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
This week’s post was written by Kari Martin, Marketing Communications Content Manager at Simpson Strong-Tie.
There are a couple of turkeys that like to hang out around our home office in Pleasanton and, no, I’m not referring to any of my colleagues — we actually have a gang of wild turkeys that comes up from the creek behind the office. Almost every day, these colorful birds feel safe enough to stroll onto the office walkway pecking for food outside our office windows and doors. It’s surprising to me that these beautiful creatures could be so fearless (or is it simply naïve?), especially around Thanksgiving time. Their presence reminds me that being fearless is important, because nothing new would ever be discovered if we were too afraid to venture outside our comfort zones.
The beauty and strangeness of the turkeys also remind me to be thankful, because everything we have in life is ultimately a gift. Their consistent return to our office is a gentle reminder as I walk into work to give thanks to you, our readers, our customers and our partners every day. Thank you!
We hope you enjoy the holiday with your family and friends. We’ll be back with another post next week.
This week’s post was written by Griff Shapack, FRP Design Engineer at Simpson Strong-Tie.
Specifying our Composite Strengthening Systems™ (CSS) is unlike choosing any other product we offer. In light of the unique variables involved with selecting and using fiber-reinforced polymer (FRP) solutions, we encourage you to leverage our expertise to help with your FRP strengthening designs. To get started, we first need to determine whether FRP is right for your project. The fastest way to do that is for you to fill out our Design Questionnaire. Our new Excel-based questionnaire collects your project information and helps you use the existing capacity check to evaluate whether or not FRP is suitable for your project per the requirements of ACI 562-16 Section 5.5.2. After the feasibility study, the questionnaire creates input sheets specifically for your project.
Open the FRP Design Questionnaire spreadsheet using Microsoft Excel. If a yellow warning appears at the top of the sheet, click “Enable Content” to ensure that the workbook will function properly. You will start on the worksheet tab named “Main”. “Main” will be the only worksheet tab when you begin, but more worksheet tabs will be created as you use the spreadsheet.
Enter the project information and your contact information in Section 1 of the worksheet. The contact information should be for the Designer that you would like Simpson Strong-Tie to work with for this project’s FRP design. See Figure 1.
Enter the FRP strengthening information in Section 2 of the worksheet. If the application will require an existing capacity check, an input form requesting the information needed for the check will appear in Section 3 of the worksheet.
For members that support gravity loads, an existing capacity check must be performed to verify that FRP strengthening is suitable before a design can be generated. For these members, the spreadsheet will generate a check table for you in Section 3 of the worksheet. Enter the number of members to be checked and the dead load (D), live load (L) and snow load (S) for each member. Use consistent units for the input. See Figure 2. The spreadsheet will calculate the demand-to-capacity ratio (DCR) for each member. The ratio must be less than or equal to 1.0.
- A result of “OK” means the existing capacity check is passed. Proceed to Step 5 below.
- A result of “NG” (no good) means the existing capacity check is failed and FRP strengthening is not likely to be suitable. However, consider contacting Simpson Strong-Tie about your design condition to ensure that this is the case.
You are now ready to create an element input worksheet for those members that passed the existing capacity check. Click “FRP Questionnaire” from the Excel menu bar. Then click the “Input Sheet” button in the ribbon bar. See Figure 3.
This will create an element input worksheet as a new worksheet tab. See Figure 4.
Enter the number of elements to be checked and fill in the design information for each member. The “No. of elements” cell features a drop-down menu with the numbers 1–5, but any number can be typed into the cell. (Each member should have passed the existing capacity check in Step 4.) See Figure 5.
If you would like to add different member types that need to be strengthened, click “Another Type of Strengthening” button in the ribbon bar. See Figure 6. This will create a new “Main” worksheet. Repeat the steps above, until all strengthening types and member data have been entered.
When you have finished inputting all required data, save the spreadsheet file and email it to firstname.lastname@example.org. You should expect confirmation of receipt from us within one business day.
From there, if FRP is a viable option, you can decide to utilize our no-cost, no-obligation design services. Our team will design a unique solution specifying the most cost-effective CSS products that address your particular needs. The design calculations, drawings, notes and specifications are prepared by Simpson Strong-Tie Engineering Services and can then be incorporated into the design documents that you submit to the building official.
Don’t know which FRP solution is the right one for you? We do. Give our new Design Questionnaire a try, and let us be your partner during the project design phase. Our new Excel-based questionnaire collects your project information and helps you use the existing capacity check to evaluate whether or not FRP is suitable for your project per the requirements of ACI 562-16 Section 5.5.2 or AASHTO FRP Guide Spec Section 1.4.4. After the feasibility study, the questionnaire creates input sheets specifically for your project. For projects in Canada designed per the requirements of CSA S806 or CSA S6, please use our fillable PDF questionnaire to collect your information.
