Unit Structural Quantities

Quantities are physical items of construction to which unit costs are applied to arrive at a total construction cost. These are relatively easy to obtain once complete working drawings and specifications have been prepared. Prior to this point, however, the estimator or engineer must use "conceptual estimating" to determine approximate quantities. Conceptual estimates require considerable judgment in addition to unit quantities in order to adjust so-called average unit costs to reflect complexity of construction operations, expected time required for construction, etc.

Typically, units of structural quantities are one-, two-, or three-dimensional, based on linear feet, square feet, or cubic feet. These result in unit quantities such as pounds per linear feet (plf), pounds per square foot (psf), etc.

8.1.4.1. Unit Weight of Structural Steel for Preliminary Estimate

The total quantity of structural steel divided by the gross area of the building has always been, and will always be, an item of great interest to building developers and designers alike. In U.S. practice, this unit quantity of steel is usually expressed in terms of pounds per square foot. In selecting a structural system, the usual practice is to look into several possible structural schemes that meet the basic architectural requirements. The deciding factor most often, then, is the unit quantity of material for the systems. Although the unit

Figure 8.28. (Continued.)

quantity of steel in a steel building may not address other factors relevant to construction cost, such as unit price for steel for a particular scheme, whether or not the scheme has composite steel-concrete vertical systems, the cost of fireproofing, and the cost of borrowing money for the period of construction, it is the unit quantity that most often pushes a particular scheme to the forefront.

Prior to the advent of tubular and megaframe systems, most of the buildings were designed using braced or moment frames as lateral load-resisting systems. Consequently, their poundage is very heavy compared to present-day schemes.

Leaving aside the pre-1960 buildings, it is of interest for conceptual estimating purposes to assemble the unit structural steel quantities for buildings that have been built within the last three decades. Figure 8.29 shows the general trend in the increase of unit quantity of steel as building height is increased.

Four distinct regions A, B, C, and D are shown in the figure. Region A is for buildings up to 30 stories, B for buildings between 30 and 50 stories, C for buildings 50 to 70 stories, and D for high-rises in excess of 70 stories. Use of this figure is best explained with respect to the following examples.

Example 1. A proposed ten-story building in a wind-controlled, low-seismic risk area. The engineer is asked to come up with a unit quantity of structural steel for purposes of a conceptual cost estimate.

Center! ine

Figure 8.28. (Continued.)

Center! ine

Figure 8.28. (Continued.)

For a ten-story building, it is seen from Fig. 8.29 that the lower and upper bounds for the unit quantity are 10.5 and 15 psf, respectively, which is a rather wide range. The engineer now has to make some judgment calls, depending on what is known about the building at this stage of the game. Some relevant questions are: 1) What are the typical

(h) Typical floor truss

Centertine of Exterior Column

3/S' Gusset Plate Welded 10 Column and Top Chord

Centertine of Exterior Column

3/S' Gusset Plate Welded 10 Column and Top Chord

Two 7/0* Diameter Botts

(i) Detail A - Exterior Wall End Detail

Two 7/0* Diameter Botts

(i) Detail A - Exterior Wall End Detail

Centerline of Interior Column

Centerline of Interior Column

in Slotted Holes (j) Detail B - Interior Wall End Detail

Figure 8.28. (Continued.)

in Slotted Holes (j) Detail B - Interior Wall End Detail

Figure 8.28. (Continued.)

spans for the floor framing? 2) Are there any undue restrictions for beam and girder depths? 3) What is the likely lateral framing system? If none of these questions can be answered with any great certainty the engineer must make some judgment calls. For the current example, the designer decides that the subject building is an average building resulting in an average unit quantity equal to (10.5 + 15)/2 = 12.75, rounded to 13 psf.

Example 2. Fifty-story building. Using a similar procedure, the engineer determines that the lower- and upper-bound values are 21 and 32 psf, giving an average unit quantity of 26.5 psf.

Example 3. Twenty-two story building. Design controlled by seismic loads. The straight line designated as S in Fig. 8.29 represents an approximate unit quantity of steel for buildings located in high-risk seismic zones (zones 3 and 4). Observe that this line stops at 25 stories, implying that design of taller buildings, in general, is controlled by wind loads. The cut-off level of 25 stories with building periods in the range of 2-3 sec is considered the threshold where wind design requirements exceed those of seismic activity. For the example building, the unit quantity of steel is given by the straight line S as 22 psf.

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