Holdown Deflection/Elongation of Strap Type Holdowns

[medeek]

Up until now I have been in the practice of specifying HDU holdowns for all my shearwalls, this makes sense when you have a concrete foundation to connect to beneath you. For second story shearwalls I've often gone the route of two HDU holdowns connected via a threaded rod. Some builders like this since it eliminates the strap on the exterior which tends to interfere with siding, however this is generally more expensive than just applying a strap across the floor line.

I'm currently updating my shearwall spreadsheet/calculator to include MSTA and MSTC straps but unlike the HDU holdowns I don't have a listed elongation or deflection at the allowable load. Has anyone ever thought about this before? I would like to be able to include this elongation in my calcs that check the deflection of the shearwall.

I have sent an email to Simpson's technical support.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[XR250]

PL/AE? Ok, maybe not that simple as the tension varies from max at the joint to zero at the end. Maybe do 1/2PL/AE.

 

[medeek]

There is the non-linear elongation of the strap itself (full of holes) and there is going to be a certain amount of fastener slip. To get an accurate value you would need to empirically test this, I would think.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

We have tools to evaluate chord splice slip in horizontal dopiaohragms. Perhaps one could adapt those procedures to the straps here. After all, a shear wall is just vertical diaphragm.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[medeek]

Interesting problem, I think there are probably too many variables at play to come up with a theoretical solution. How does the deflection change with fastener type, wood species, or installation on top of the sheathing? Since I'm installing the straps across a 14" I-joist floor there will be even more stretch. I could load up a model of the strap in ALGOR or ANSYS and get a fairly accurate prediction of the strap elongation but the nail slip in the holes and bending of the nails in the wood is something else.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[XR250]

With a regular hold-down, do they take into account the subfloor or rimboard crushing when the movement is determined?