusted Advisor: Communication and Selling Skills for Structural Engineers.” As this was a summit for the leaders of the structural engineers associations from across the country, I wasn’t sure how many people would find it valuable to spend their time learning about a very nontechnical topic. To my surprise and delight, the seminar ended up being standing-room only, and I was able to field some great questions from the audience about how they could improve their selling and communication skills. In the many conversations I had with the conference attendees after my presentation, the common theme was that engineers felt they needed more soft-skills training in order to better serve their clients. The problem, however, was finding the time to do so when faced with the daily grind of design work.

Structural Engineers In a Training for Seismic Retrofits
Presenting at the NCSEA Summit, I’m the tiny person in upper left hand corner.

When I started my first job as a design engineer at a structural engineering consulting firm straight out of school, I was very focused on improving and expanding my technical expertise. Whenever possible, I would attend building-code seminars, design reviews and new product solution presentations, all in an effort to learn more about structural engineering. What I found as I progressed through my career, however, was that no matter how much I learned or how hardworking I was, it didn’t really matter if I couldn’t successfully convey my knowledge or ideas to the person who really mattered most: the client.

Contractors discussing building plans with an engineer.
Contractors discussing building plans with an engineer.

How can an engineer be most effective in explaining a proposed action or solution to a client? You have to be able to effectively sell your idea by understanding the needs of your client as well as any reasons for hesitation. The importance of effective communication and persuasion is probably intuitive to anyone who’s been on the sales side of the business, but not something that occurs naturally to data-driven folks like engineers. As a result of recent legislation in California, however, structural engineers are starting to be inundated with questions from a group of folks who have suddenly found themselves responsible for seismically upgrading their properties: apartment building owners in San Francisco and Los Angeles.

Imagine for a moment that you are a building owner who has received a soft-story retrofit notice under the City of Los Angeles’ Ordinance 183893; you have zero knowledge of structural engineering or what this term “soft-story” even means. Who will be your trusted advisor to help you sort it out? The City of Los Angeles Department of Building and Safety (LADBS) has put together a helpful mandatory ordinance website that explains the programs and also offers an FAQ for building owners that lets them know the first step in the process: hire an engineer or architect licensed in the state of California to evaluate the building.

Simpson Strong-Tie Structural Engineer Annie Kao at a jobsite.
Checking out some soft story buildings in Los Angeles. The Los Angeles Times has a great map tool.

I’ve had the opportunity to be the first point of contact for a building owner after they received a mandatory notice, because it turns out some relatives own an apartment building with soft-story tuck-under parking. Panicked by the notice, they called me looking to understand why they were being forced to retrofit a building that “never had any problems in the past.” They were worried they would lose rent money due to tenants needing to relocate, worried about how to meet the requirements of the ordinance and, most importantly, worried about how much it was going to cost them. What they really wanted was a simple, straightforward answer to their questions, and I did my best to explain the necessity behind retrofitting these vulnerable buildings and give an estimated time frame and cost that I had learned from attending the first Los Angeles Retrofit Resource Fair in April 2016. With close to 18,000 buildings in the cities of San Francisco and Los Angeles alone that have been classified as “soft-story,” this equates to quite a number of building owners who will have similar questions and be searching for answers.

To help provide an additional resource, Simpson Strong-Tie will be hosting a webinar for building owners in the Los Angeles area who have received a mandatory soft-story retrofit notice. Jeff Ellis and I will be covering “5 Steps to a Successful Retrofit” and helping to set a clear project path for building owners. The five steps that Simpson Strong-Tie will be recommending are:

  1. Understanding the Seismic Retrofit Mandate
  2. Partnering with Design Professionals
  3. Submitting Building Plans with the Right Retrofit Product Solutions
  4. Communicating with Your Building Tenants
  5. Completing Your Soft-Story Retrofit

We encourage you to invite any clients or potential clients to attend this informative webinar, which will lay the foundation for great communication between the two of you. As part of the webinar, we will be asking the building owners for their comments, questions and feedback so we can better understand what information they need to make informed decisions, and we will be sure to share these with the structural engineering community in a future post. By working together to support better communication and understanding among all stakeholders in retrofit projects, we will be well on our way to creating stronger and more resilient communities!

For additional information or articles of interest, there are several resources available:

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

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

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

 

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

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

 

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

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

safer-cities-ca

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

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

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

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

1994-northridge

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

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

1933-earthquake-shot

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

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

oes-inspectors-program

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

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

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

For additional information or articles of interest, please visit:

New LSSJ Hanger Strengthens Jack Rafter Connections

When our company is considering a new or improved product, we like to start out by talking to our customers first. That’s what we did recently with a connector improvement project for attaching jack rafter hangers in roof framing – and we got lots of feedback!

