CISC Project Solutions Cost Estimating Method for Steel Building Structures. Canadian Institute of Steel Construction

Transcription

1 CISC Project Solutions Cost Estimating Method for Steel Building Structures Canadian Institute of Steel Construction

2 Copyright 2012 by Canadian Institute of Steel Construction All rights reserved. This book or any part thereof must not be reproduced in any form without the written permission of the publisher. June 2012 ISBN

3 Introduction This publication supersedes the section on Estimating the Cost of Structural Steel in A Project Analysis Approach to Building Costs, published in 1971 and its metric version published in Project Solutions (originally known as Project Analysis Division) published this do-it-yourself method for estimating the cost of erected structural steel to assist designers and estimators in the selection of an economical steel framing system for buildings. The method has been widely used by construction professionals as well as members and staff of CISC. In this new publication, CISC Project Solutions Cost Estimating Method for Steel Building Structures, several key parameters used in the costing method have been revised to reflect the pertinent changes that have taken place as the industry advanced since These include the elimination of the irrelevant cost premium on 350 MPa (50 ksi) steels and premium on imported wide-flange sections. Welded wide-flange sections, once very popular products, are now covered under Plate Girders and Built-up Column H-shapes. A simple feature has been added to account for the cost difference between key members in conventional construction, as defined in the Building Code, and those used in more ductile seismic force-resisting frames. This is strictly a cost estimating method and it should not be confused with the method for Computation of Units as defined in the CISC Code of Standard Practice. General Description Structural steelwork is divided into a number of appropriate categories that define the types of steel section used, as well as their specific use in the frame. A connection factor reflecting connection cost is applied to the net mass of each member in each category to produce a gross mass. Next, a cost factor reflecting the use of each specific member in a frame is applied to the gross mass. Finally, a Cost Index, calculated regionally and periodically from marketplace estimates of the cost of a standard sample building, is applied to produce a calculated estimated cost for the erected steel frame. Variations in building typical bay size have a direct effect on the number of connections and member mass in a building or in fabricator vernacular, the pieces per tonne. A Bay Size Modifier (BSM) provides a means of assessing the influence of any significant deviation from a 9 m x 9 m bay (or any bay with an area of 81 m 2 ). The BSM curve is illustrated in Figure 1. 1

4 Definitions Member Length (L) Grid dimensions (center-to-centre of supporting members) are used to define member lengths. Net Steel Mass (NSM) The net steel mass is computed as the product of member length and nominal mass per unit length. Connection Factor (Confac) Each member must be connected to others. Material (or shop supplies in lieu of) and both shop and field labour allowances are required. A series of connection factors applicable to various categories of members has been developed as listed in Table 1. Gross Steel Mass (GSM) The gross steel mass is computed by applying a connection factor to the net steel mass for each category of member. GSM = NSM x Confac. Member Classification Pre-fabrication steel sections vary significantly in their base cost, depending on the method of manufacture and mass per unit length. Likewise, the utilization of these members in various frame components will significantly influence their end cost. For example, a heavy W section used as a secondary beam requiring two simple connections and with a substantial steel mass would have a significantly lower unit cost than a light W section used as a diagonal member in a braced frame, requiring two complex connections and with a fraction of the mass of the previous member. Thus a number of classifications have been developed, segregating member types and member applications. Complexity or Cost Factored Mass (CFM) As discussed under Member Classification, the type of pre-fabrication steel section and its end use in a frame can have a significant impact on cost. A Cost Factor (Cosfac) is applied to each category to calculate a Cost Factored Mass (CFM). CFM = GSM x Cosfac. This interim step provides a measure of the influences of design complexity and pre-fabrication steel cost. For example, alternative components performing a specific function can be compared a truss having a low mass per unit length, but high connection and cost factors, may be compared with a rolled structural member having a higher unit mass but lower connection and cost factors. 2

