Composite (ie Trex) Decking and Lateral Loads

[rn14]

I was wondering what you are all doing when you design a deck as a cantilevered horizontal diaphragm but using composite decking? The ESR report for Trex specifically says it may not be used as a diaphragm (section 5.1). Am I misinterpreting the ESR report when it states "Trex must not be used as a component of trusses or structural diaphragms..."? Are you placing some type of horizontal bracing under the joists or decking? Sheathing perhaps?

 

[msquared48]

Horizontal X bracing under the decking or vertical X bracing at the edge of the deck between the columns.

Don't know why you are using Trex decking. They went extinct 90 million years ago.

Mike McCann, PE, SE (WA)

 

[KootK]

Better website than your average stegosaurus: Link.

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.

 

[rn14]

It seems quite popular here because of the low upkeep requirements. Pretty interesting that they don't list an allowable diaphragm value, it obviously has some capacity. Scary thing is that it weighs a lot more than standard lumber so in cases where people are just pulling up the old decking and replacing it with this or a similar product there may be problems in the future. I also found it interesting that although they claim it is not to bu used as part of a diaphragm that is exactly what was done in this test Link.

 

[msquared48]

Well, if I remember correctly, for I have seen this material Eons ago, it is not a solid section as with standard wood planking. I do agree with the force couple theory, but suspect that the screws may not be fully laterally constrained, making them more susceptible to bending and failure. I believe Trex claims no degregation from UV rays, but I am skeptical on that too.

Mike McCann, PE, SE (WA)

 

[msquared48]

Also... The material is probably much more temperature sensitive than wood which would affect any possible diaphragm resistance, making it less reliable in that capacity.

Mike McCann, PE, SE (WA)

 

[XR250]

I have never considered any decking as a diaphragm. I am sure nailed wood decking act as a giant vierendeel truss to some degree, but I would not count on it. They may change the decking out at a later date and use a hidden fastening system which would have about zero diaphragm capacity.

 

[dcarr82775]

The Trex decking uses little biscuits between the planks to anchor it down. The biscuits allow the large thermal movements to occur and eliminate any reasonable though of using it for a diaphragm.