© 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without permission of the publisher.
Chapter 3 ECONOMICAL SELECTION OF COLUMNS Transverse stiffeners and web doubler plates are ex- welding transverse stiffeners and web doubler plates that tremely labor-intensive detail materials due primarily to is predominant in their cost. the fit-up and welding that is associated with their use. An equivalent column weight change is tabulated from Additionally, issues such as restraint, lamellar tearing and these estimated costs based upon a mill price of $425 per welding sequence must be addressed when transverse ton, which is a median value in the common range of stiffeners and/ or web doubler plates are used. As such, from $400 t o $450 per ton FOB, 67 , and a 14-ft floor-to- they add considerable cost in spite of their disproportion- floor height. The tabulated values are calculated as the esately low material cost. If transverse stiffeners and web timated cost times 2000 lb per ton divided by $425 per ton doubler plates can be eliminated and an unreinforced col- divided by the 14-ft length. The resulting value is the esumn can be used, significant cost savings can often be re- timated maximum per-foot column-weight increase that alized. Additionally, the elimination of column stiffening could be made to eliminate that element of the column will simplify (and thereby economize) connections that stiffening without increasing cost. In fact, because the tabare made to the weak axis of the column. ulated values do not consider other intangible economic In wind and low-seismic applications, the specification benefits, such as the simplification of connections that are of column sizes that eliminate transverse stiffeners is en- made to the weak axis of the column, the tabulated value couraged. In high-seismic applications, however, trans- should be considered conservative. verse stiffeners will normally be required, as discussed As an example, consider a W1490 column with fullpreviously in Section 2.3. depth transverse stiffeners (Case 5, Table 3.1) at each In wind, low-seismic, and high-seismic applications, beam flange (2 pairs total) and one web doubler plate the specification of column sizes that eliminate web dou- (Case 8, Table 3.1). The total of the tabulated column- bler plates is encouraged. Web doubler plates require weight-change values for this column stiffening arrangesignificant welding intothe column flange-to-web fillet region (k-area), which is an area of potentially lower notch ment is 40 lb/ft 82 lb/ft 122 lb/ft. Thus, if any heavier W14 up to and including a W14211 column toughness (<strong>AISC</strong>, 1997b). The shrinkage that accompa- could be used without transverse stiffeners and a web dou- nies the cooling of these welds typically can distort the bler plate, it would likely be more economical than the cross-section and overwelding in this region carries the W1490. In most cases, the actual increase in column potential for cracking. Additionally, the weld joint may re- weight required to eliminate column stiffening will be quire the use of a non-prequalified detail as discussed in much less than the maximum calculated and a significant Section 4.4.3. economic benefit can be realized. When the required column-weight change exceeds the 3.1 Achieving Balance Between Increases in Material Cost and Reductions in Labor Cost sum of the tabulated values, some engineering judgment must be used. If the comparison is unfavorable, but still In Table 3.1, estimated costs are given for some arbitrarily close, the use of a heavier column might still be justiselected transverse stiffener and web doubler plate details fied by the aforementioned intangibles. Alternatively, the as illustrated in Figure 3-1. These estimated costs were designer may still find it advantageous to investigate the determined by averaging the cost estimates5 provided by possibility of eliminating the web doubler plate only ( or several fabricators and rounding the result to the near- transverse stiffeners only in some cases). est five-dollar increment. When comparing these typical As an example, consider again the W1490 column details toactual details, it should be noted that the compar- at each beam flange (two pairs total) and one web dou- than the thicknesses or overall dimensions of the plate ma- bler plate (Case 8, Table 3.1). If any heavier W14 up to with full-depth transverse stiffeners (Case 5, Table 3.1) ative weld types and sizes are of much greater significance terials. It is the labor involved in cutting, profiling, and 6FOB stands for “free on board,” which indicates that the quoted price 5 assumes delivery to the indicated location. In the above case, the indi- The estimated costs are predicated upon the material and labor costs cated location is the mill itself; subsequent shipping would incur addithat existed at the time this <strong>Design</strong> <strong>Guide</strong> was written (circa early 1999). tional cost. Because it is anticipated that labor costs will continue to rise at a faster rate than material costs, the user may find it advantageous toperiodically 7Because mill prices fluctuate, the designer may find it advantageous to inquire with local fabricators to determine a more current estimate of periodically inquire with fabricators, steel mills, or other shape suppliers these costs. to determine the current range of mill prices. 13 © 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without permission of the publisher.
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