Composite Beams vs. Bar Joists

[cjd97]

Has anyone ever did a cost comparison of a composite floor beam system versus a bar joist system? Possibly on a square foot basis? Looking for the cheapest alternative. I know beams are better for vibration, but that shouldn't be a problem, I have plenty of interior partition walls on the second floor.

Please comment.........

 

[lkjh345]

cjd97:

No exact cost numers here, but expereince with several buildings and comments from contractors regarding this question in the past:

Only looking at the structure itself, using floor joists is indeed cheaper. Not only are the joists themselves much lighter, but I usually end up using a shallower deck/concrete system. When I use composite beams, I tend to use 2" or 3" composite deck with either 5" or 6" of total concrete fill. With joists, I tend to space them closer together, so end up with 1 1/2" deck with 4" total of concrete or thereabouts.

The problem is running the mechnaical ducts. If you are very lucky, and have a fairly simple building, where the web members of EVERY joist on a section of floor will end up in the same place, it is possible to run the ducts between the web members and this works out pretty well.

The problem comes if not all the joists line up, so that the mechanical must either snake his ducts through the joists, which is pretty expensive, or run the ducts under the joists which rasies the floor-to-floor height of the building.

Also keep in mind the largeest duct you can run between the webs of the joists you are considering, and then check with the mechanical. if the largest size you can run between the webs of the joists is not large enough for mechanical, and they need to run multiple lines of ducts for this reason, you are losing ground pretty quickly.

According to a couple of design/build contractors, unless large portions of the floors use the same joists with the same lengths (so that the ducts can be run through the joists), it is cheaper to use the composite beams when the cost of the structure AND the mechanical system or rasied floor to floor height is considered.

Just my experience. Others may have different opinions.

 

[Lutfi]

ikjh345,

In my experience, they rarely run ducts through joist web openings. These opening are often times are too small to allow ducts with 2 inches of insulation to pass through them. You need to be using 48 deep joists before this advantage can be used. Additionally, it has been my experience that running ducts between joists will no work due to presence of bridging.

From experience do not forget to allow for two (2) inches of fire proofing to the joist depth if you are to suspend ducts underneath the joists of beams.


Regards,
Lutfi