Steel Joist web configuration

[penpe]

I'm designing cable tray bridges using two steel joists. Wind loads and the need for cable tray support between the joists require horizontal steel members between the joists. I'd like to space those so that they intersect the chords where the joist's web members intersect the chords. How can I determine that spacing?

 

[DaveAtkins]

This is difficult--check a manufacturer's website, but in my experience, there is a lot of variation.

DaveAtkins

 

[JedClampett]

You might want to just go with a typical detail where they add a diagonal (usually a 2 x 2 x 1/4) at the members. All the joist manufacturers have this on their drawings.

 

[JAE]

I would also suggest you check with a local joist manufacturer. Some years ago Vulcraft used to publish "P" and "Y" dimensions showing the first panel point space and subsequent interior panel spacings.
(see image below)

However, they quit doing that for the reason DaveAtkins stated...variation due to facbricator preference/design, spans, and loads.

You could ask them the feasibility of actually specifying a mandatory panel spacing and forcing them to design joists around that requirement.

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[MotorCity]

Don't forget to provide the joist supplier with the lateral/wind loads on the joists. They will need to account for that in the joist design.

 

[penpe]

Yes, DaveAtkins, I'm finding wide manufacturer variation: New Millenium said about 3 feet, Vulcraft said about 5 feet. Probably need to go the Beverly Hillbilly route and have joist supplier put additional web members where the horizontal pieces end up by my design, (Thanks, Jed). And, MotorCity, I'm confused by your post; because I'm putting truss-like bracing in to handle the lateral loads. Not sure why I'd provide lateral loading info to the joist supplier, except spacing info needed to have them add web members where my braces land.

 

[dik]

Have the joist supplier provide the bracing, just stipulate the loading or code requirement for loading.

Dik

 

[dhengr]

Penpe:
Maybe the better way to state (or start) your question would be to give much more info. on your basic cable tray. What does its end view cross section look like, a well proportioned sketch and dimensions and loads would help a lot? How far will it span btwn. cross framing members, under your various cable loading conditions, and what are those loads, in detail? You already know that your must be able to span 3' to 5' btwn. panel points for off-the-shelf stl. jsts. What are your desired spacings btwn. the two stl. jsts. for your various tray sizes and loads? Now you know your load per foot of cable tray and the stl. jst. span length, include some lateral loads, and you call some stl. jst. suppliers with this info. and ask them what depth they need for these point loads at every (or every other) of their std. panel points. What are those dimensions over the length of the jst. they are proposing and what is the out to out widths of the t&b chords on that jst.? Ask them if your point loads can actually be 6 or 8" either side of the actual panel point. Now you are buying their least expensive std. stl. jst., maybe even run with another job number at their shop, with only a few changes on the fit-up table. Stl. jst. orders of six or eight of an odd-ball size or length jst. aren’t even worth the set-up cost for most of those guys.

Now, my structural framing scheme for under the cable tray would be something like this: On the inside of each stl. jst. I would have a square, bent “J” shaped hanger, 6 or 8" wide, with the vert. leg hanging down, and a little longer than the jsts. are deep. Near the bot. of this vert. leg, punch two bolt holes, to miss the jst. chord members and match up with other bot. holes (see below).. The top horiz. would have a dimension to match the out to out widths of the t&b chords, and it would have a bent down outer leg to hook over the top chord. This hanger would have two punched holes in the horiz. leg to match the slot down the mid-width of the stl. jst. for fixing bolts, large square washers and nuts. Welded to these two primary hanger members would be a small WF cross framing member, which fit your cable tray and determined the spacing of the stl. jsts. For the jst. bot. chord make an identical bent hanger shape, but the vert. leg is only about 10" long and points up. It also has two punched slotted holes in its vert. leg, for some vert. ht. adjustment and tightening onto the jst. Under the small WF and inside the two side hanger vert. legs I would bend a light angle iron brace, which hit the WF at about its 3rd points and matched up with the bot. holes in the two hanger pieces on each side, like kickers from a beam to a column, but continuous under the WF. Now, you don’t have to do any field welding to put this together, maybe some field drilling to bolt the tray to the hangers and WF.

 

[XR250]

How much per foot does the cable tray weigh? Isn't the rule of thumb that if the load is less than 200 lbs, no reinforcing is req'd?

 

[CANPRO]

What is the span of the joists? Joist suppliers make their money on producing the same joist over and over again, if you ask them to make two special joists you bet you're going to pay a premium. It might be cheaper and provide you much more control if you design the truss yourself and get a local fabricator to build it for you - depending on the span.

 

[penpe]

The span is 52 feet. There will be a 36" cable tray on top, a 24" cable tray, and a 24" cable bus 2 feet lower. Total load of 254 lb/ft + 38 lb/ft snow. Lateral load due to wind is 54 lb/ft. I'm finding two 24KCS4 joists will do the trick, though one supplier told me that the rule of thumb is minimum 1" of joist depth for each 2' of span.

dhengr, sounds like you've done this before. Thanks for the design suggestions, but I will probably keep it similar to the design our company has used before.

 

[dik]

For that span, I'd typically be running a 30" or 36" deep truss... not 24". If I wasn't leaving the design to the truss supplier, I'd design it so a local fabricator could manufacture it.

Added: I usually use the depth in inches is equal to 0.7 x span in feet. For loading I usually use tray weight + 100 psf for cables + snow... Too often, I've encountered cable tray attracting added loading over a period of time. In one mining installation, there was nearly a foot of fine caked dust, almost like concrete.

Dik

 

[NS4U]

@JAE can you point me to the document for those excerpts you posted? It seems like it could come in handy.

@OP my experience is that web members intersect top chord at around 2’ oc. It is usually around twice that (4’ oc) at bottom chord. This is because Sji allows you to neglect top chord bending where the spacing Between panel points is less than 24 inches. But at the very end of the joists it is different but is less than 24”.

 

[JAE]

NS4U - This was a copy I got from another engineer at a local Vulcraft office many years ago. They quit publishing it since...well...per the discussion here about variability in webs and they apparently didn't want to offer data when the data was not always correct. So what you see is what you get unless someone else here has the source document from which it came.

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[KootK]

I did something similar recently and learned a couple of interesting things:

1) SJI now has an official stance on how much concentrated load can be considered negligible between panel points:Link. A paltry 100 lbs.

2) The latest SJI spec has dropped the business about not needing to consider transverse loads on panel points if those panel points are 24" o/c or tighter. Obviously, this need not apply to legacy stuff.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.