Joist questions

[WWTEng]

Bottom chord extensions: I always show b.c. extension when the joist is bearing on a column. Otherwise if the architect does not ask for the extension to support the ceiling, I don’t ask for it either. From what I understand, you need bottom chord extensions when the joist is part of a frame and resists axial loads. Any input?

Camber for long span joists: Say I have a 70’ long 40LH roof joist. Who decides how much camber should be there in the joists. Should I as engineer dictate LL deflection (say L/240 or L/360), indicate the loads and let the jst mnfr decide what the camber should be or should this info be on my design documents?

Thanks.

 

[cmoran]

WWTEng,

1. You are correct that joists as part of a frame (or resisting axial load) will need to have bottom chord extensions at the columns.

2. If you look in the Vulcraft Joist catalog they have tables for approximate cambers of joists. However, if you will have special situations where stricter deflection criteria will need to be used (brick, folding partitions etc....) It would be in your best interest to provide a joist diagram with the loads and design criteria you would like them to follow. Camber is their deal let them design for it.

Hope that helps!

 

[IsaacStructural]

1. Bottom chord extensions are also useful for laterally bracing the bottom chord of the support girder. This is very useful in drop in beam systems in which the girder line will have areas in negative bending.

Licensed Professional Engineer (Illinois)

 

[dik]

You can have bottom chord extensions in the form of a 1/2" dia round bar that extends to the wall line. This is used to secure ceiling T-bar sections from to allow the ceiling to extend to the wall.

Dik

 

[BAretired]

Bottom chord extensions are useful for ceiling support, but I don't think they should be considered as part of the structural system.


BA

 

[NS4U]

Bottom chord extensions are not typically attached to tye supporting column since joists need to be pinned at te ends. Whe they are attachef to the column they are assumed to buckle and therefore cary no load. My understanding is that purpose of chord extensions are for constructability only.