We heard from installers that they really wanted a hanger that could be easily adjusted in the field for different slopes and skews. We were asked whether we could design a hanger that could be installed after the rafters were already tacked into place to support construction sequencing and retrofit applications. Also, having a hanger that could be installed from one side was a popular time-saving request.

Our Engineering innovation team took all this feedback and closely evaluated our current selection of hangers. After much consideration, the team decided that rather than adapt one of our existing hangers, they would try to  come up with an all-new design that would satisfy our customers’ most pressing needs.

After months of designing and testing prototypes in the lab and in field trials, the answer was yes. The result is our new LSSJ field-adjustable jack hanger. It’s an innovative field-slopeable and field-skewable hanger that features a versatile hinged seat. This new design allows it to be adjusted to typical rafter slopes, with a max slope of 12:12 up or down.

What is a jack hanger and why does it provide a better connection than nails alone? 

There are two basic types of wood roof construction: framed roof construction (stick framing) as shown above, and truss assembly. The main difference is that stick assembly takes place onsite, while trusses are prefabricated and ready to place. In the United States, the number of truss-built roofs versus stick-frame roofs is about two to one. The LSSJ jack hanger is used for stick-frame construction and provides a connection between the jack rafter to either the hip rafter or the valley rafter as shown below.

The LSSU hanger connects the jack rafter to the hip rafter
The LSSJ hanger connects the jack rafter to the hip rafter

Connecting a 2X jack rafter to a hip is hardly new. The hardest thing is making a good compound miter cut – something an experienced framer can figure out (and most engineers marvel at). In many parts of the country, these are simply face-nailed into place.  Often there isn’t a lot of engineering that goes into that connection.  However, a closer look raises a couple of questions.

Random Nail Placement

Where exactly are those nails going? When there’s no seat support for the rafter, the allowable shear is reduced per the NDS depending on where the lowest nail on the rafter is. This is based on the split that develops at the lowest fastener. The LSSJ provides a partial seat which not only meets the bearing requirement of section R802.6 of the IRC but also delays the type of splitting found in a nailed-only connection.

Consistent Nail Placement

The LSSJ conforms to the bottom of the jack rafter slope and ensures consistent nail placement on both the rafter and the hip.  Consistent nail placement promotes consistent performance based on testing (or as consistent as wood gets)!  The highest nail on the hip is located near the neutral axis if the hip is one size deeper than the rafter.  This assures that not all the load is focused at the bottom of the hip.

A Closer Look at the LSSJ Jack Hanger

Some of our customers may be familiar with our current product, the LSSU, which is used for the same connection. Here’s a closer look at the improvements that the LSSJ offers.

LSSU and LSSJ
LSSU and LSSJ
lssu-lssj-installation
LSSU and LSSJ Installation
lssu-lssj-Skewing
LSSU and LSSJ Skewing

You can see the differences and improvements just by looking at these hangers, installations and load tables. Here’s a different way of showing the advances and benefits of the LSSJ:

LSSJ Improvements
LSSJ Improvements

One of the greatest improvements is the fact that there are fewer nails to install in the LSSJ, and the loads are very similar if not better.

In addition to the LSSJ, Simpson Strong-Tie offers a full line of connectors for wood-framed sloped roofs, including:

 

We look forward to hearing from you about our newest innovation. For more information about the LSSJ hanger, please see strongtie.com.

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

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

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

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

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

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

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

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

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

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

 

 

 

How to Pick a Connector Series – Truss Hangers

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

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

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

Typical THA29 Installation

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

hgus2102-installed

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

The wrong hanger selection for the application
The wrong hanger selection for the application

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

Example of a short heel installed in a tall hanger.

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

hthgq-installation

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

Incorrect hanger selection – this skewed hanger requires the carried member to be bevel-cut whereas the truss is square-cut.
Incorrect hanger selection – this skewed hanger requires the carried member to be bevel-cut whereas the truss is square-cut.
Not all skewed hangers can be used with square-cut members (trusses).
Not all skewed hangers can be used with square-cut members (trusses).

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

HTU Hanger
HTU Hanger

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

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

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

 

Can Decorative Hardware Add Structural Strength?

At Simpson Strong-Tie, we really try to listen to our customers. Our products are developed with your needs in mind.

Last year, at my daughter’s college orientation, I found myself in an interesting conversation with one of the other parents. It turned out that he owns a deck-building company. When he found out that I’m an engineer at Simpson Strong-Tie, his first question was “why don’t you guys make some nice-looking connections that I can use on my decks?”

Ugly Connector
Ugly Connector

I had to choke back a laugh because that’s exactly what I was working on at the time. What he didn’t mention (but we knew he also needed) were connectors that are fast to install, suitable for outdoor use and structurally rated for engineered designs. We also knew code approval was critical to help building departments approve the designs.

