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post tension bottom chord reinforcement

post tension bottom chord reinforcement

post tension bottom chord reinforcement

(OP)
I have a 8' o.c. wood truss roof system receiving a heavier HVAC. In addition to other reinforcements, I am thinking of utilizing a post tensioned system to reinforce the bottom chord. The system would use something similar to simpson holdowns fastened to the bottom chord, but custom fabricated and heavier, and a steel cable between the two 'holdowns' that can be pulled to a select tension with a calibrated device.

My question is the proper tension force to apply. After the new HVAC is installed, the bottom required tension will have increased approximately 3500 lbs. But the tension will likely be applied before the HVAC is installed, and also while there is not live load on the roof. The region has no snow.

I would sincerely appreciate your experienced thoughts regarding this.

RE: post tension bottom chord reinforcement

More than likely, that would void the warranty for the trusses.

Normally, too, it's a nono to engage any connector to the bottom chord of the truss, let alone the top chord, and you would haved to do both to get that scheme to work - basically a two point harped configuration. This could be a real issue.

Better to add another truss or Microlam/GluLam/Parallam to do the job if possible considering any Mechanical/electrical clearance constraints.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

RE: post tension bottom chord reinforcement

(OP)
Thanks,
I I think I'll investigate a different solution, and definitely like you suggested.

RE: post tension bottom chord reinforcement

If the roof purlins are placed on top of the truss then the trusses should be fairly easy to repair for the additional load. But the repair should be done by someone with experience in repairing wood trusses. Also adding addition trusses will be easy.

If the purlins are hung from each face of the truss, then it would be much more difficult to repair or add trusses, without removing the purlins. So other means may be better to use.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

RE: post tension bottom chord reinforcement

If you are adding 3,500 lbs to the bottom chord - my guess is that the top chord is also in trouble... I think as better solution is needed - as noted

RE: post tension bottom chord reinforcement

(OP)
Agreed to the above, and thank you!

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