steel joists 3

[bjb]

What is the benefit in extending the bottom chord of a joist and welding it to either the column or a steel beam if the beams run over the top of the columns? This is for a shear wall building, so the joists are not participating in any moment frames. The design of the columns is based on the height from the top of foundation to the joist seat. If you are extending the bottom chord and welding it to the steel, shouldn't you be telling the joist manufacturer to design the bottom chord for fixed end moments?

 

[JAE]

Here's all the reasons I'm aware of:

1. Older engineers used to do this to "add" lateral stiffness to the framing in a nebulus sort of way - just a judgement thing. - Usually specifying that the connection must be made after all roof dead loads are in place.

2. The extension to the bottom of the beam flange would add lateral stability to the beam and prevent lateral buckling at the beam/top plate connection where the beam would rotate off the column.

3. The extension of the bottom chord provides full connections for hung ceiling grids.

All of the above do have the same effect that you mentioned - the addition of fixity to the joist end and should be noted for the joist manufacturer.

 

[bjb]

Thanks for the reply JAE. I agree that extending the bottom chord and welding it adds lateral stability when you are cantilevering a steel beam over the top. For the types of projects that I usually work on, I usually don't cantilever the beams over the column by more than 12", so my negative moments are very small. I just cantilever that much so that I'm not butting 2 beams on a column. I also always put stiffeners in the beam web, so that I'm not just relying on the stiffness of the web for lateral stability. Regarding hung ceiling grids, you can specify ceiling extensions and not weld them to the columns. For these reasons, I don't like specifying that a bottom chord extension be welded to a column unless I really do need it to brace something. I think that doing it just "to do it" adds unnecessary complexity. I think that in many cases the joist manufacturer is not being told to consider fixed end moments when the bottom chords are extended and welded.

 

[KarlT]

I believe that the extension of the bottom joist chord to the column (tie joists) is typically done with a slotted connection in Canada.

From the Cdn Steel code commentary....

The function of tie-joists is mainly to assist in erection & plumbing of the steel frame. Either the top or bottom chord is connected by bolting and after plumbing the columns, the other chord is usually welded. If the designer wants to use the joist to brace the columns, or resist lateral forces on the finished structure, the appropriate moments and forces are to be shown on the building design drawings to enable the joist manufacturer to design the joists and joist-to-column connections properly. In cantilever suspended span roof framing, joists may also be used to provide stability for girders passing over columns.

 

[LPPE]

Isnt it also an OSHA requirement for erection stability?

 

[bjb]

I know that at columns the joist has to have a bolted connection for erection stability, but I'm not aware that the bottom chord also has to be positively attached to the column.

 

[MBones]

A bottom chord extension attached after dead loads have been applied provides support for the bottom flange of beam to resist Wind/Earthquake loads.

 

[bjb]

Another point to consider is that since attaching the bottom chord creates a fixed end moment in the joist, this moment will also act on the column in an amount proportional to its stiffness.

I agree that extending the bottom chord to a beam running over the top of a column will brace it. In my experience, I have not seen many engineers give consideration to the fact that attaching the bottom chord creates an end moment in the joist and the supporting structure, even for vertical loads.

 

[bjb]

I just received this email from an engineer at a joist manufacturer:


Joist bottom chord extensions should not be welded at the column
unless they are part of a moment frame system. SJI joist are designed as
simple span members, the fixity that would be produced is not
investigated unless the design requires it, then the moments would have
to be given to the joist manufacturer for his analysis.
OSHA requires that joist at columns must have their bottom chords
extended to a column stablizer plate for safety concerns during
erection. They are not to be welded.

Carl Pugh, P.E.
VP Engineering

 

[denoid]

In our office, we don't extend the joist bottom chord at the column, due to the arguments above about inducing unwanted axial loads into the joist chords. However, we usually do add a small angle brace (L2x2x3/16) from the two or three joists nearest the column to the bottom flange of the beam to prevent lateral out of plane rotation of the top of column/bottom of beam. This is especially done when the roof beams are continuous over the columns, and is done even when beam web stiffeners are in place. Maybe it's a little bit "belt and suspenders", but it's pretty cheap, too.

 

[JoistsioJ]

Can anyone kindly recommend a reference book on steel joist design?

Thanks a lot

 

[Lutfi]

Dr. James Fisher done an outstaying job along with Vulcraft in the book titled: “Designing with Vulcraft steel joist, joist girders and steel deck". I recommend that all structural engineers get a copy of it by contacting Vulcraft. I think it is around $14.99, a bargain in my opinion. The link to their page is

http://www.vulcraft.com/

and click on the order link.

While there request or download the joist manual. Also you may contact SJI (steel Joist Institute) for their publications and design guides.

I use a simple but handy program that I bought few years back from Spencer Engineering. What I like about it is the vibrations analysis. Here is the web site

http://www.spencer-engineering.com/software.html

Good luck

 

[JoistsioJ]

Very helpful.Thanks.

 

[SimpleStructures]

If you do not have net wind uplift on the joist and have accounted for the proper column "brace" point as I think you have, bottom chord joist extensions are not required (not sure about the OHSA requirement given above, I'll have to look into that).