Learn more at strongtie.com/products/rps/css/frp-engineering-design.
Advanced FRP Design Principles
In this free webinar we will dive into some very important considerations including the latest industry standards, material properties and key governing limits when designing with FRP.
This week’s post was written by Jhalak Vasavada, Research & Development Engineer at Simpson Strong-Tie.
When we launched our new, patent-pending MPBZ moment post base earlier this year, the evaluation of the moment capacity of post bases was not covered by AC398 – or by any other code, for that matter. There wasn’t a need – there were no code-accepted connectors available on the market for resisting moment loads.
We proposed adding moment evaluation to the AC398 and presented our research to the ICC-ES committee in June. After a thorough review, which included a public hearing, the provision was approved. Here are some details about the revisions to this acceptance criteria.
What is AC398?
AC398 is the Acceptance Criteria for cast-in-place cold-formed steel connectors in concrete for light-frame construction.
Acceptance criteria are developed to provide guidelines for demonstrating compliance with performance features of the codes referenced in the criteria. ICC-ES develops acceptance criteria for products and systems that are alternatives to what is specified in the code, or that fall under code provisions that are not sufficiently clear for the issuance of an evaluation report.
The criteria are developed through a transparent process involving public hearings of the ICC-ES Evaluation Committee (made up entirely of code officials), and/or online postings where public comments were solicited.
How is the moment load evaluated?
The MPBZ moment post base is a cast-in-place post base designed to resist uplift, download, lateral and moment forces. This blog post in February describes how it works, how it was tested and includes a design example. Since the MPBZ falls under the specialty inserts category of cast-in anchorage, it is not covered by the provisions of chapter 17 of ACI 318-14. Therefore, the MPBZ was evaluated based on AC398 for anchorage to concrete.
Our engineers worked closely with ICC-ES and the American Wood Council to develop evaluation criteria for moment. This revision to the criteria for moment evaluation and testing was posted for public comments on the ICC-ES website, and then presented by our engineers at the ICC-ES committee hearing last June. The presentation included the design, use, testing and load rating of the MPBZ. Following the hearing, and a thorough review, the committee approved the proposed revision to AC398.
What are the revisions to AC398?
With reference to moment evaluation, a few of the key changes to AC398 are:
- Moment Anchorage Strength: Similar to tension and shear anchorage strength, the available moment anchorage strength shall be determined using the equation
Where F = applied horizontal test force used to determine moment strength (lbf)
D = vertical distance from top of concrete member to the applied lateral test force F (ft.) (moment arm)
Other terms are as previously defined for tension and shear anchorage strength equations.
- Rotation: Testing of moment base connectors subject to an applied moment shall include measurement and reporting of the connector rotation as determined by the relative lateral displacement of gauges positioned 1″ and 5″ above the top of the connector.
- Side Bearing: Testing of moment base connectors that rely on bearing of the wood member against the side of the connector to resist moment loads shall address wood shrinkage.
Learn more about the MPBZ in our free upcoming webinar.
Join us live on December 6 for an interactive webinar on the MPBZ moment post base, its evaluation, its testing and its applications. In this webinar, we will discuss MPBZ moment post base product features, product development, design examples and much more. Attendees will also have an opportunity to ask questions during the event. Continuing education units will be offered for completing this webinar. Register today here.
Upcoming free MPBZ webinar.
Join Simpson Strong-Tie R&D engineer Jhalak Vasavada, P.E., and Simpson Strong-Tie product manager Emmet Mielbrecht for a lively and informative discussion of this new product.
This week’s post was written by Clifton Melcher, Senior Product Manager at Simpson Strong-Tie.
Structural engineers concerned with building envelopes are always looking for better solutions that help isolate the cladding from the primary structure in conditions where large building drift is a concern. Simpson Strong-Tie has an answer with a unique and innovative solution, the new DSSCB (drift strut sliding clip bypass).
The DSSCB is used to anchor cold-formed steel framing to the primary structure in bypass applications. The DSSCB is a clip that slides inside standard struts that most engineers and contractors are already familiar with. These struts will typically be attached to structural steel. However, there is also a cast-in-place strut option referred to as a strut insert. Many different manufacturers of these struts exist, but three common manufacturers are Unistrut®, PHD and B-line. The strut and strut insert requirements for the DSSCB can be found in the Simpson Strong-Tie DSSCB flier (F-CF-DSSCB17).
The DSSCB has many design features that make it easy to use, cost-effective and designer-friendly.
