Steel joist loading

[penpe]

We have designed a cable tray bridge that spans about 56 feet. Based on design loads joists required are (2) 30K11 - about 7 feet apart. For support of cable trays C6's span between the joists - at about 4 foot spacing. Our drawings direct the fabricator to place the C6's at the joist panel points, but they didn't do that. The C6's are spaced at about 3'-10" while panel points are 4'-0" apart. My first concern is bending stresses in the bottom chord where a C6 ends up halfway between panel points. The actual section modulus is unknown but some sources suggest it would be (2)-L2x2x3/16. (We're working on getting hard data about section properties). Double angle properties listed in AISC9 provide S and I. Using cable tray design loads the bottom chord is not over-stressed by a long shot, and deflections are less than L/500.

What else should I check? I know that concentrated loads should be placed at panel points, but with (14) C6's spanning between the two joists it would seem that the load is fairly well distributed.

We'd probably have the fabricator remove/replace the C6's to coincide with panel points, but would result in unacceptable delay. But if delaying prevents structural failure it's worthwhile, of course!

 

[structSU10]

Did you combine the bottom chord tension with the bending loads? Did you check that the bottom chord welds can transfer the force from the bottom chord to the top? Typically joists are designed for the load applied at the top chord, and welds for the web loads - it may not be a problem, but something to considered.

One additional solution is to add web members from the C6 connection point at the bottom chord up to the next panel point at the top chord, designed to carry the force in tension.

 

[Triangled]

I got lost in your description and cannot envision cable trays in relation to the framing plan and specifically the supporting joists, but I am imagining/guessing the trays are perpendicular to the joists - a thing I've done on several occasions. (But C6 is pretty stout, and I am not quite sure what you mean by "spanning" 56' ....)

assuming globally the loads have been addressed correctly, and that your only concern is local stresses in elements of the joist --
I think you are correct that bending in bottom chord is your only concern.

It may be a simple thing to add some minor bending reinforcements to the bottom chords and thus prevent the delay you mention.

 

[JoshPlumSE]

If I remember correctly, this situation (point loads between panel points of bottom chord) is covered in Fisher, West and Van De Pas', Designing with Vulcraft.

My belief is you merely check the bottom chord (I'd use AISC criteria) for bending + axial. If it fails then you've got to add a vertical or diagonal at that location.

 

[MotorCity]

We do not specify that joists be loaded at panel points for the same reason as you have found out......you have no control over the panel point locations.

If we have concentrated loads, we provide a cartoon diagram that shows an additional web member be added at the load and taken back to a panel point.

The other option would be to designate it as a "Special Joist" and let the manufacturer design it.

Here is what I would do given your situation. Look up the allowable uniform load on the joist and compute the moment between panel points.
Decouple the moment to determine the allowable axial load in the chords.

Next, place your C6 reaction between panel points follow the same process to determine the axial loads in the chords.

Compare the results to see if any further action is needed.

Or if you wanted to play hardball, put it back on the contractor to verify the joists are ok per the manufacturer since they did not follow your drawings or call you for clarification.

 

[KootK]

Recent editions of SJI require modern joists to be designed for 100 lb point loads anywhere withing any panel without the need for reinforcement. With the cable tray parallel to your joists and running midway between them, that should give you an available capacity of about 50 lbs per foot of cable tray if the joists are new or new-ish.

 

[JLNJ]

Do these C6s also provide lateral stability to the individual joists and/or the whole structure?

Is this structure exposed to wind?

 

[gte447f]

I would require that the contractor add diagonals to transfer the point loads to top chord panel points. If it's new construction, then I would give them the 100 lbs leeway that KootK mentions above.

 

[ukbridge]

I can't comment without some kind of drawing/sketch.

 

[SteelPE]

I don't understand why you are even entertaining the idea of helping out the fabricator/contractor. I guarantee you that on top of your requirement of the contractor placing the supports for the trays at the panel points, there is a note on the joist shop drawing indicating the maximum load that can be placed in-between the panel points without reinforcing. Seems like Mr fabricator wasn't paying attention. I would refer to that note and the note on your drawings and tell the fabricator to install additional web members to be in compliance with the design.

This isn't the first time I have run into similar issues, and unfortunately it will not be the last.

As a side note, for future reference, you could calculate the maximum load applied to the chord by the cable trays and have the joist manufacturer design the bottom chord of the joist for the maximum point load applied anywhere along the bottom chord. YMMV with that approach.

 

[penpe]

Thanks to everyone that weighed in. We ended up insisting that the fabricator put the members spanning between the joists at the panel points, as was originally specified on our drawings. The engineer that approved the shop drawings may have to take some heat for the delay, but a structural failure due to this oversight would have been much worse.