The Outdoor Accents® connectors we designed include some basic T’s, L’s, angles and post bases with a nice architectural feature of decorative edges from our Mission Collection®. The steel has our ZMAX® (G185) galvanizing (which is twice as heavy as our standard G90) to resist corrosion and a black powder-coat finish for aesthetics.

outdoor-accents-group outdoor-accents-strap

But the real innovation is in the fastener. Architectural connectors and big bolts go hand in hand, but big bolts are expensive, time consuming and often structurally unnecessary. To solve the installation issue, we designed a decorative washer that looks like a washer and nut and perfectly fits our SDWS22DBB Structural Wood screw.

stn22-installation-with-sdws

We named it the shear tube nut (STN) because the extended tube increases the shear area in contact with the connector.

shear-tube

Together with the SDWS22DBB screw, this solution looks like a bolted connection but installs with the speed and ease of a self-tapping screw. Structurally as well, the hardware is comparable to a bolted connection with a shear capacity of 470 lb. per fastener when used with metal side plates, i.e., connectors.1  The solution has also been tested and load rated for use directly on wood, so it can be used for a variety of other connections such as joining multi-ply beams, knee braces, etc.

In order to be code approved, the SDWS22DBB screws were tested with and without the STN in both wood-to-wood and metal-to-wood per AC233 Acceptance Criteria for Alternate Dowel-Type Threaded Fasteners. The connectors and fasteners, including STN, were tested as assemblies per ASTM D7147. Code agency reviewers quickly saw the benefits of the design and issued evaluation reports verifying the loads. The Outdoor Accents® connectors and SDWS22DBB screws are recognized under IAPMO UES ER-280 and ER-192, respectively. The smaller APA21 angle uses our new SD10112DBB screw, which is listed in ICC-ES ESR-3046.

My deck-builder friend will be pleased to see the new connectors are now available at select Home Depot stores.

outdoor-accents-project

I can’t wait to see what he thinks of them and to get his ideas for the next big project. How about you?  What would you build with these new architectural products?  Let us know in the comments below.

  1. Ref. IAPMO UES ER-192 Table 6A steel side member DF = 470 lb.; 2015 NDS Table 12B 3 1/2″ main member, 1/2″ bolt, DF perpendicular-to-grain = 510.

Outdoor Accents®

Add Beauty and Strength to Your Custom Outdoor Living Structures.


Bucket Lists for Structural Engineers and Some Resources for Helping Cross Post-Frame off Your List

Bucket lists are mentioned regularly today, which got me to thinking  – what about a bucket list for structural engineers? ASCE and others have put together lists of engineering wonders of the modern world, so those seem like a good start for sights to see. But for a practitioner, I’d propose the next most obvious things to add would be working with each of the common structural building materials and system types. For engineers working with buildings, the “list” would include the various types of steel, concrete, wood and masonry materials, and then the different respective building systems.

Maybe this list can also offer a refreshing perspective when you’re wading into uncharted territory; a new material or system presents the chance to cross another item off your list! For most engineers, I would guess a post-frame building will be one of the final remaining items on their list. Post-frame is rightly known for its historical origins in agricultural buildings; however, today there is more developed design information, and post-frame buildings are being built for many different uses. If you do find yourself looking at post-frame for the first time, there are a few resources to be aware of that can help guide and inform your experience.

post-frame

Post-frame buildings comprise a primary framing system of wood roof trusses or rafters that are supported by large solid-sawn or laminated lumber columns. The secondary roof purlins and wall girts support the roof and wall sheathing. The columns are either embedded into the ground or anchored to concrete piers, walls or slabs. The buildings offer efficiency in materials, construction time and costs, and energy. An engineer can design a post-frame building in compliance with the IBC, with allowances for high-wind and seismic conditions.

Two free resources that are good starting points for an engineer considering post-frame are the American Wood Council’s Design for Code Acceptance (DCA5) – Post Frame Buildings, and the Post-Frame Construction Guide by the National Frame Building Association (NFBA). The DCA5 gives a brief overview of the pertinent section of the IBC that relates to post-frame. The Post-Frame Construction Guide is a 20-page document that describes the components of a post-frame system, fire performance, examples of common details and different building uses, and a summary of resources for additional information.

A manual for purchase that is an excellent resource is the NFBA’s Post-Frame Building Design Manual – Second Edition. The manual presents a comprehensive scope of content including sections on code provisions, guidance for design, diaphragm design, post design and foundation design. Lesser-known IBC-referenced standards that are commonly utilized in post-frame, such as ASABE EP 484.2 for diaphragm design and ASABE EP 486.1 for shallow post foundation design, are covered by the manual.

What do you think of the idea of a bucket list for structural engineers? Would you already be able to cross off post-frame building from your list? Let us know by posting a comment.

Concrete Anchorage for ASD Designs

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

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

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

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

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

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

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

ICC-ES-equations

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

IBC-ealier-later

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

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

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