- The DSSCB clip has uniquely formed inserts that twist into place easily with minimal friction
- The clip features squaring flanges that help keep the clip square inside the strut
- Shoulder screws (included) prevent over-drilling and increase overall capacity
- Pre-engineered design offers clip, strut and anchorage solutions
- Pre-punched slots provide a full 1″ of both upward and downward deflection
- Sight lines facilitate proper screw placement
The DSSCB is also a hybrid clip and accompanies both slide applications as well as fixed applications. In addition to vertical slots, the clip also has round circular holes for fixed-clip conditions. This will make the clip more versatile and limit inventory.
Another great use for this product is for panelized construction. The DSSCB makes it a snap to anchor finished panels to the slab without having to waste time drilling and installing anchors. Locking panels into place is also simple with a DSHS connector clip that can be easily slid into place and attached with only one (1) #10 screw.
Accommodating for building drift and commercial panel construction just got easier with the Simpson Strong-Tie DSSCB!
Load required at bypass slide condition attached to steel with ASD reactions of 450 lb. tension (F2) and 422 lb. compression (F3) – based on CFS DesignerTM software or hand calculations
Stud member = 600S162-43 33 ksi at 16″ o.c. – based on CFS Designer software or hand calculations
Per page 4 of the DSSCB flier (F-CF-DSSCB17), allowable F2 = 785 lb. and F3 = 940 lb. for slide-clip connector (shown below)
Per page 7 of the DSSCB flier (F-CF-DSSCB17) allowable loads of F2 = 475 lb. and F3 = 2,540 lb. for strut allowable anchorage with 1″ weld at 12″ o.c. using a 13/16″ strut (shown below)
Note that, at a strut splice (if required), maximum load is not to exceed F2 of 865 lb. per note 6 on page 7 (shown below)
6. For any connector occuring within 2″ of channel strut splice, load not to exceed — F2 = 865 lb. and F4 = 785 lb.
Check connector and strut/anchorage:
F2 (tension): Pmax = 450/ minimum of (785,475) = 0.95 < 1 ok
F3 (compression): Pmax = 422/ minimum of (940,2540) = 0.45 < 1 ok
Q: How are the products sold?
A: The clips are sold in kits of 25. For the DSSCB43 and DSSCB46, one polybag of 83 screws is included. For the DSSCB48, two 55 screw polybags are included. The DSHS will be sold separately from the clips and come in bags of 100. The struts will not be sold by Simpson Strong-Tie.
Q: Can I use the 1 5/8″ x 1 5/8″ strut for the fixed-clip application?
A: No, the fixed-clip application was tested only with the 13/16″ x 1 5/8″ strut. The 1 5/8″ x 1 5/8″ strut would overhang more, which we calculate could reduce capacities.
Q: When should I use the DSHS clip?
A: The DSHS clip should be used where you want to fix the clip in place in the F1 (in-plane) direction. This clip will most likely be used for panelizing, but could be used for stick framing as well when adjustment is required before locking the clip in place.
Q: Why are there two tables that I need to use to determine my connector capacity?
A: One table is for the capacity of the clip, and the other table is for the capacity of the strut/anchorage. Two tables give the designer more flexibility in the design as well as an understanding of what is controlling the failure.
Q: How do I accommodate load requirements at a strut splice?
A: Note 6 to the Strut Channel Allowable Anchorage Loads to Steel table states the capacity of the strut with a clip directly at the splice. The values are based on assembly testing. Refer to page 7 of the flier.
Q: How do I accommodate load requirements at the strut end?
A: Note 10 to the Strut Channel Allowable Anchorage Loads to Steel table states that the connector load is to be located a minimum of 2″ from the end of the strut channel. Note 2 to the Concrete Insert Allowable Load Embedded to Concrete table gives a reduction capacity for end conditions. Reference pages 7 and 8 of the flier.
Q: Why do we show an F1 load on a drift clip?
A: The drift clip without the DSHS does not support any load in F1 direction. F1 load is only supported if a DSHS clip is used in conjunction with the DSSCB clip. This is also noted (note 4) on the DSSCB Allowable Slide-Clip Connector Loads and the DSSCB Allowable Fixed-Clip Connector Loads tables. Refer to pages 4 and 6 of the flier.
Q: How do I accommodate higher concentrated loads at jambs exceeding my typical stud loads?
A: Note 7 to the Strut Channel Allowable Anchorage Load to Steel table gives the capacity of the strut/anchorage if the strut is welded directly at the clip. Refer to page 7 of the flier.
Q: Can I drive PAFs into my strut?
A: No. The shot pin tool will not fit inside the strut channel.
Q: If I want to attach my strut to the steel edge angle with screws, what brand should I use?
A: Simpson Strong-Tie makes great fasteners, and we would recommend these fasteners (#12-24 Strong-Drive® Self-Drilling X Metal screw). However, you can use any brand fastener provided they meet our Pss and Pts capacities minimum nominal strength values in General Notes for Allowable Connector Load Tables on page 8 of the flier.
Q: At a double-stud condition, is it acceptable to double the capacity if I use two (2) clips?
