Choosing which bays to be braced 3

[GalileoG]

Hello all,

How does one usually know what is the optimum location of the braces? What considerations can one take into account when deciding on which bay is to be braced. I suppose trial and error on an analysis model would do but I was curious if there are rules of thumbs out there about this (one being for structures supporting cranes, bracings are best to be placed at the end bays.)

Also, if at one wall I add a second braced bay, does that help at all with the drift or does it simply allow me to use lighter members (since we have braced two bays instead of one)

Thanks!

 

[pba]

Clansman,
I'm guessing this is a crane operating in a single bay steel portal frame building. For a 150 ton crane (overhead travelling type), I'd go with Hokie66 and use separate columns to support the gantry beams and brace between them. You don't say how long the building is but conservatively provide movement joints every 40m (135ft ?) These should go between pairs of columns and through the gantry beam and rail. The crane braces are designed for the stop force and any thermal forces. The gantry carries a compression force...

For a building housing a large crane, there is likely to be a big door. If is often convenient to locate a bracing bay adjacent to large doors as it helps in stabilising the area. It then makes sense to position bracing at the other end as well. It is possible that this reduces the sizes of logitudinal members as the gable end wind forces go directly to the beaces. My experience would indicate that this is rarely of significance.

There really isn't a single right answer for the question. Whatever solution you adopt pay attention to load paths and make sure all your design loads can be taken down to the foundations.

 

[hokie66]

Reminds me of my ol' dawg. Wanders in, innocently drops a big one, then disappears.

 

[civeng80]

hokie66

I know 150 ton crane is quite a large one but why would you have separate columns for this ? Or do you mean a stepped column? I hope we are on the same wavelength here and looking at portal frames spaced evenly with a gantry.

Also what's a dawg (LOL) ?

 

[hokie66]

civeng80,

We are on the same wavelength. And stepped columns can certainly be used. I prefer using a separate UC column, rotated 90 degrees to the building column and braced to the building column with flexible or vertically sliding braces so that the shortening of the crane column does not affect the building column. This arrangement avoids the need for some of the fabrication involved in the stepped column, and sometimes the end clearance on the crane requires a big step.

The object of my barb knows what a dawg is, but as he is a hit and run contributor, may not read about it.

 

[civeng80]

Thanks hokie66!

Always a pleasure reading your comments.

Cheers!