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Post tensioned wood beam

Post tensioned wood beam

Post tensioned wood beam

(OP)
My client has a roof structure over a busy retail store with existing 6x16s at 8' o.c. Spanning 30' in southern California which does not know how to spell snow.  He wants to add HVAC loads to the roof.

The underside of the roof is very congested with ducts and conduits etcetera such that a system to sister strengthening members to the existing beam seems undesirable.

I have heard of engineers using some kind of post tensioning system to increase capacity in this condition.  

As explained to me, a kind of Simpson hold down is attached up onto the bottom face of the beam at each end, a kind of "chair" is attached up onto the bottom face of the beam at midspan, and a cable is run from hold down to hold own over the chair and tensioned such that an upward force is induced at midspan at the "chair".

Is this system sound?  Any advice on design procedures?  I'd sure appreciate your comments on this.   

RE: Post tensioned wood beam

It's been discussed many times here in the forum.  Do a search...

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Post tensioned wood beam

Have you checked the current capacity??  6 x16's will carry quite a bit of load!!

RE: Post tensioned wood beam

Agree with both "Mike's".  The system you are thinking about is better done with rods and turnbuckles instead of cables, but can be done with either.

RE: Post tensioned wood beam

(OP)
Thank you.  I have checked the capacity and they are undersized given the dead loads being supported.   They were plainly designed before the 1991 code change that hit wood hard.
Also, still searching for the right "search" words......

RE: Post tensioned wood beam

Sister joists??   

RE: Post tensioned wood beam

That works too brother...

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

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