A: It is acceptable to double the capacity of the DSSCB slide-clip or fixed-clip table loads (pages 4 and 6 in flier). However, the load should not exceed the load listed in the Strut Channel Allowable Anchorage Loads to Steel table (page 7 in flier). If a load is exceeded, please follow note 7 on page 7 of the flier by adding a weld connection directly at the concentrated load. This will allow you to have a wider anchor spacing for your typical studs and only reinforce the higher concentrated loads with connections directly at these locations.
This week’s post was written by Jacob McAuley, Associate Regional Marketing Manager at Simpson Strong-Tie.
Every October, millions of people across the globe participate in earthquake drills as part of an event called the Great ShakeOut in order to improve their earthquake preparedness. This year, the Great ShakeOut took place on October 19 and involved more than 60 countries. In addition to the earthquake drill, participants in the event often take part in other activities such as seminars, Q&As and more. At Simpson Strong-Tie, we practiced earthquake drills at each of our major branches, and, in our Pacific Northwest region, we were part of a Reddit Ask Me Anything event (an online live Q&A) to talk about earthquake safety and answer people’s questions. Below, I discuss our participation in both of these activities.
If you’ve ever participated in an earthquake drill, you know that they can be a bit surreal. Sure, at certain points jokes are made and people laugh, because after all, it is pretend. But at other points during the drill, you will probably recall the very real threat that an earthquake presents, and you will wonder, “Would I really be prepared?” That’s why these drills are so important and why we as a company participate every year – because in order to react quickly, you need to be well-practiced. Here’s a little about the drill we did.
The Great ShakeOut earthquake drill lasts only about 20 minutes and is fairly easy to prepare for and participate in (learn about how to participate here). At our Pleasanton office, where I work, at 10:19 am we heard a recording over the intercom that announced the earthquake drill was now under way. Each of us immediately did what the announcement instructed us to do: Drop, cover and hold on. The website of the ShakeOut expounds the three steps as follows: “Drop to the ground, take Cover under a table or desk, and Hold On to it as if a major earthquake were happening (stay down for at least 60 seconds).” They also recommend that during the drill, while you’re under the table, you should look around and imagine what would happen in a major earthquake and consider various potential hazards around you (e.g., items that might fall, break or cause a fire, such as lighting fixtures, unsecured bookcases or other furniture, computer or television screens, unstrapped water heaters) and how you might make them safer. Once the drill was over, another announcement indicated that we exit the building and go to a predesignated meeting area, where selected trained employees were responsible for taking roll of their assigned groups. Once everyone was accounted for, we went back in and resumed our usual workday.
Then, continuing in the spirit of preparedness, at 12:00 noon (PT), our very own Emory Montague, R&D engineering manager, joined Pacific Northwest earthquake-preparedness experts for a lively two-hour Reddit Ask Me Anything forum. The conversation is still live so be sure to check it out. The group of experts included a seismologist, two geologic hazards managers, a structural engineer (Emory), two geologists and an earthquake and volcano coordinator. Questions could be asked by anyone, and topics ranged from building safety to predictions about earthquakes to necessary earthquake supplies and more. Overall, the Ask Me Anything event was a success and helped make this a sobering and very informative Great ShakeOut day.
This week’s post was written by Caleb Knudson, R&D Engineer at Simpson Strong-Tie.
It’s been said that the World Wide Web is the wave of the future. Okay, maybe this is slightly outdated news, as it’s been 25 years since Bill Gates penned his internet tidal-wave memorandum, but it’s a good lead-in to this week’s blog topic – web apps. More specifically, those apps that have been developed to address the wall-bracing requirements defined in the International Residential Code® (IRC). Designers and engineers have no doubt noticed that over the last several code cycles, the wall-bracing provisions in the IRC have become increasingly complex. To help navigate these requirements and calculate the required bracing length for a given wall line, Simpson Strong-Tie introduced the Wall-Bracing-Length Calculator (WBLC) a few years back, as discussed in an earlier blog post. I’ll also mention that the WBLC has since been updated to the 2015 IRC.
Those familiar with the wall-bracing provisions in the IRC know that there are twelve intermittent wall-bracing methods and four continuous-sheathing methods to address wall-bracing requirements. Each of these methods may be used in most applications, and, while some provide advantages over others, the code-based methods provide Designers with quite a bit of flexibility. However, there may be cases where the site-specific conditions are beyond the scope of the IRC, or there just isn’t enough available full-height wall space to accommodate the required wall-bracing length. These cases are most likely to occur at large window openings or at garage fronts.
Let’s take the following example of a house on Lake Washington – assuming the house is being designed in accordance with the IRC. Presumably, one might prefer to have unobstructed lake views, which of course means lots of large picture windows and not much room left for braced wall panels. Let’s also suppose you’ve got a brand-new Chris Craft that you’d like to protect against the weather when it’s not in the water – this means wide garage doors and, again, not much room for conventional wall bracing.
