Ok, been asked to supply a poprosal for a 2-story office building. I've been involved with numerious jobs in the past using a bar joist floor with concrete deck. The architect shows to be a composite floor using steel beams. I can certainly provide this, but what are the benefits to a composite floor using steel beams vs. a non-composite floor using bar joists? The floor members need to span approx. 31'-0".
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Compsite Floor vs. Bar Joist Floor
- Thread starter ars001
- Start date
civilperson
Structural
Lesser floor-to-floor height due to lesser structural system depth. Better acoustic and vibration properties for human comfort and perception. Equal framing cost, higher HVAC cost, (may not apply to two storey roof mounted units).
I generally agree with CivilPerson.
I have done multi-story building both ways. Joist manufacturer's tout that their system is lighter and cheaper, and it may have a small cost advantage, however, by the time you stiffen the joist systems enough to take care of the floor vibration, a lot of this saving disappears. Even 'low-end' contractors in my area tell me the savings, in the end, are pretty minimal using the joist system.
Composite beam/Composite slab systems are also a lot more forgiving for accepting unanticipated loads after they are constructed. Most of our building projects now days go out with early structural packages. Either Foundation Packages with Steel Mill Order Packages, or 'Core and Shell' packages that are just the basic structure of the building and its enclosing skin. No interior partitions, mechanical units, etc yet. (This is the 'Structural Engineer, please work your ass off to get something out the door to buy every other discipline some time to get their act together.' routine.)
Joists sytems are difficult to deal with after they are erected and the architect suddenly tells you the client wants an area designated for high density file systems, or something like that. There are ways to strengthen the composite beam/composite slab system. But you are in a difficult spot with the joist system.
For my two cents, I always try to stear a mutli-story building towards composite beams/composite deck if I can.
I have done multi-story building both ways. Joist manufacturer's tout that their system is lighter and cheaper, and it may have a small cost advantage, however, by the time you stiffen the joist systems enough to take care of the floor vibration, a lot of this saving disappears. Even 'low-end' contractors in my area tell me the savings, in the end, are pretty minimal using the joist system.
Composite beam/Composite slab systems are also a lot more forgiving for accepting unanticipated loads after they are constructed. Most of our building projects now days go out with early structural packages. Either Foundation Packages with Steel Mill Order Packages, or 'Core and Shell' packages that are just the basic structure of the building and its enclosing skin. No interior partitions, mechanical units, etc yet. (This is the 'Structural Engineer, please work your ass off to get something out the door to buy every other discipline some time to get their act together.' routine.)
Joists sytems are difficult to deal with after they are erected and the architect suddenly tells you the client wants an area designated for high density file systems, or something like that. There are ways to strengthen the composite beam/composite slab system. But you are in a difficult spot with the joist system.
For my two cents, I always try to stear a mutli-story building towards composite beams/composite deck if I can.
JedClampett
Structural
In addition to the excellent points made above, I feel that when I use bar joists, I lose some control of the design. The joist manufacturer gives me a joist that is good for the load I specified. But is it conservative? Where is its weak link? If the load is 10 psf more, where do i reinforce? And the joist suppliers are notoriously bad at record keeping. There's almost no way to track down their calculations if the original submitted copies are not available.
If you look at various threads in this forum, how many threads start out with, "I have a building I'm evaluating, and I can't figure out the type of joists..." or "What is the capacity of an XYZ brand joist manufactured in 1959?" With composite designs, you can either measure and analyze or use the drawings.
But I'm ranting.
If you look at various threads in this forum, how many threads start out with, "I have a building I'm evaluating, and I can't figure out the type of joists..." or "What is the capacity of an XYZ brand joist manufactured in 1959?" With composite designs, you can either measure and analyze or use the drawings.
But I'm ranting.
Just to confuse issues, smore ramblings...
Does the floor require a Fire Resistance Rating? if so, by the ceiling, by spray, or by assembly? I've also used OWSJ in composite manner... many tests have shown, the puddle welds attaching the deck to the joists generally cause composite action for a significant portion of the live load. As your live load increases, the cost benefits of composite design become more significant.
A typical assembly might be: Steel composite purlins at 10' o/c framing into composite girders with a 3" deck and 3-1/2"semi-lightweight conc topping. The beams and girders have firespray... The deck assembly does not... another might be OWSJ @ 2' o/c with 2-1/2" topping on V-rib form... dramatically different.
There are a pile of assemblies, with various deck thicknesses, joist/purlin spacings, and different toppings. The cost of each varies from locale to locale.
OWSJ may have problems with firespray. If you have sufficient floor area, you might want to do a prelim design of a couple of framing schemes and have them costed... The entire assembly should be costed.
If all else fails, the framing using OWSJ may be more economical, but will provide a greater depth of structure. If it's a small area, then run with what the Arch has sketched advising him that you haven't done any cost comparisons. If you have a large floor area, then the added savings warrants a bit of investigation.
Dik
Does the floor require a Fire Resistance Rating? if so, by the ceiling, by spray, or by assembly? I've also used OWSJ in composite manner... many tests have shown, the puddle welds attaching the deck to the joists generally cause composite action for a significant portion of the live load. As your live load increases, the cost benefits of composite design become more significant.
A typical assembly might be: Steel composite purlins at 10' o/c framing into composite girders with a 3" deck and 3-1/2"semi-lightweight conc topping. The beams and girders have firespray... The deck assembly does not... another might be OWSJ @ 2' o/c with 2-1/2" topping on V-rib form... dramatically different.
There are a pile of assemblies, with various deck thicknesses, joist/purlin spacings, and different toppings. The cost of each varies from locale to locale.
OWSJ may have problems with firespray. If you have sufficient floor area, you might want to do a prelim design of a couple of framing schemes and have them costed... The entire assembly should be costed.
If all else fails, the framing using OWSJ may be more economical, but will provide a greater depth of structure. If it's a small area, then run with what the Arch has sketched advising him that you haven't done any cost comparisons. If you have a large floor area, then the added savings warrants a bit of investigation.
Dik
michfan
Structural
- Jan 4, 2007
- 107
Did you ever consider precast plank? I believe a 10" deep would easily span 31'. Good fire rating, easy installation despite being such a heavy system. Plenty of capacity - load table I checked showed 170 psf total capacity for 10" at 32 feet span. Just a thought.
FWIW:
If you have a contractor on board, check with them on using lightweight concrete.
A lot of contractors do not like it as it is difficult to finish, and will charge your client extra if it is used on a job. I have run the 'numbers' several times, with typical bay framing scenario's given to contractors, and the saving from lighter steel, and thinner slabs (due to greater fire rating) usually does not pay for the contractor's extra costs to deal with the lightweight.
If you have a contractor on board, check with them on using lightweight concrete.
A lot of contractors do not like it as it is difficult to finish, and will charge your client extra if it is used on a job. I have run the 'numbers' several times, with typical bay framing scenario's given to contractors, and the saving from lighter steel, and thinner slabs (due to greater fire rating) usually does not pay for the contractor's extra costs to deal with the lightweight.
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