Composite Steel Beam Design - lateral restraint of beams - fire protection of steel beams

[JohnnySm]

Dear All.

5 storey steel frame building (residential). Steel beam with steel/concrete deck flooring kingspan or something.

1. is it sufficient to assume that if the decking trays are positioned and cut correctly, so as to ensure a trough of the deck lies on the top flange, can beams running parallel to the span of the decking be assumed laterally restrained with the use of designed shear studs? i know this is the case with beams spanning perpendicularly.

2. The beams are not designed for composite action but have been designed for full lateral restraint only FYI.

3. in terms of fire protection i know a lot of fabricators assume top flanges of steels where shear studs are located should arrive on site unprotected? is this because they assume i have designed for composite action between the slab and bean? i have not... furthermore with trapazoidal decking this leaves approximately 50% of the top and untreated flange exposed after installtion? should i request these voids to be filled in on site? this seems like something i think the site workers would want to avoid? is it not more preferable to just have the steels fully protected during fabrication? or does this conflict the shear stud installation process? is it standard to request voids filled in on site?

Kinest Regards
JS

 

[dik]

Might repost this under Structural Engineering forum. That being said:

1. is it sufficient to assume that if the decking trays are positioned and cut correctly, so as to ensure a trough of the deck lies on the top flange, can beams running parallel to the span of the decking be assumed laterally restrained with the use of designed shear studs? i know this is the case with beams spanning perpendicularly.

The flutes of decking spanning over beams will be in intimate contact with the top flange. Decking spanning parallel to beams are usually stopped on the edge of the flange to allow placement of headed studs. The beams are laterally supported once the concrete has hardened.

2. The beams are not designed for composite action but have been designed for full lateral restraint only FYI.

Why? You have headed studs, you may as well take advantage of this. In addition if you have a 5 storey building there is likely significant economy is using composite construction for the purlins and girders.

3. in terms of fire protection i know a lot of fabricators assume top flanges of steels where shear studs are located should arrive on site unprotected? is this because they assume i have

The top of the flanges should not be coated where in contact with deck. If interior, there may be some economy in not providing a coating. There are some assemblies using semi-lightweight concrete that don't require the decking to be fire sprayed.

designed for composite action between the slab and bean? i have not... furthermore with trapazoidal decking this leaves approximately 50% of the top and untreated flange exposed after installtion? should i request these voids to be filled in on site? this seems like something i think the site workers would want to avoid?

Usually any beam fire spray closes off the space between the flange and deck.


is it not more preferable to just have the steels fully protected during fabrication? or does this conflict the shear stud installation process? is it standard to request voids filled in on site?

Unless you are using an intumescent material, fire spray is almost always field applied

Dik

 

[JohnnySm]

Dik,

Thanks for the reply. I have since posted this thread in the correct forum. My mistake...

1. Sorry, are you saying that once concrete has hardened both directions of steel work can be regarded as restrained?

2. Yes you are correct. However my employer has asked for the structure to be designed this way.

3. My question refers to beams being fire rated using intumescent paint during fabrication. The top flanges will arrive on site untreated... therefore the exposed section of the flange directly between the bearing troughs of the decking is deemed exposed? would this normally be closed off on site? can i assume the contractors will perform this? i assume not?

 

[dik]

1. Sorry, are you saying that once concrete has hardened both directions of steel work can be regarded as restrained?

Not familiar with the project, but, generally once the concrete is hardened the purlins and girders are considered as being laterally supported.

2. Yes you are correct. However my employer has asked for the structure to be designed this way.

You may ask him to re-consider since there are financial benefits, and, it's part of engineering. For a 5 storey building the savings can exceed your engineering fees. [Added]The benefits are better as your design live load increases.[Added] In addition to deflection issues member sizes can be reduced. Mock up a small 'floor area' and send the composite and non-composite design to a steel fabricator for a quick costing. I think you will be surprised. I've attached F902 FRR... as a possible FRR.

http://productspec.ul.com/canada/document.php?id=BXUVC.F902

3. My question refers to beams being fire rated using intumescent paint during fabrication. The top flanges will arrive on site untreated... therefore the exposed section of the flange directly between the bearing troughs of the decking is deemed exposed? would this normally be closed off on site? can i assume the contractors will perform this? i assume not?

Intumescent coating is very costly and subject to site damage. It is almost never shop applied. If field applying headed studs, you may have some difficulty. The girder studs can be shop applied with the deck 'running up to them'. An intumescent coating can be applied to the girders, avoiding the headed studs... the cost adds up.

I'll move any comments from now on to the structural engineering forum, where you may get better responses.

Dik