So what do we do now?
Thankfully, the International Residential Code provides some guidance. Section R301.1.3 states that when a building, or portion thereof, is outside the scope of the IRC, the element(s) may be designed in accordance with accepted engineering practice. The code goes on to state that the extent of the design shall be such that the engineered element(s) are compatible with the performance of conventional methods prescribed in the code. That creates some additional options for our tool box. We could design a site-built shearwall; however, due to aspect-ratio limitations defined in the Special Design Provisions for Wind and Seismic (SDPWS), we still may not be able to get the lake views and wide garage we want. The next option, and one we’ll focus on here, is the code-approved prefabricated Simpson Strong-Tie® Strong-Wall® shearwall.
In an earlier blog post, as previously mentioned, we introduced the Strong-Wall Bracing Selector (SWBS) and defined just how we determine equivalence to conventional bracing methods. We further described the benefit of using the selector in conjunction with the Wall-Bracing-Length Calculator (WBLC). To refresh your memory, when Designers start with the WBLC to determine required wall-bracing-lengths for up to seven parallel wall lines, they can export those bracing lengths as well as project and jobsite information directly to the SWBS with the click of a button. The SWBS will then provide a list of Strong-Wall panels that provide an equivalent bracing length, evaluate their anchorage requirements, and return a list of pre-engineered anchor solutions for a variety of foundation types.
On to the present: We just launched the Strong-Wall Bracing Selector web app version 2.0, and there are a few new features worth noting.
First, I’ll mention that all Strong-Wall solutions have been evaluated according to the 2015 I-Codes. Next, and hopefully this doesn’t come as too much of a surprise, the original wood Strong-Wall shearwall (SW) is being phased out with guaranteed availability through December 31, 2018. In light of this planned obsolescence, we have removed the SW solutions from the latest version of the bracing selector.
Here’s the good news – and this is big: We’ve now added the new Strong-Wall wood shearwall (WSW) to the app and recommend this as a replacement for the SW in all applications. In the interim, while the original wall is still available, version 1.0 of the bracing selector app may be used if an SW bracing solution is required.
Lastly, we’ve provided the Designer with a bit more flexibility and control over the Strong-Wall bracing solutions provided by the app. If you recall, version 1.0 provided a solution using the minimum possible number of Strong-Wall panels to satisfy the bracing length requirement. We’ve changed that in version 2.0; Designers may still select a solution using the minimum number of panels, but they may also select the exact number of Strong-Wall panels to satisfy their wall-bracing-length requirements. Typically, it’s desirable to address the bracing requirement with the minimum number of Strong-Wall shearwall panels possible. Sometimes, however, it may be advantageous to increase the number of panels used, in order to decrease the Strong-Wall panel width used for a solution or to reduce anchorage requirements, i.e., lesser footing dimensions and anchor embedment depths. Stated a little differently, we’re providing the option to find the right balance between the braced wall panel design and the anchorage design – i.e., the Goldilocks zone for prescriptive wall bracing.
So now that we’ve reviewed just why a Designer may need to specify a Strong-Wall shearwall in prescriptive applications and how the Wall-Bracing-Length Calculator and Strong-Wall Bracing Selector web apps help to navigate this process, we’re interested to see what you think. Is there any additional functionality that you’d like to see in the future, or are these apps just right for your design needs? Let us know in the comments below.
I recently had the pleasure of presenting a webinar with Rob Madsen, PE, of Devco Engineering on our CFS Designer software, “Increase Productivity in Your Cold-Formed Steel Design Projects.” The webinar took place on September 28, and a recording is available online on our training website for anyone who wasn’t able to join us. Viewing the recording (and completing the associated test) qualifies for continuing education units and professional development hours. The webinar covers how to use the CFS Designer software to design complex loading conditions for beams, wall studs, walls with openings, and stacked walls using cold-formed steel studs, tracks, built-up sections, and even custom shapes. We received some excellent questions during the webinar, but due to time constraints were only able to answer a few during the live webinar. Rob and I did get a chance to answer all the questions in a Q&A document from which I’d like to share some excerpts. The complete Q&A webinar list can be accessed here, or through the online recording.
Where can I download the CFS Designer program?
Please visit strongtie.com/cfsdesigner to download a free 14-day trial version of the software or to purchase a license. Webinar attendees should check their email for a special discount code. There are different licensing options based on the number of users.
Is the price for the software an annual subscription fee or is it a one-time purchase price? Is there any maintenance cost?
