Roof Bracing with Large Building Length to Width Ratio

[JAX91]

I am currently designing a building addition located in Arkansas. The building footprint is approximately 440'x128', giving it a length to width ratio of 3.44. In AISC Steel Design Guide 7, James Fisher recommends a maximum length to width ratio of 3.00 for both diaphragm and horizontal bracing design. He says if the ratio is larger than 3.00, an alternate bracing method is recommended. Unfortunately, there is no mention of any alternate bracing methods. I considered placing a braced frame down the center of the building to give the diaphragm a two-span condition, but I would imagine the owner would not be happy with that idea. Does anyone have any other recommendations?

On a side note, I found an article in Structure Magazine's Engineer's Notebook which was written by James Fisher in May of 2004. This article gives a maximum length to width ratio of 4.00 for diaphragm and horizontal bracing design.

 

[MiketheEngineer]

Both are rules of thumb - usually based on experience.

Find the loads and forces and then design accordingly...

 

[paddingtongreen]

The controlling "problem" is deflection in the middle of the building. You can, probably, handle that with increased bracing depth in the middle.

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

 

[JAE]

JAX91 - with very long diaphragms you start to get larger second order effects, where the deflections of both the diaphragm and the vertical bracing system combine to create additional lateral load on the frame due to the leaning.

With a "bending" diaphragm, many of the center columns will have greater lean, and thus greater PDelta forces that will add to your lateral resisting system design. Keep those effects included in your calculations.

 

[hokie66]

For this shape building, I would probably use portal frames in the 128' direction, and brace it in the other direction.

 

[JAX91]

Thank you all for your inputs. I was able to talk the architect into placing a double row of columns near the middle of the building. The double row will be spaced based on the allowable seismic gap for the drift of the structures, and it will essentially break the diaphragm into two separate buildings. The building was also borderline too long to not have an expansion joint, so the double row of columns can be used to accommodate building expansion as well.

 

[ash060]

I agree with hokie66

I would have some big frames in the 128' directions. Break up the diaphragm until it is what you want it to be. Metal building guys do that kind of stuff all the time

 

[hokie66]

With the double row of columns, you still have to provide bracing in the vertical plane to pick up the diaphragm forces.