Steel Joists-Is mid-span bearing permitted?
Steel Joists-Is mid-span bearing permitted?
(OP)
I am reviewing plans from an architect to determine what structural engineering services are required. Additional uniformly distributed load will be hung from steel bar joists. The architect is proposing installing additional steel beams at the mid-span of the bar joists to account for the increased loading. I have little experience working with bar joists, but I suspect that since this would change the stress ditribution in the joists it would not be permitted unless the joist was analyzed for the new condition. The joists are old and there is not time or budget to allow for this. Any comments on this practice. The additional load will be about 6 to 10 psf. I do not know if there is additional capacity available in these joists.






RE: Steel Joists-Is mid-span bearing permitted?
RE: Steel Joists-Is mid-span bearing permitted?
Without the center support, the moment in the middle is +WL/8; with an unyielding support, the moment would be -WL/32 using the original W and L. The new, unyielding beam would carry 5/8ths of the load, the end reactions would reduce from 1/2 to 3/16ths of the total load.
But then, the beam is not unyielding, it will deflect so you will have to find the properties of the joists for an analysis program.
The one possible "out" that I see is that if you could support the floor temporarily, you could remove a section of the bottom chords and then push the new beam up to the top chord, in effect, converting them into two shorter joists. If you are lucky, the diagonals will handle the change. If you could push the beam through from one side, that might obviate the need to prop the floor.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Steel Joists-Is mid-span bearing permitted?
My experience has been that most joists are already pretty much "maxed" out in the original design leaving very little "fat". That is why you see such weird spacing - such as 5'- 5 3/4'' which does not divide equally into the building length - but does "max" out the joist capacity.
Good Luck
RE: Steel Joists-Is mid-span bearing permitted?
Second, the web members will be seeing load at midspan that they were never designed for and I can say for sure that the actual shear will fall outside the acceptable limits for deviation from a simple shear diagram. Even if you can get the beam to the top chord and cut the joists to have two simple spans shear will still be a problem for the same reason-there will be shear where the webs were not designed for it.
RE: Steel Joists-Is mid-span bearing permitted?
There are couple of write-ups available that discuss the strengthening of bar joists. Unfortunately, I do not have the information to list here, but I do remember finding them through a search of the internet. A typical strengthening case would be welding a rebar into the inside corners of the top-chord angles.
With the case of the "old" joists, there is a manual available from the Steel Joist Institue that lists joists from the past 75-80 years. This would help you establish a base capacity for the joists.
Hope this helps...
RE: Steel Joists-Is mid-span bearing permitted?
It also has been done changing single span joists into a 2 span continuous member with all the appropriate checks for strength, stability and connections.
Smaller joists are much harder to reinforce since the members are smaller.
It can get involved!
RE: Steel Joists-Is mid-span bearing permitted?
These joists are manufactured with no "fat" like Mike says. This would mean that the webs are very skimpy at the mid span and probably will not be okay for shear if the single span is broken up into 2 spans.
Maybe worth contacting the joist manufacturers to see if they have a solution.
RE: Steel Joists-Is mid-span bearing permitted?
RE: Steel Joists-Is mid-span bearing permitted?
No buddy ever thanks you for saving them a few dollars in design when the floor fails and legal issues begin.