There’s no annual maintenance fee or subscription fee. You pay only a one-time fee for the license. CFS Designer is based on an update-and-upgrade program. All updates to the program are free to licensed users and occur every few months to correct software bugs and add functionality. Upgrades, which include new design modules and updated code information, will require an additional purchase. Simpson Strong-Tie anticipates releasing upgrades on a two-year cycle, and the next upgrade has a projected release of early 2019. If you elect not to upgrade your version of the software, the current version you have will still work, but will not have the new upgrade features.
Is CFS Designer fully compliant with AISI S100-12?
CFS Designer is compliant with AISI S100-12. You can also access earlier versions of the AISI Specification in CFS Designer by selecting Project Settings/Code and selecting the version.
Are load inputs in ASD or LRFD? Do the load combination factors have to be applied prior to entering loads in the program? Should factored or unfactored loads be input?
The current software is all in ASD (allowable strength design). The next upgrade version will feature up to eight stories of stacked x-bracing and shearwalls, which will be in LRFD. Everything else will be in ASD. The stacked x-brace and shearwalls will be LRFD because of the ACI requirements for concrete. We will also make it much more clear in this version which input is ASD and which is LRFD.
What is a web stiffener? How would you use one at a stud, header, or jamb?
A web stiffener is typically a stud or track piece that is used to support the wall stud or joist from crippling at a point load or bearing support. There are different ways to design a stiffener at different locations. Some examples include using a cut piece of stud or track attached to the stud or using a clip attached to the beam. Essentially, a web stiffener is a member that is added to the stud to help stiffen the stud from crippling.
Does this program take into consideration the cold work of forming in the design/analysis?
Yes, per AISI the program’s Project Settings default is to include cold work of forming in the design and analysis.
We generally try to size our cold-formed members to avoid the need for web stiffeners, just to save on construction and material costs. Something that helps quite a bit with the web bending and crippling calc is the bearing length. Are there code requirements for bearing lengths, or is this simply based on how much bearing we anticipate the member to have at its supports?
There are no specific code requirements for calculating bearing length for web crippling; the calculation is usually based on engineering judgment and connection detailing to determine how much bearing there will be at the support. A reasonable bearing length may be the length of the connection clip you are using for the attachment. Since web crippling is a “bearing” phenomenon, where attachments are made through the web, provided the attachment is not isolated near a flange, you may not need to consider web crippling. For stud-to-track types of connections, it’s common to use the track leg length as the bearing length.
Does this software give any stud-to-stud connection calculation like stud tearing and shearing? Checks?
The studs are designed per the AISI code for shear, moment, web crippling, axial load, and the related code-required interactions. Net-section rupture near connections is not checked by the CFS Designer™ software.
What is the difference between flexural bracing and axial bracing?
Flexural bracing is bracing that is used to increase the moment capacity of the stud, and axial bracing is bracing that is used to increase the axial capacity of the stud. These might be the same for your design, but we have given the user the ability to designate different spacings.
Do you have recommendations for how to properly terminate bridging at the end of the wall?
We agree that termination of bracing is often overlooked by engineers and should definitely be considered in design. Accumulation of bridging forces should also be considered. AISI S100-12, D3.3 and AISI S240-15 D3.4 provide methods of estimating brace forces. Simpson Strong-Tie has provided some suggestion in our cold-formed steel typical details sheets that show our SFC clip as one method to properly terminate a line of bridging.
Can the kicker connection be used on the underside of concrete fill over metal deck?
Yes! The SJC kicker connection has been tested and code listed to support diagonal brace loads. Simpson Strong-Tie has also provided a wide range of anchorage solutions for the kicker application that include connecting to the underside of concrete fill over metal deck. Concrete over metal deck may be normal weight or sand-lightweight with f’c of 3,000 psi minimum and 2.5″ minimum slab height above upper flute. Minimum deck flute height is 1.5″ (distance from top flute to bottom flute). Please visit strongtie.com for more information and design tables.
Why do some engineers use steel posts welded to a base plate for low wall applications?
For walls that are not top-supported, some Designers use a welded steel post at a certain spacing and infill with cold-formed steel studs and a top track. Simpson Strong-Tie has developed an innovative moment-capacity connection called the RCKW rigid kneewall kit, which can support many of these same conditions using cold-formed steel studs and eliminate the need for structural steel.
Are there any plans to expand the software capabilities?
We have a long list of enhancements and additions for the software and will continue to make the software more efficient, more user friendly, and with additional design capabilities.
Thanks again to everyone who joined us for the webinar and sent us questions. For complete information regarding specific products suitable to your unique situation or condition, please visit strongtie.com/cfs or call your local Simpson Strong-Tie cold-formed steel specialist at (800) 999-5099.
