Connector deformation or deflection for rigid diaphragm analysis

[Cayroo]

I am doing an analysis of a wood-framed rigid diaphragm and I am trying to find the value for da, the displacement of the tie-down or deflection at highest allowable design load. The Simpson Company publishes some but not all of their holdown and strap values. Does anyone know a source for the values that they do not supply, specifically MST, MSTI, and ST straps,and LSTHD and PAHD strap-tie holdowns?

Breyer, Fridley and Cobeen's book call it "chord anchorage slip" value. The 2000 IBC Structural/Seismic Design Manual, Vol 2, calls it displacement due to the tiedown elongation at capacity. The Simpson catalog refers to it as "holdown deflection at highest allowable design load". Simpson goes on to say that this movement is strictly due to the holdown deformation under a static load test conducted on a steel jig.

Yet Simpson has not performed the test on many of the straps and holdowns that we specify. How do other engineers handle this question?

Do you engineering each strap or holdown for deformation or displacement by the mechanics of the material or do you use a standard value, say 1/8" or 1/16" for the deformation on these holdowns or straps or is there some third party testing lab that has performed these tests?

Thank you.

 

[rlflower]

Yeah, this gets technical.

I suppose you are already familiar with UBC Standards Section 23.223 "Calculation of Shear Wall Deflection", which employs the da variable you are referring to.

By the way, do you know that Simpson has resolved this problem of anchorage distortion and slip with its new product, the PHD anchor? PHD stands for "pre-deflected holdown".

Simpson provides the da value for the HD and HDA series, but this deflection is rendered non-existant in the case of the PHD.

Now, about the MST straps, etc. It seems Simpson is being rather coy about this one. ICBO NER-413 specifies values for these straps; however, The City of Los Angeles has published its version of this approval, RR-25119, which deletes these values and requires that these values be determined by an engineer.

OK, so now we look at the National Design Specifications for Wood Construction, which outlines how to calculate the lateral capacity for a nail connection gage metal to wood.

But this fails to answer your question about slippage in straps. If I were you, I would contact Simpson directly. Ask them to send to you a sample of their MST products. Compare the stamped hole diameter provided in the strap to receive the nail with the diameter of a common-wire nail (and, while you are at it, check the diameter of a comparable "common-wire" machine-applied nail). The difference is the total allowable slippage of the assembly.

Gald to help.

 

[Cayroo]

Thanks for your reply rlFlower. I am not so sure Simpson has dealt with this issue with its pre-deflected holdown. Why would they list in their 2003 catalog the amount of holdown deflection at highest allowable design load? Why is this deflection almost as great as the other Simpson holdowns for a similiar amount of load?

I would like to get a copy of the ICBO report that you mention, NER-413, do you know how I can obtain one?

The nail slippage question is not what I am after, as you said we can calculate that information by hole and nail diameter. The actual strap or holdown deflection for those that are not given in the Simpson catalog are those that I need. Anyone have any other ideas?

 

[rlflower]

Hmmm... I have the 2002 catalog. You're right! They do list the da value; but it shows how the PHD8 with 0.060 da has about the same capacity of a HD8A with 0.111 da.

For the NER report, go to the Simpson web site, or go to my web site

http://home.att.net/~rlflower/My_Business.html

click on International Conference of Building Officials. These are the guys that publish the NER series reports.

In the case of the strap, I am not sure what other source of da there can be, other than elongation of the strap itself.