braced frame in freezer building 1

[tmr4791]

I am designing a braced frame in a freezer warehouse building (the building will be built at ambient temperatures, and then brought down to -10 degrees. It's an ordinary concentrically braced frame (R=3.25, design category D). The building is approximately 300 feet long. The braced frame has double angle tension-only members. I'm concerned about the thermal forces potentially buckling the braces when the building shrinks and likewise increasing the design tension forces on the tension braces(I notice that ASCE 7 does not combine thermal loads with earthquake forces??) I've never done this before, but would it be a good idea to use long slotted holes in the bolted braces, and go with slip critical bolts? The bolts could be hand tightened at first. After the freezer is brought down to -10 degrees, the bolts could then be fully tightened.

 

[msquared48]

Regardless of ASCE7, I would definitely combine thermal forces with seismic, unless you take the measure you describe to, literally, take them out of the equation. It makes perfect sense to me.

Furthermore, I believe that it is also in line with the intent of the code, which is more important than the governing euations alone.

Mike McCann
MMC Engineering

 

[JAE]

Another point is to place your braces near the mid-point of your overall building - as opposed to placing a brace at a far corner or two braces at each end where thermal forces would build up across the full length.

For a single, centered, X-brace bay -

Keep in mind that you have the following:
1. Shrinkage of the horizontal beam at the roof.
2. Shrinkage of the column lengths.
3. Shrinkage of the diagonal braces.

The only thing that theoretically may not move as much is the horizontal distance between column bases although you could argue that the column stem-wall or grade beam is shrinking a bit as well.

Across 40 feet for an 80 degree drop you get about 1/4" of horizontal shrinkage. For an 18' high roof the column shrinks about .11 inches. On the diagonal the brace shrinks .27" - exactly the amount of movement that the corner point moves diagonally.

So if everything is shrinking, do you really get slacking in your diagonal?

 

[Compositepro]

Thermal stresses are not caused simply by the temperature change. It is caused by different materials having different thermal expansion coefficients and by different parts being at different temperatures. If an all steel frame changes temperature It will expand and contract uniformly without stress. But what about the outside sheathing or the concrete floor?

 

[paddingtongreen]

It gets colder than that in the wheat belt of North America, but I don't think they do anything special.

Timing has a lot to do with the outcome of a rain dance.