750
Cost and Time Analysis: First Cost
Cost base:
.60/OGSF
Example design:
.79 + (23.05-15.95) = .89/OGSF
Difference:
$
.71/OGSF
751.2
BUILDING SUBSYSTEM TARGET COSTS
751.2.1
Structure
The target cost range for structure is between .60 and .30/OGSF, or
from .00 to .45 per square foot of structural framing. These figures
are higher than the comparable cost base range of .86 (Martinez) to
.10 (Atlanta) due to increased floor-to-floor height, longer spans, higher
uniform live loads, and the addition of a three-inch topping slab.
The major variables affecting these costs are:
1. Building Height. Up to four stories, unit costs change very little. From
four to nine levels, the unit cost increases from five to ten percent.
2. Beam Spans. When the longer of the modular spans (49'-6" to 58'-6")
are used rather than the shorter spans (40'-6" and 45'-0"), unit cost will
increase from three to six percent. The cost will also be affected by
the efficiency of the structure in terms of the number of continuous
spans and the use of cantilevers. A single span is the least efficient
whereas beams continuous over three or more spans are the most
efficient. The cantilever is most effective for economy of structure
when used in conjunction with the longer beam spans which are also
continuous over two or more supports.
3. Shear Elements. In high seismic load zones, the cost of shear
resisting elements will be approximately
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.55 to
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.80 per square foot
of structural framing for a four-story building, and
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.90 to
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.20 for a
nine-story building. In low seismic load zones, these costs will be
reduced by approximately 50%.
4. Generic Design Option. Generally speaking, the concrete options will
cost up to fifteen percent less than steel. However, the shorter
construction time required for steel framing may under some
circumstances provide a lower cost.
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