Drive a New Path: Resisting Uplift with Structural Fasteners

Structural screws are designed and tested to do hard work, but that doesn’t make them hard to use. In this post, Simpson Strong-Tie structural engineer Bryan Wert explains how the load-rated strength, versatility and easy installation of the code-listed Strong-Drive® SDWC Truss screw and SDWF Floor-to-Floor screw make it a cinch to create a continuous load path to resist wind uplift. Learn more during our May 2 webinar.

Winter’s finally shedding her blanket and unveiling springtime in Texas. There’s now a short window of picture-perfect weather where my purchases at Home Depot are no longer foam hose bib covers to protect outdoor faucets from freezing temperature, but aren’t quite yet tiki torches and floats for the pool for hot and humid summer days. I find myself in the garden center looking at the freshly delivered trees, shrubs and flowers, along with just about every other adult in my city. This year, my wife’s decided we need to surround our outdoor living space with hanging planters displaying perky red, purple, yellow and blue flowers.

Upon returning home to get started on the honey-do list I find that instead of simple screw-in hooks for the hanging planters, we’ve instead purchased extension arms with S-hook do-hickeys and — lucky me — they come with their own installation screws. I try installing the screws through the predrilled bracket into the wood cedar beam at my patio’s perimeter and the screw turns maybe two revolutions before refusing to embed further. Now I have to find a drill bit to match the undefined screws. At best, this will double my time for installation and at worst cost me a second trip to the store to replace the common Phillips-head screw as it cams out upon installation. In the end, I find myself somewhere in between — predrilling holes, then hand-driving each screw into place.

This project deepened my appreciation for the full line of Simpson Strong-Tie® structural screws with either hex-head or 6-lobe drive-heads. The Simpson screws make driving easy, thanks to their proprietary SawTooth™ point that helps them start fast with reduced torque, and, the best part, require absolutely no predrilling.

One of the most exciting product groups in the growing Strong-Drive® line is the combination of the SDWC Truss screw and the SDWF Floor-to-Floor screw with TUW take-up washer. Models within these two lines of screws create a new and innovative method of creating a continuous load path for wind uplift resistance.

The Strong-Drive SDWC screw is suited for a plethora of fully tested and load-rated connection types. These include ledger-to-rim, sole-plate-to-rim and almost all connections needed to complete a load path to resist uplift forces. The SDWC is load rated for stud-to-bottom-plate or stud-to-top-plate connections, as well as fastening trusses and rafters to top plates. The fully threaded shank engages the entire length of the fastener providing a secure connection. It’s tested in accordance with ICC-ES AC233 (screw) and AC13 (wall assembly and roof-to-wall assembly), is code listed under IAPMO-UES ER-262 and meets 2012 and 2015 IRC® and IBC® code requirements for several common framing applications.

Where the SDWC Truss screw’s capabilities end, the Strong-Drive SDWF Floor-to-Floor screw’s begin. This screw’s designed to simplify the wind uplift–restraint floor-to-floor connection while providing superior performance over the life of the structure. The unique design of the SDWF enables it to attach upper and lower walls together from the top, spanning the floor system, and requires no predrilling to provide a secure connection within the continuous uplift load path of the structure.

The innovative take-up washer (TUW) plays a key role in the long-term performance of the SDWF when installed between the screw head and the sole plate of the upper floor. The specialized threaded portion under the head of the screw ratchets up through the matching threaded tabs of the TUW as the structure settles in response to shrinkage and construction loading. The interlock between the tabs of the take-up washer and the threads under the head of the SDWF prevents the screw from sliding back under load, providing a simple yet reliable means of shrinkage compensation up to 3/4″ per story.

As I sit back on my patio with a cold drink in my hand and admire my handiwork, I dare not tell my wife about the versatile, labor-saving SDWC and SDWF screws. Revealing the existence of these innovative wind uplift–resisting continuous load path screw connections might result in a much longer honey-do list that could even include deploying Strong-Drive® fasteners to build a whole new house. But while I won’t be telling my wife about it, if you’re an engineer, builder or code official interested in learning more about how to use fastener systems for uplift restraint, check out our upcoming one-hour webinar:

Drive a new path: Resisting uplift with structural fasteners

May 2 at 11:00 a.m. PT / 2:00 p.m. ET.

