So, What's Behind A Structural Connector's Allowable Load? (Holdown Edition)

So, What’s Behind A Structural Connector’s Allowable Load? (Holdown Edition)

This is Part 1A of a four-part series I’ll be doing on how connectors, fasteners, anchors and cold-formed steel systems are load rated.

I envisioned doing a four-part series on how connectors, fasteners, anchors, and cold-formed steel are load rated. After writing the first installment on connectors, I realized that connectors are a bit more complicated, since the testing and evaluation for joist hangers (or similar devices) is different than testing for holdown devices. And I wanted to discuss holdowns. So without belaboring the apology for my numbering system, this will be part 1A of the series – still discussing wood connectors, but focusing on holdowns and some of the unique requirements in their load rating.

AC155, Acceptance Criteria for Hold-Downs (Tie-Downs) Attached to Wood Members, was first developed in 2005 to better address boundary conditions, deflection limits, and wood post limits. Prior to AC155, holdowns were evaluated based on testing on a steel jig with a safety factor of 3.0 and an NDS bolt calculation. Deflection at the allowable load was simply reported so that it was available for use in design, but there was not a deflection limit that affected the load rating.

Bolted Holdown - Steel Jig Test
Bolted Holdown – Steel Jig Test

In the steel jig setup shown, the jig keeps the holdowns stationary while the rectangular bar underneath the holdowns is pushed downward to simulate an uplift force. This was (and still is) an effective method of testing the capacity of the steel body of a holdown, but it does not tell you a lot about the deflection of the holdown when installed on a wood member. Since story-drift is such a critical component to shearwall performance and the deflection of holdowns has a significant effect on the total drift, this needed to be address in the holdown test standard.

Screw Style Holdown Steel Jig Test
Screw Style Holdown Steel Jig Test

The wood jig test setup used for screw style holdowns is an improvement, since the holdown is tested on wood. However, the setup shown will load the post in compression. Having the post end grain bearing on the test bed eliminates several possible failure modes one might observe if the post is loaded in tension.

Holdown Post Failure
Holdown Post Failure

The critical requirements found in AC155 are:

(1)    Deflection limits based on assembly tests on wood members

(2)    Test setup to match the intended use (i.e., load the post in tension for a holdown)

(3)    Steel jig testing to evaluate strength of the holdown

HD5A Setup Per AC155
HD5A Setup Per AC155

There are some slight differences when evaluating nailed or screw style holdowns and bolted holdowns, which are outlined below:

Nailed/Screwed Holdowns:Bolted Holdowns:
Fastener CalculationFaster Calculation
Deflection Limits:Deflection Limits:
¼” Deflection Limit on Wood Post¼” Deflection Limit on Wood Post
0.185” Deflection Limit on Steel Jig0.185” Deflection Limit on Steel Jig
Deflection Limits at Strength LevelsDeflection Limits at Strength Levels
Ultimate Load LimitUltimate Load Limit
     Steel Jig Test ÷ 2.5     Steel Jig Test ÷ 2.5
     Wood Assembly Test ÷ 3.0

Since shearwall deflections are calculated at strength levels, the deflection limits are also at strength level. To convert to allowable stress design numbers, you divide the deflection limited load by 1.4 and the corresponding deflection is measured from the average load-deflection curve. In addition to a steel jig test limit, nailed and screwed holdowns have an ultimate load limit based on the wood assembly test. This requirement is included to evaluate potential wood failures around the fasteners.

What are your thoughts? Visit the blog and leave a comment.

– Paul

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Paul McEntee

Author: Paul McEntee

A couple of years back we hosted a “Take your daughter or son to work day,” which was a great opportunity for our children to find out what their parents did. We had different activities for the kids to learn about careers and the importance of education in opening up career opportunities. People often ask me what I do for Simpson Strong-Tie and I sometimes laugh about how my son Ryan responded to a questionnaire he filled out that day: Q.   What is your mom/dad's job? A.   Goes and gets coffee and sits at his desk Q.   What does your mom/dad actually do at work? A.   Walks in the test lab and checks things When I am not checking things in the lab or sitting at my desk drinking coffee, I manage Engineering Research and Development for Simpson Strong-Tie, focusing on new product development for connectors and lateral systems. I graduated from the University of California at Berkeley and I am a licensed Civil and Structural Engineer in California. Prior to joining Simpson Strong-Tie, I worked for 10 years as a consulting structural engineer designing commercial, industrial, multi-family, mixed-use and retail projects. I was fortunate in those years to work at a great engineering firm that did a lot of everything. This allowed me to gain experience designing with wood, structural steel, concrete, concrete block and cold-formed steel as well as working on many seismic retrofits of historic unreinforced masonry buildings.

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