5 Metric Cost Index (MCI) The Metric Cost Index is calculated regionally, at regular intervals, from averaged fabricator-submitted cost estimates of a standard sample building. It is not the steel unit cost but a base upon which to build a total steel estimate by applying connection and cost factors, for each specific project. The regional cost index (and local cost index within some regions) is available from CISC offices. Bay Size Modifier (BSM) The Metric Cost Index is computed from a building with a standard 9 m x 9 m bay configuration. Any significant change in typical bay size could affect the final unit cost of the structure. For example, a 6 m x 6 m typical bay configuration would result in a much increased number of joints to be detailed and fabricated, and a much increased number of pieces for each tonne of erected steel framework. Consequently a graph indicating appropriate factors has been developed and is shown in Figure 1. (Note: bay size is a function of bay area.) Estimated Steel Cost (C) By assembling steel member masses in appropriate categories, applying connection and cost factors, collating and multiplying by the regional cost index, (and a bay size modifier factor if appropriate), it is possible to calculate an estimated in-place cost of a steel frame. Member Cost (C) $ = Theoretical length (m) x Nominal Mass (kg) per unit length x Confac x Cosfac x MCI x BSM NSM GSM CFM 3

6 Connection and Cost Factors Table 1 Member Classification Connection Factors Cost Factors (Confac ) (Cosfac ) Simple Continuous Simple Continuous Construction Construction I Construction Construction I 1. Beams a) W-Shapes II < 50 kg/m M III M b) W-Shapes II > 50 kg/m IV c) C, M or S Shapes < 50 kg/m M III d) C, M or S Shapes > 50 kg/m Plate Girders Columns a) W-Shapes up to 55 kg/m M III b) W-Shapes over 55 kg/m c) HSS (Hollow Structural Sections) d) Heavy Built-up H-Shapes & Box-Shapes Spandrel Beams a) W-Shapes < 50 kg/m M III b) W-Shapes > 50 kg/m Trusses V (parallel chords & without gusset) a) T-Shape Chords & Angle Web Members b) Angle Chords & HSS Web members c) all W Shapes d) HSS Chords & Angle Web Members e) all HSS f) all Angles Bracing a) W-Shapes 1.10 VI 1.75 VII b) Angles 1.15 VI 1.40 VII c) HSS (Hollow Structural Section) 1.15 VI 2.20 VII d) Flat Plates III, IV 4

7 Connection and Cost Factors (Cont d) Table 1 (Cont d) Member Classification Connection Factors Cost Factors (Confac ) (Cosfac ) 7. Roof Purlins a) W-Shapes up to 50 kg/m b) W-Shapes over 50 kg/m c) HSS (Hollow Structural Section) Stub Girders (use mass of main girder only) 9. Girts a) Hot Rolled Sections b) Cold Formed (F y > 300 MPa) c) HSS Sag Rods $0.015 x MCI each c/w nuts 11. Open-Web Steel Joists and Bridging CSA G W Steel Loose Lintels (Nominal fabrication) Bolts Included in connection factors 14. Welds Included in connection factors 15. Stud Shear Connectors a) Shop-Applied $ { MCI / 1600 } each b) Field-Applied $ { MCI / 700 } each 16. Ancillary Steel Attached i.e. Hangers, Hung Spandrel Angles, Roof and Floor Opening Frames, etc. Notes: I ) For Cantilever and Suspended Span (Gerber) Construction, use factors given for simple construction. II ) Add camber cost where applicable: $0.025 x MCI each for a minimum of 20 typical beams III ) M = mass in units of kg/m IV ) Increase Cosfac by 20% for non-conventional (Ductile, Moderately Ductile & Limited Ductility) seismic moment-resisting frames V ) Including up to 10% allowance for bridging VI ) Increase Confac by 10% for non-conventional (Ductile, Moderately Ductile & Limited Ductility) seismic braced frames VII ) Increase Cosfac by 35% for non-conventional (Ductile, Moderately Ductile & Limited Ductility) seismic braced frames 5

8 Bay Size Modifier Figure BAY SIZE MODIFIER (BSM) BSM = ( A 0.53) 2 where 20 m 2 A 200 m BAY AREA (A), m 2 Note: Metric Cost Index based on 9 m x 9 m (81 m 2 ) bay size. Multiply Cost Index by BSM when typical bay size is larger or smaller. 6