By attending this webinar, you should be able to:

  • Explain how a threaded fastener system works to establish a continuous load path for uplift restraint
  • Identify threaded fastener solutions for roof-to-wall, stud-to-plate and floor-to-floor connections
  • Describe the benefits of using Strong-Drive structural fasteners compared to traditional continuous load path connection methods
  • Recall design considerations when specifying fastening systems for resisting uplift

Continuing education credits will be offered for this webinar.

  • Participants can earn 1 professional development hour (PDH) or — by passing the accompanying test — 0.1 continuing education unit (CEU).

Q&A About MPBZ Moment Post Base

This week’s post was written by Jhalak Vasavada, Research & Development Engineer at Simpson Strong-Tie.

This past December, Simpson Strong-Tie hosted an interactive webinar in which product manager Emmet Mielbrecht and I discussed the development, testing, evaluation and applications of our new moment-resisting MPBZ moment post base. During the one-hour webinar, we explained the testing and evaluation criteria for new product development, test procedures, installation recommendations, allowable loads and the rotational stiffness of the connection. We also included a design example. In case you missed the discussion, you can watch the on-demand webinar and earn PDH and CEU credits here.

As part of the live webinar in December, Emmet and I led a lively Q&A session with the attendees. What follows is a curated selection of those questions and answers. Click here for more answers to participant questions. 

What is the most common ultimate failure mode?

It is concrete breakout.

How is wood shrinkage addressed?

We have evaluated wood shrinkage by testing, however, it is in review with ICC-ES. Additional information shall be made available upon approval from ICC-ES.

What was the actual strength of the concrete being used in the test (not the design value)?

It was 2500 psi, +/- 10 percent.

Breakout/pryout failure seems to govern allowable loads. Are there plans to test connection with adequate reinforcement to ignore breakout failure and achieve higher allowable moment loads?

Based on the overwhelming requests for higher loads we will be testing MPBX for:

  • Higher strength concrete
  • Reinforced concrete
  • Greater edge distances

Given overlap of steel, is one direction stronger than the other and Simpson uses the weak direction for tbl?

Yes, one direction is stronger than the other and the weak direction allowable loads are listed.

Technically, the stand-off tabs and side friction will also aid in the vertical load transfer, just extremely minimal.

Correct. Our tested loads are actually higher than the screw calculations. The code requires we use the lower of the two loads, so we use the SDS screws calculated capacity only.

Why is the 4×4 stiffer than 6×6?

The stiffness in the graphs is relative to the stiffness of the post. The 6×6 post is much stiffer, so the post base is less stiff as a percentage of the 6×6 post stiffness. The actual stiffness of the MPB66Z is stiffer than the MPB44Z.

Why a F1 value for wood? And you can use the higher wood values with a proper concrete design?

The F1 values listed in the allowable load tables are the lowest of concrete and wood assembly allowable loads.

Why aren’t uplift loads for the wood connection published?

The uplift loads are limited by the lesser of the wood or concrete capacity. We lumped those together under the concrete. This will be clarified in future publications.

Relative to deflection associated with rotation at the base is that considered elastic? In other words, when the load is removed will the deflection return to zero?

Yes. Deflection associated with rotation at the base due to applied loads within allowable load range is considered elastic.

Ready to learn more about MPBZ moment post base? See the rest of the Q&A questions here and you can watch the on-demand webinar and earn PDH and CEU credits here.

Watch a 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 MPBZ.

A New Way to See Whether FRP Is Right for Your Project

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.

Getting Started

Step 1

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.

Step 2

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.

Step 3

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.

Figure 1. Project information and FRP strengthening information.

Step 4                                                                                                                        

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.

  1. A result of “OK” means the existing capacity check is passed. Proceed to Step 5 below.
  2. 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.

Figure 2. Existing capacity check.

Step 5

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.

Figure 3. “Input Sheet” button.

This will create an element input worksheet as a new worksheet tab. See Figure 4.

Figure 4. Element input worksheet.

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.   

Figure 5. Element input worksheet.

Step 6

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.

Figure 6. “Another Type of Strengthening” button.

 Step 7

When you have finished inputting all required data, save the spreadsheet file and email it to css@strongtie.com. 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.

Learn more: Webinar – Introducing Fabric-Reinforced Cementitious Matrix (FRCM)

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

Continuing education credits will be offered for this webinar.
Participants can earn one professional development hour (PDH) or 0.1 continuing education unit (CEU).