Joist Bearing on Steel Channel

[T_Bat]

I've got a situation with a enclosed second floor area that occurs in the field of a roof. The roof is occupied space with a pedestal paver system. To accommodate the finish "floor" elevations I have some conditions with hi/low framing. The change in elevation is exactly 10". In a few areas the transitions in elevation occurs away from a column so I'm providing a "stacked" beam. I've gone back and forth between making the stacked upped beam a W10x15 vs a C10x15.3. I guess my concern is the bearing length I can get with the channel. Vulcraft recommends a minimum of 2-1/2" which I will get - but barely. Has anyone else come across this or feel strongly one way or another?

I've uploaded a quick detail. The channel will be stitch welded down to the supporting beam and I'm providing a stiffener at each joist.

 

[SlideRuleEra]

Why not just use the W10x15 you are already considering and avoid the short bearing length (and perhaps stiffeners) of the C10x15.3?

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[RHTPE]

I agree with SRE - stack the W10s.

I would also recommend that the 2x2x1/4 angle not get welded until the slab placement is complete to avoid some unusual loading of the joist webs, as well as kicking the lower W10 towards the exterior.


Ralph
Structures Consulting
Northeast USA

 

[KootK]

Another option I've used in the past. Probably just me paranoia but your channel to wide flange connection just feels a little "hingey" to me. Your concept may well be more cost effective, I'm not sure. Welding the joists down might be better too depending on the situation.

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.

 

[T_Bat]

Thanks for the help everyone. You're right - I was thinking channels would be cheapest but once I started drawing it just didn't feel right. I'm going with the W10's. I appreciate your comments.

RHTP - I'm not quite sure I understand what you mean by "kicking the lower W10 towards the exterior". Could you clarify?

 

[KootK]

I suspect that RHTPE is referring to minimizing the pesky axial compression that will develop in the brace as a result of gravity loads.

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.

 

[T_Bat]

Oh - I misread that. I thought the suggestion was to add another kicker. I see now you meant that the slab dead load will tend to "kick" out the beam via axial load in the kicker - which makes a lot more sense. Thanks for the insight everyone.

 

[RHTPE]

KootK is correct.

Way back in my early years (when I detailed OW steel joists), this kind of connection, as well as bottom chord connections to columns, was a real headache for our chief engineer. When these kinds of connections are fixed before all dead loads are applied (even some live loads), the effect on the joist web members often created stress reversals. Not good if the members were not properly sized. Of course, not fixing the connection created other problems when the EoR was relying them to provide lateral stability during joist erection.



Ralph
Structures Consulting
Northeast USA

 

[KootK]

That's actually one of the first details that caused me grief when I got started. To this day, it still bothers me how it essentially creates a fixed connection between the joist and whatever the supports it. Folks seem to lie to deceive themselves into thinking that a shallow angle brace somehow improves matters. Sometimes I try to use the beam bottom flange as a wind girt instead but that works poorly and is a hassle from a calculation perspective.

@Ralph: can I ask what the preferred solution is from the joist supplier perspective?

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.

 

[RHTPE]

KootK - It has been many years since I have had dealings with the manufacturers of OW joists. As things have evolved since, I may not be current. It really depends on the underlying purpose, both from the EoR's perspective AND from the steel erector's perspective.

The steel erectors look for a quick, practical and effective way to introduce as much stability in the partially erected structure. One solution was a tab welded to the column that fit between the joist bottom chord members. Obviously bolting to the tab was preferred (horizontal slot in the tab), but accuracy in joist fabrication is nowhere near the same tolerances as structural steel. Just the presence of the tab helped with side-to-side stability, but until clamped or connected, it was not the ideal simulation of a moment frame.

The EoR's were looking for a moment frame for wind (lateral) loading. Most were very understanding of need to wait until all dead loads were applied. Some tried to dream up temporary bolted 'hand-tight, weld after DL application' connections, but had minimal acceptance of the fact that OW joist tolerances were not on par with the kind of connection they dreamed up. OW steel joists are somewhat of a commodity product, produced rapidly, and not with the kind of attention to detail one can get from structural steel.

This phase of my career was well over 40 years ago so perhaps the situation has improved some.



Ralph
Structures Consulting
Northeast USA