A couple years ago, I did a post on selecting holdown anchorage solutions. At the time, we had created a couple engineering letters that tabulated SSTB, SB and PAB anchor solutions for each holdown to simplify specifying anchor bolts. About a year later, a salesperson suggested we tabulate SSTB, SB and PAB anchor solutions for each holdown. You know, to simplify specifying anchor bolts…
This conversation reminded me of the difficulty in keeping track of where design information is. In the C-C-2017 Wood Construction Connectors catalog, we have added this material on pages 62-63. Which should make it easier to find. I thought I should update this blog post to correct the links to this information.
A common question we get from specifiers is “What anchor do I use with each holdown?” Prior to the adoption of ACI 318 Appendix D (now Chapter 17 – Anchoring to Concrete), this was somewhat simple to do. We had a very small table in the holdown section of our catalog that listed which SSTB anchor worked with each holdown.
During the good old days, anchor bolts had one capacity and concrete wasn’t cracked. ACI 318 stipulates reduced capacities in many situations, different design loads for seismic or wind, and reductions for cracked concrete. These changes have combined to make anchor bolt design more challenging than it was under the 1997 Uniform Building Code.
This blog has had several posts related to holdowns. So, What’s Behind a Structural Connector’s Allowable Load? (Holdown Edition) explained how holdowns are tested and load rated in accordance with ICC-ES Acceptance Criteria. Damon Ho did a post, Use of Holdowns During Shearwall Assembly, which discussed the performance differences of shearwalls with and without holdowns, and Shane Vilasineekul did a Wood Shearwall Design Example. So I won’t get into how to pick a holdown.
Once you have determined your uplift requirements and selected a post size and holdown, it’s necessary to specify an anchor to the foundation. To help designers select an anchor that works for a given holdown, we have created different tables that provide anchorage solutions for Simpson Strong-Tie holdowns.
Two tables on pages 62-63 in the Wood Construction Connectors catalog summarize holdown anchorage solutions. The tables are separated by wood species (DF/SP and SPF/HF) to give the most economical anchor design for each post material. The preferred anchor solutions are SSTB or SB anchors, as these proprietary anchor bolts are tested and will require the smallest amount of concrete. When SSTB or SB anchors do not have adequate capacity, we have tabulated solutions for the PAB anchors, which are preassembled anchors that are calculated in accordance with ACI 318 Chapter 17.
The solutions in the letters are designed to match the capacity of the holdowns, which allows the contractor to select an anchor bolt if the engineer doesn’t specify one. They are primarily used by engineers who don’t want to design an anchor or select one from our catalog tables. We received some feedback from customers who were frustrated that some of our heavier holdowns required such a large footing for the PAB anchors, whereas a slightly smaller holdown worked with an SB or SSTB anchor in a standard 12″ footing with a 1½” pop-out.
To achieve smaller footings using our SB1x30 anchor bolts, we reviewed our original testing and created finite element (FEA) models to determine what modifications to the slab-on-grade foundation details would meet our target loads. Of course, we ran physical tests to confirm the FEA models. With a 6″ pop-out, we were able to achieve design loads for HD12, HDU14 and HHDQ14.
The revised footing solutions for the heavier holdowns require less excavation and less concrete than the previous Appendix D calculated solutions, achieving desired loads while reducing costs on the installation.
Part of the fun of structural engineering is that there are always new problems to solve. Let us know what holdown anchorage challenges or solutions you have to share!
This week we want to let you know about a new resource, the Building Strong blog. It’s very different from the SE Blog in that it ranges beyond the topics important to structural engineers to cover issues and various perspectives that help construction professionals of all disciplines design and build safer, stronger structures as efficiently as possible. We developed the new industry blog to highlight issues and topics that are of special interest to construction and building professionals. Through semi-monthly articles, the blog will cover topics ranging from rising labor costs to innovative technologies and the changing landscape of the building industry.
The Building Strong blog will cover topics on:
- Safety, codes, and compliance
- Residential and commercial construction
- Decks and outdoor living
- Building resilience
- Emerging trends and industry insights
- Collaborations and giving
- Pro tips
This week’s post was written by Darren Conrad, PE. Engineering Manager, Truss at Simpson Strong-Tie.
With Hurricane Irma wrapping up, the cleanup after Hurricane Harvey’s devastation underway in Houston and more big storms already churning in the Atlantic, it seems like a good time to discuss hurricanes and high wind. There is a great deal of good information out there to help us better understand hurricanes and their impact on people, structures and other property. To improve awareness of wind speeds and their measurement, this article will discuss a commonly misunderstood aspect of hurricane wind-speed reporting.
When a storm is approaching, you will hear meteorologists report wind speeds. They often refer to storm categories. These categories attempt to generalize expected damage to structures based on the wind speed of the storm. The wind speed for a given storm is a measure of the severity of the storm and the danger it poses to life and property. But how do meteorologists determine the wind speed that they are reporting? It seems so concrete and certain, but anyone who has been outside during a storm or windy day knows that wind isn’t constant at any one location over a period of time. It varies continuously in magnitude and direction over time. So how can something so variable be the subject of knowledge that is precise enough to be useful? How do we understand wind-speed measurements and make sure that when comparing them we are doing so in such a way that they are comparable? That is a great question.
