Lateral Support - Steel Beam

[PT999]

I am looking at a one story building with roof rafters supported on cmu's (2 ft high) , and just below that, a steel beam. (RR's are perpendicular to cmu's)

Do I have lateral support for the steel? Do the cmu's really provide the lateral support when the real lateral supporting members are not directly connected to the steel

 

[jechols]

Are ceiling joist present? Is there thrust at the top of your cmu wall from the rafters? If thrust is present I would be concerned about torsion. If the beam is relatively short the wall would probably brace it, but then you probably wouldn't be concerned about unbraced length. Maybe you could provide a sketch and upload it to the forum.

J

 

[Lion06]

What is supporting the steel beam? Is there CMU on four sides? Are the roof rafters bearing on the steel beam? Is there any positive connection between the roof rafters and the steel beam?

 

[DaveAtkins]

I would not count on the rafters bracing the beam (they are 2 feet above it).

DaveAtkins

 

[csd72]

I agree with dave, I would not count on any bracing for that beam, in fact, it is top flange loaded which id the worst possible case.

csd

 

[msquared48]

Now, if you had wood or steel kickers going from the steel beam to the rafters, then the lateral bracing would be accomplished. You could weld in stiffeners to the beam to bolt the brading to, and tither bolt or neil the kickers to the rafters. However, for this solution, you might see a small portion of sloping surface at the wall/ceiling intersection. This may be objectionable to the client.

For the record, the nomenclature "roof rafter" is, well, overly descriptive. Rafters are found in the roof structure, but not at floors as the inclusion of the word "roof" might imply. Just a minor point in passing.

Mike McCann
McCann Engineering

 

[PT999]

The building is three sides cmu walls. The front side (with the beam) is 60 ft of storefront, with steel columns at 15 ft o.c. and masonry at the very end supports. So beam spans are 15 ft.

The roof structure is lumber framing, gable type, with ceiling joists and collar ties. All framing 90 degrees to cmu wall (except of of course the 2 end sides - but no beam there)

So, there is lateral support for the cmu's just above the steel beam. The question again, is does this lateral support work its way down the 2 ft of 8" cmu's (unreinforced)

The conservative answer of course is no, but I am just checking if this is absolutely correct.

 

[jechols]

PT999,

Sounds like to me that the beam is definitely unbraced. How does the wind load on the beam wall get back to MWFRS? You might have a 60' wind beam. The load path as I understand it would be storefront spans vertically to foundation (or knee wall) and to underside of beam, then where? From your description I would be worried that hinge point exists at the cmu/steel beam interface, unless the storefront spans horizontally to the columns and the columns are full height and are tied to the roof structure.

J

 

[msquared48]

A B S O L U T E L Y !!!!!!!!!!!!

Mike McCann
McCann Engineering

 

[hokie66]

I agree with jechols also. You definitely have a stability problem. The steel beam could be located at the roof structure level, with the fascia section hung from the beam or supported on another beam. The top of the storefront could be braced with the lower beam or a horizontally spanning channel. It is hard to see how it is prudent to include 3 courses of unreinforced CMU in this system.

 

[csd72]

hokie66 makes a good point. Using a few courses of masonry with no dead load over is never a good idea. It will also add cost and time to the project as the contractor has to stop erecting steel, get bricklayer over for 3 courses (on scaffolding), allow mortar to set then continue erectingsteel (after removing scaffolding).

Also, how are the joists connected to the CMU?

I have used the detail many times where we have hung a light gage (girt/purlin type) section from a supporting member to act as a window support. If you can get light gage to work, use a channel.

csd

 

[jheidt2543]

I have seen similar details used with vertical rebar dowels welded to the top flange of the beam and block "threaded over the dowels onto the beam, then the cores grouted and the top course was a bond beam.

While this would seem to be much stiffer, I wonder if it really solves the steel beam bracing problem.