Open Web Steel Joist: Extension of Bottom Chord

[sunyaer]

I've never dug deep into the details of open web steel joist design. One thing I've been thinking is the extension of the bottom chord. I found the attached paper on the internet, which discusses in length about the bottom chord extension.

My question is, when the bottom chord extends to a support, especially a masonry wall, is there a need to calculate the force that the bottom chord would exert on the wall laterally in out of plane of the wall?

If yes, I would check the wall for out of plane stiffness and strength. What would engineers usually consider with regard to extension of bottom chord of open wen steel joist?
Thanks.

 

[BAretired]

Typically, in building construction, open web steel joists are designed as simple spans. Joists occurring on column lines may be designated as "tie joists" and have their bottom chord connected to the column, usually with a slotted hole. This provides some stability during erection but the joists are not designed as continuous members. Tie joists differ from typical joists only by having the bottom chords extended.

It has not been my usual practice to extend bottom chords to masonry walls but if that was done, the wall would need to be designed for the moment resulting from the lateral force from the chord extension.

The paper cited in the OP seems to be addressing continuous bridges where all joists have bottom chord extensions. I may have missed it, but I don't believe the paper is addressing typical building construction where only selected joists are tie joists.

BA

 

[sunyaer]

I found a detail called on a drawing for bottom chord extension at each end of all open web joists in a gym area. By doing this, are the OWSJ'S being designed as continuous, and is the intent of the design to reduce deflection or for other purpose?

 

[BAretired]

You would have to ask the designer why he used that detail. I'm pretty sure the joists are not being designed as continuous. I'm not fond of that detail because joist end rotation under snow load would tend to open up a horizontal crack on the outside face of the wall.

If the joist deflects L/360 under snow load which is permitted by code, the end rotation is approximately 4L/360L = 1/90 radians. The width of crack in a 290mm wall would be more than 3mm unless the wall is suitably reinforced.

BA

 

[sunyaer]

BAretired, thank you very much for your input.

I agree with your comments.

The paper I attached in the OP is more about the contribution of bottom chord extension to floor vibration, which is not for typical building construction. The bottom line is that when the bottom chord extends to support, the support needs to be designed for the forces induced.

I have an impression that people tend to think that tying things more rigidly would make the structure stronger, while forgetting the assumption that makes the calculation valid.

I didn't quite understand that detail, and I wouldn't use it even the wall could be reinforced to resist the force.

 

[BAretired]

Neither would I, sunyaer.

BA

 

[RickyTickyTavi]

I actually had to do repairs on a 1980's era tilt-wall building. The original engineers got by the minimum thickness (5.5 in) by extending the bottom chord of all the joists, thus lowering the unbraced length of the wall. We were debating if they even bothered to check the axial loads on the joists.

 

[sunyaer]

I think it needs to be checked. I would never use the bottom chord of joist to support a wall. It would be done by using diagonals to transfer the bracing load to diaphragm.

 

[BAretired]

Diagonals would create the same problem on the wall when the roof was loaded with snow. The wall should be able to span from floor to roof without diagonals or bottom chord extensions.

BA