The good news is that even though wind is variable, we have a commonly accepted way to measure wind speed and know something about a wind field or event that is occurring at a time and place. This is done by averaging measured wind speeds over specified lengths of time, or picking the highest average wind speed that occurs for a specified averaging interval from a longer period of time. A great resource for understanding how wind speeds are measured and reported can be seen here. From this explanation, it can be seen that a reported wind speed is meaningless without a specified averaging time. The shortest averaging intervals will yield the highest reported wind speeds. The longer averaging times will capture more peaks and lulls and yield lower reported wind speeds. The most common averaging intervals used to report wind speeds are three seconds, one minute and two minutes. Some countries even use a ten-minute averaging interval for reporting wind speeds. So the question arises, which average is correct? And the answer is, none of them and all of them. They are just different ways of looking at measured wind data. That is not very comforting, but one thing we can know is that none of them can be truly interpreted or compared without understanding this idea of averaging time. To make it more confusing, meteorologists and building codes do not use the same averaging interval when reporting or specifying wind speeds. This can lead to misunderstandings.
In general, you will hear meteorologists report sustained wind speeds when covering an approaching hurricane. They might also mix in some peak gusts, but for the most part they focus on sustained wind speeds. Sustained wind speeds for tropical cyclones use a 60-second averaging time. Sustained wind speed is also used by the Saffir-Simpson scale to roughly quantify the likely damage that the wind from a storm might cause typical buildings and other structures. There are criticisms of the accuracy of the Saffir-Simpson scale method, but it is widely used by the public to generalize about the severity of tropical cyclones; therefore, it is likely that the public might and commonly does attempt to compare reported sustained wind speeds to building-code-specified three-second-gust wind speeds to determine if their house or structure will withstand the storm. There is danger in making that comparison.
We need to be careful when comparing the reported sustained wind speed for a storm with the three-second-gust design wind speeds referenced in building codes and design standards. They are not the same and need to be converted before they can be compared for equivalence. After seeing the following example, one could easily see the possibility of the public or a public official comparing the sustained wind speeds reported by the weatherman to the wind speeds used by building codes and design standards and drawing conclusions that may underestimate the force and effect of the storm.
Let’s take a hypothetical situation where a building jurisdiction has adopted a wind speed of 130 mph three-second-gust design wind speed for structures built in that jurisdiction. There are various methods to convert wind speeds between different averaging times, and many factors that may need to be considered when doing that. One method for converting is the Durst method referenced in ASCE 7. Another more recent method recommended by the World Meteorological Organization provides a pretty straightforward conversion between sustained wind speed and three-second-gust wind speeds for near-surface applications. So for the sake of simplicity, we will use it for this example. If we convert a reported sustained wind speed of 130 mph to a three-second-gust average wind speed using this method, it equates to a three-second-gust wind speed for Off-Sea of 160 mph (Off-Sea is appropriate for an approaching hurricane). The adopted130 mph three-second-gust wind speed converts to 105 mph sustained wind speed. This difference could lead individuals in the path of the storm to underestimate its severity if they are not aware of the difference between averaging intervals for wind speeds. They could see the sustained wind speed of 130 mph being reported by the weather service when the storm is over open water and assume that their structure, or structures in their jurisdiction, will stand up fairly well. This would be a serious underestimate, since those structures would need to be designed to resist a 160 mph three-second-gust wind speed using ASCE 7 in order for that to be true. To say that a different way, one might think that their structure was designed for a Category 4 storm (130 mph sustained), when in fact it was actually designed for a Category 2 storm (105 mph sustained) using the Saffir-Simpson scale. Hurricane Irma at its maximum sustained wind speed of 185 mph would equate to a 227 mph three-second-gust wind speed using this conversion method. From a roof anchorage, lateral design and load path design perspective, the difference between 130 mph and 160 mph can be substantial, especially when the building is located on flat open terrain where Exposure C or Exposure D are appropriate assumptions for the design.
There is a lot more background and detail to this very complicated discussion, but the general point is to know your averaging times when comparing reported wind speeds, so as not to underestimate a storm’s force. If a storm is headed your way, hopefully you have already selected the proper hurricane tie for your structure; you have a well-defined and properly designed continuous load path; and you are protecting your exterior openings from windborne debris. Remember, the objective is not to protect the window or door product itself. Unless you are in the insurance business, you are preventing the breach of the opening to keep wind from pressurizing the structure, increasing loads on the structure and potentially causing catastrophic failure.
Know how to secure your structure against high winds, and be safe.