Bracing Columns using Joists

[nhrohr]

When framing a big box building with joist girders and joists, I have been told to place a joist at the column in order to brace the column. However, with the joist girder framing into the column it seems that the joist girder would brace the column and I would not need to place the joist exactly at the column.

The reason that I am interested in other professional's opinions is that if my maximum spacing is 6'-0" oc, I could eliminate joist if I don't need to place one at every column, instead just space at 6'-0" oc across the building to maximize spacing.

Let us assume our shears are taken out using concrete shear walls.

 

[BAretired]

You would not eliminate a joist but you might change the spacing slightly. Normally, the column would be braced in one direction by the girder joist and in the other direction by a tie joist at the column. Joist spacing would be determined by the distance between columns but would not exceed the maximum spacing determined by the deck.

Joist spacing should align with panel points of the girder joist.

BA

 

[msquared48]

Yup...

Mike McCann
MMC Engineering

 

[SteelPE]

I always do the best I can to get the column "tied in 3 directions". Usually when using joist girders, this means I will have a tie joist at the column. As mentioned above, you will need to place the joists at the girder panel points.... so the joists will need to be aligned across the building. Having reasonable equal spacing helps with costs (I imagine) however, you may increase costs if you can not "standardize" your girders. This may happen if you hold a joist spacing of 6'-0" o.c. across the building when your girder length is 40'. Also, you may increase your erection costs as joists now need to be erected in tandem in order to provide proper support for the column in accordance with OSHA.

In the end, it probably doesn't matter which way you do it as the costs increases in erection and detailing will be offset the material savings.

 

[nhrohr]

Thank you for your input!

 

[TDIengineer]

If your girder has a beefy enough bottom flange, it may be able to brace the column in the opposite direction from a joist that is slightly offset assuming the bottom flange can span and carry the bracing load. Sometimes I have left my purlins offset from the column line by a foot or so in order to keep the spacing consistent or because of some other constraint.

The appendix in AISC 360-05 has the nodal brace loads.