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!