Pretensioned Awning System

[SteelPE]

I designed a building last year that is currently 95% complete. The building is used to design and manufacture products and has a 2 story office with a large single story manufacturing area. One architectural feature was to have the second floor exit onto a very large roof deck which was placed above the manufacturing area.

Now that the owner is almost complete, the owner would not like to add some pretensioned awnings to shade portions of the roof deck. The awning has an area of 700 square feet and covers approximately 1/3 of the roof deck. My contract has been amended to include the design two columns to support this canopy (support opposite of these columns will be provided by the building structure). Part of my new contract requires the loads from the canopy to be given to me by the manufacturer. I have talked to the manufacturer and all I get from them are to “design the columns for #2,500” and I have little confidence that these are the correct loads to use.

Has anyone designed a similar system before and if so, how would I go about calculating the loads in the system so I can properly design my support?

As a side note, I am considering giving a load rated system and then tell the manufacturer he cannot exceed a certain load.

 

[ARKeng]

Yes I've designed something similar before. You are wise to question.

Like you we got vague info from a "supplier" that said something like the "design for 2500#" you're getting. Later, when we got the 3rd party engineering submittal during construction, it was something like 2500# for the pre-tension alone. But the load went up to like 7500# under D+W load cases. And as I recall, we found something in their engineering that required revisions and then loads were even higher. Then, their QC on installation was terrible and we got field reports saying the pretension was higher than the assumed range.

I would not attempt to calculate the load yourself as you'll likely just waste time. The force in the cables is partly a function of how much sag deflection you want to allow. So even if you come up with good reasonable approximations for doing it, if you make a wrong assumption there that doesn't match their engineer's, you'll be way off.

Honestly, based on my experience, I would tell them you require their engineering submittal before proceeding with your design. Or, at the very least, require a stamped letter from their engineer stating a maximum load will not exceed XXXX#. If they refuse... refer to your contract.