Here are the keys to your dilemma; the closely spaced joist system allows
for the use of a very thin slab, typically 3" on 9/16" metal deck. The
composite system requires a much heavier slab for the greater spans
involved. So at face value, independent of the other considerations you
made below and all things (meaning the steel tonnage) being equal the joist
system should be the more economical just because of the reduced slab
thickness required. The problem is that with the thinner slab transient
vibrations become a problem and one typically has to increase the joist slab
thickness to avoid this problem. The typical composite slab thickness
usually checks for a conventional office grid of 30'x30'. The other primary
reason a composite system is more economical than a joist floor system is
because it lends itself to fireproofing as compared to a joist system.
Typically the same composite slab thickness required for gravity loads also
satisfies the fire rating without sprayed on fireproofing where as the joist
slab does not and therefore has to be sprayed. Wide flange beams are easily
rated with spray on fireproofing, joists are much, much more difficult to
rate with spray on fireproofing.
Matthew Stuart, PE, SE, PEng
Atlantic Engineering Services
From: EMAIL SERVER
Sent: Thursday, March 22, 2001 10:44 AM
To: MATTHEW STUART
Subject: FW: Cost Analysis
From: Jason Kilgore [mailto:jkilgore(--nospam--at)leok.com]
Sent: Thursday, March 22, 2001 10:35 AM
Subject: Cost Analysis
We currently have a disagreement in our office about relative pricing of
different floor construction methods for office-building type usage (fire-
proofed or not). Most of our work is in the Kansas City area, where unions
rule supreme and labor costs are high.
Both of the owners of my company are adamant about one of the methods,
several of the younger engineers disagree (but are out-ranked, so the
method is used).
If the disagreement continues, I'm going to do a mock building design using
the two methods and have a few construction companies price it.
Here are the two sides of the issue (very generalized). Any comments?
Use WF girders, steel bar joists at 2'0" - 2'6" o.c., with light-gage form
Advantages: Easier installation, lighter & easier-to-handle pieces, cheaper
deck, no shoring.
Disadvantages: Short spans require heavier joists for fire-rating, fire-
proofing more labor-intensive (requires wrapping each joist in mesh),
more pieces to install and fire-proof
Use WF girders, WF beams at 6'0"-8'0" o.c., heavier composite decking.
Advantages: Far fewer pieces, WF beams usually meet min. sizes for fire-
ratings, easier and faster to fire-proof.
Disadvantages: Pieces are larger, installation more difficult, deck far more
expensive, sometimes requires shoring for casting.
Jason W. Kilgore, P.E.
Leigh & O'Kane, L.L.C.