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Cable Bracing in Pre-Engineered Metal Buildings

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waytsh

Structural
Jun 10, 2004
373
For all of you pre-engineered metal building engineers out there (and others who care to weigh in) I would be interested in hearing your reasons for, or against, using cable bracing in lieu of rods or angles. I know there are many manufacturers who will not use them and even more that will. I am also interested in knowing whether or not those who do use cables are using a prestretched cable. I have been told by a cable supplier that all pre-engineered metal building manufacturers who use cable are using a non-prestretched cable. Is this accurate? Also, what limitations would you impose on the use of cables, i.e. crane buildings, seismically controlled structures, etc.? I would appreciate your perspective.

Thanks,

waytsh
 
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"your reasons for, or against, using cable bracing in lieu of rods or angles."
For: They're cheap and have worked OK since ancient times (well, slight exaggeration).
Against: No clue other than situations for which the load is too high.

The cables are pulled tight, but I don't believe anybody actually measures the force either directly or indirectly. The analyses assume only the tension cables are effective, so there's no reason to try and figure out when some go slack.

There might be reasons to avoid them, but I'm not aware of any. Not claiming to be an expert, though, even though I've worked with MBs for a long time.

I'll say one thing, though: Beware being afraid of something just because you wouldn't use it in a conventional bldg. For example, I recently measured the web members of a 30K7 joist and some were something like 1" angles around 0.1" thick. A conventional bldg engineer wouldn't think of using that in a truss!
 
Thanks for weighing in. I certainly agree that cables could be more economical particularly if you consider the erection time that can be saved. I too have worked with MB's for a long time and have designed more than I can count. Until recently I have been using exclusively rods or angles for my X-bracing but times are getting tougher and clients are pushing to save money where they can. Because of this I am being forced to take a hard look at incorporating cables into more of my designs.

I do not have a problem using the cables for many applications where they are able to meet the deflection requirements of the building. The biggest problem I have is if they are not pre-stretched as described in ASCE 19-96 “Structural Applications of Steel Cables for Buildings”. I believe they need to have this initial elongation taken out of them to “set” the final length of the cable and establish the minimum modulus of elasticity. Without doing this it becomes impossible to accurately calculate the longitudinal deflections of the building which is even more critical now with the 13th Edition of AISC.

I know you’re thinking, “Well then just specify a pre-stretched cable.” Unfortunately it appears from the suppliers that I have contacted so far that pre-stretching cables increases the cost to near that of rods thus eliminating one of the biggest benefits.

Do you know if the cables in the buildings you have looked at were pre-stretched?
 
I doubt prestretching cables in preengineered buildings is something which you can really have confidence in, and I would have the same concerns. I want to rely on a given P giving a given PL/AE.
 
We used 50 x 1.6mm flats that were tensioned.
 
Our company does not use cables due to both the stretching under load and the lack of return to original length after load is removed. Not sure pre-stretched would necessarily solve all of that. As noted above, with the new second-order requirements in AISC proving cables work becomes even harder.
That said, for a smallish (say 40'x100' building) cables probably work fine. Unfortunately, people want to stretch (pun intended) the limits and apply the same philosophy to 300' wide buildings where second-order becomes severe.
 
Doing a little more internet research I cam across a reference stating that the Florida Building Code does not allow the use of cables as tension members in pre-engineered metal buildings. I do not have a copy of the code does anyone know if this is accurate? Are there any other codes out there that might preclude the use of cables in PEMB's?
 
The Florida Code, actually the "Dade/Broward" high velocity hurricane zone mandates angle bracing for "habitable" structures, permits rods (and I guess cables) for other applications.
In answer to "asixth" we regularly do modular buildings over 300' wide. One of our engineers came to me the other day to discuss expansion joints in a proposed 650' x 2000' building. While certainly not a large percentage of our mix we have done quite a number of million square foot structures.
 
Are we talking non-pre-stretched, pre-stretched, or both not being a good idea? I would think that cables that have been stretched to obtain a reliable MOE would be ok for very limited applications,such as small buildings with screw down panels and no cranes, as long as the pre-stretching force and the loads fall within the limits specified by ASCE 19.

Somehow these manufacturer's are justifying the use of even non-pre-stressed cables and I do not understand how. I have sent a question into MBMA to see if they have a position but seeing as how there is such a divide on the topic I doubt they will comment.

I'll let you know what I hear.
 
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