Steel Framed System Selection 5

[271828]

Hello all,

I have a question--more like a poll, because I know what I think. Say you are selecting the system for a steel-framed building. It needs a 1 or 2 hour fire rating and the columns can be spaced on a 30 ft square grid.

What system would you select? Slab thickness, fire protection, composite or not, etc.

 

[Lion06]

I don't have my USD catalog in front of me, but it would likely be 3" composite deck with 3.5" of concrete (or 4.5", I can't remember off the top of my head what is needed for two hour rating with no fireproofing - we never fireproof deck unless it needs some ridiculous rating. I haven't seen it done in our office yet.).

It would also most likely be composite (though this would depend on the loading and deflection requirements) and the % composite would be in the 70% range. That can get you out of some holes when the architect changes something late in the game. All you have to do is add studs to the beam instead of bumping up the size of the steel.

 

[haynewp]

2" unprotected deck plus 3 1/4" SLW concrete with composite beams at 7.5 ft on center, unshored.

 

[JKStruct]

We use LWC to get the fire rating while minimizing slab thickness. So our thicknesses would either be 5 1/4" or 6 1/4", depending on metal deck depth (either 2" or 3"). That is usually driven by allowable spans for concrete placement, and I'll use the 2-span condition in the deck manuals and select 2 or 3 inch based on the grid spacing.

Agree with StrlEIT, never fireproof the deck.

I also always design as composite.

 

[271828]

Thanks guys.

Those concur with my experience and opinions also. I just had a disagreement with a good friend about this. His opinion is that it's silly to go composite for such small bay sizes.

I, and everybody in the firms I worked for in two major cities, would use either a 5.25" lightweight concrete slab on 2" deck, sprayed beams, unsprayed deck, D916 system or the corresponding normal weight concrete system. The beams and girders usually end up about W16x26 or 31 and W21x50 or 62, respectively. Beams spaced at either 7.5' or 10'. Beams and gdrs often cambered 3/4".

 

[271828]

By 5.25" slab, I mean 5.25" total, so 3.25" solid.

 

[WillisV]

I'd probably go 6 1/4 LW composite with the 3" deck and beams spaced at 10'.

 

[jike]

Costs of NW concrete, LW concrete and fireproofing will vary all over the country. Therefore, if you do not know which way to go in your part of the country from previous experience, here is what I would suggest:

I would estimate the cost to see which of the following options is cheaper:

Based upon 2 hour floor requirement:
1) 3 1/4" Lightweight over deck
2) 4 1/2" Normal weight over deck
3) 2 1/2" Normal weight over deck plus fireproofing

In our area, at one time #1 was cheaper but more recently #3 is cheaper. You might also want to consider the use of the building, whether it will undergo extensive future remodeling, etc.

 

[271828]

jike, what kind of beams?

 

[jike]

I would generally use composite beams for offices, hospitals, schools, etc. but may avoid composite if industrial with lots of floor holes (either now or in the future).

 

[271828]

OK. Thanks everybody.

 

[BAretired]

In my location, I would select 2.5" + 1.5" Normal weight concrete on 1.5" x 22 gauge deck with open web steel joists spaced at 5' or 6' centers. Beams would be non composite. But as stated before, economy of structure varies across the continent.

Best regards,

BA

 

[271828]

for a 2 hour fire rating?

 

[jike]

If you have an "open concept" type of office building, you will want to run a transient vibration analysis before you lock yourself into a slab thickness. Sometimes, I would add an inch of concrete to dampen the floor movement for certain spans.

 

[lkjh345]

The OP doesn't state the overal size of the building.

Whether to use composite or non-composite beams can vary somewhat depending on how many studs you anticipate using.

An owner in a steel erection firm (who formerly worked as a structural engineer doing office buildings, schools, hospitals, etc)) has told me several times that if you do not have approximately 1,000 studs on the job, it is probably cheaper to go with non-composite beams and pay for the extra steel. Setting up the stud welding machine, and testing it at each site, and doing test studs, etc is a surprisingly expensive item, and its is his judgement that your don't overcome this expensive on very small projects that need just a few hundred studs.

 

[271828]

Assume a decent sized building.

 

[BAretired]

Assume a 1 hour fire rating?

Best regards,

BA

 

[slickdeals]

@271828,
It must be noted that you would be better off going with composite system, but DO NOT CAMBER.

 

[271828]

Why the blanket statement to not camber?

BA: 1 or 2 hour rating.

 

[slickdeals]

For a 30' span, the cost of cambering a beam will exceed the cost of making the beam slightly heavier to meet the same deflection criteria and you will get more value for money than cambering.. For spans longer than 40-45', there will be a savings from cambering.