Expansion Joint for a Tall Warehouse

[QC-engr]

I'm needing help deciphering the actual need for an expansion joint in my current project...

Project Description:
Footprint of 775' x 335', and 75' tall warehouse with no floors/mezzanines.
Construction is rigid insulation over metal decking over joist and girders.
The cladding is simply metal siding and CFS girts.
I'm employing ordinary concentric braced frames - all bundled at the mid-lengths of each wall. All four exterior walls are braced.
Wind definitely governs.

Before anyone asks, YES, I've read report #65.
My project is set in mid-Pennsylvania. According to the methodology of that report, the temperature delta (approx 44 degree F) would produce an overall expansion of 2.65" in the 775' distance. The report figure recommends I have an expansion joint at 517'. If I include the heating/AC (which I feel is allowed in my case) 15% addition, I can get up to 595'. This is still short of my 775' building length.

My questions/thoughts/ramblings:

1. All of my steel is UNDER insulation and not exposed to the outdoor shifts. Would my temperature delta actually be the temperature swing of the the thermostat window - more like 15 degree F. This would lower my overall expansion/contaction from 2.65" to 0.88".

2. What IS the overall purpose of expansion joints? From what I read its to either/or (a)inhibit too much deflection/drift/sway as to not interfere/harm with architectural/brittle materials and (b)to "release" internal forces caused by these temperature changes.

(a) I don't have brittle finishes. If recognizing the 2.65" expansion, that would mean my two outer walls (opposite of the 775' direction) would each drift out 1.33". That equates to a very tight H/631!​

(b) The ends of the 775' direction are not bound by any other confining structure(s). The expansion of the steel would not be bound up against other items. There is no stress since the expansion is not restricted - only the roof level expanding outward.​

Whats the harm of letting it expand freely - with no expansion joints?

 

[bob33]

There should not be a problem with the main structure in this case, but I would check the girts. The girts want to expand the full amount but the lower ones are restrained by the columns. I would also check with the wall cladding contractors to see if they are okay with these kinds of movements.

 

[QC-engr]

Had some time to think about it and share with co-workers...

One possible reason for expansion is during construction phase:
During this time, the decking is still exposed to possible direct sun. At this time it can absorb the heat more readily and expand easily. The exterior perimeter beams are also exposed but (this is MY opinion) due to their mass and not being as thin as decking, won't absorb the heat as quickly, thus won't expand as quickly as the roof decking. Ergo, the roof decking will expand faster than all of the perimeter (bounding) beams and thus create a confining effect the expanding decking will want push against.

Thoughts?

 

[Compositepro]

The main issue is that the ground/foundation does not change temperature or move much. Railroad tracks are an example of long steel beams on the ground. Extreme temperature changes will cause tracks to move.

Here is an example:

https://www.youtube.com/watch?v=jJeiYaSFCf0

If you do use an expansion joint it will work best to minimize stress when placed in the center of the span.