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Remove columns and doubling or tripling girder spans

Remove columns and doubling or tripling girder spans

Remove columns and doubling or tripling girder spans

I am working on an industrial/office building that is to be modified to allow for basketball courts and indoor track. It involves restructuring the roof framing, including removing eleven existing steel columns. The idea currently is for a new W-girder to be framed right below the existing W-girders spanning two bays and in several cases even three bays. The existing roof structure consist of 30-ft x 47-ft bays, with the open web steel joists spanning the 47-ft span direction, and for the most part W27-girders the 30-ft direction. The roof is flat.

Has anyone had a project removing columns and installing girders or trusses below (or above) existing W-girders? The maximum new girder span would be 90-ft, and tributary roof width 47-ft.

Some concerns I have are:
- Ponding with that long of a span and a flat roof. Cambering could have issues as well, by shedding water to adjacent bays.
- The added settlement of the roof due to higher loads on soil compacting it to deeper soil depths - and again adding to the potential of ponding.
- I question whether I can rely on a braced girder condition by welding on braces.

One thing the Architect and I pondered on, is to remove the 24,000 SF existing roof structure in the area in question, and re-frame above the existing roof level and incorporating a pitch.

In the track area instead of W-girders there are existing open web girder trusses. An additional constraint there is no allowable depth to add structure below. I am figuring having to remove the girder trusses and replace with a new W-girder or girder truss.

If anyone has worked on a similar project I would appreciate what has worked for you, or what you would do differently next time.


Eric McDonald, PE
McDonald Structural Engineering, PLLC

RE: Remove columns and doubling or tripling girder spans

One idea is to use the existing beam as a bottom chord of an exterior truss or, if that is not feasible, add 9' or 10' deep trusses over the existing girders, then hang the existing structure from the new truss in the areas where columns are to be removed.

With the additional depth, it should be possible to incorporate new roof deck with adequate slope to drains.


RE: Remove columns and doubling or tripling girder spans

Thanks BA retired and JAE!
There are several new ideas and opportunities for me.

Eric McDonald, PE
McDonald Structural Engineering, PLLC

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