Steel Joists-Is mid-span bearing permitted?

[mjr6550]

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.

 

[SLTA]

Yes, it would change the truss stress distribution. The trusses will have to be analyzed for the new support and/or the new load. How can there be no time for that if you're just now determining structural services required? Seems like a red flag to me. Good luck!

 

[paddingtongreen]

Can you take a video of the system crashing down?

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.

 

[MiketheEngineer]

You can try steeljoist.org. They have a simple to use inquiry sheet and will try to help you identify an old joist.

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

 

[Lion06]

I can almost gaurantee that it won't work - for two reasons. First, you'll be putting the bottom (tension) chord into compression over the new support. The bottom chord was never designed for that plus the unbraced length will likely be much higher than for the top chord so buckling of the bottom chord is a potential problem, but so it LTB of the section (depending on the ubraced length).
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.

 

[rwpe]

I would recommend against adding the mid-span support.

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...

 

[jike]

It is possible to do if you cut the bottom chord, make 2 simple span joists, check the bending forces in the chords, check the stress reversal in the webs, check the welded connections, etc.

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!

 

[slickdeals]

jike,
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.

 

[jike]

The joists MUST be re-analyzed and reinforced for the NEW condition. Look particularly for stress reversals in the web, connections and bending stresses as stated previously.

 

[ivorm]

Is the floor area very big? You could add beams between the existing joists to essentially cut their tributary width in half. You get into some trouble with joist bridging and bracing the top flange of the new beam but that's what I usually try to do. I never mess around with joists, their always maxed out and if they're old they might not even be up to today's code.

No buddy ever thanks you for saving them a few dollars in design when the floor fails and legal issues begin.