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Est im at in g Co n cret e V o lu m es f o r Co Co m p o sit e Dec De ck s M BA
053000 Metal Decking and 053100 Steel Decking. Decking. Structural steel is part of Division 05—Metals subdivisions 051000 Structural Metal Framing and 051200 Structural Steel Framing.
Introduction
Methods of Measurement
Many buildings incorporate composite decks into their design. Comp osite decks decks consist consist of a con crete slab poured over metal decking supported by structural steel beams as shown in in Figure Figure 1. 1. The concrete, metal deck, and structural steel all become part of th e structure structure and work together to support the loads in a composite deck. When pouring the
Concrete for the deck is measured by the cubic yard (cubic meters). Feet may be converted to meters by multiplying the feet by 0.3048, and cubic yards may be converted to cubic meters by multiplying the cubic yards by 0.7646.
By Steven J. Peterson, PE,
concrete, the metal deck must be supported until the concrete has cured and achieved sufficient strength to support itself. itself. As the concrete is is placed, the weight of the concrete increases the load on the structural steel beams, which causes the beams to deflect. This deflection increases the amount of concrete needed for the slab. The purpo se of this paper is to give give the reader an understanding of how to estimate the quantity of concrete needed for a composite deck.
Estimating Quantities The estimating of concrete for composite decks is similar to the estimating of slabs on grade with two exception s. Firs First, t, th e th ickness of the slab varies because the bottom of the slab is poured over an intentionally uneven surface (the metal deck). Second, because the structure supporting the slab deflects under the weight of the concrete, the slab slab at th e center of the span will be thicker than the concrete nea r the supports unless unless measures are taken to avoid this. To accoun t for th e uneven surface
CSI Divisions and Subdivisions Raised concrete decks are part of Division 03—Concrete subdivisions 033000 Cast-in-place Concrete, 033100 Structural Concrete, and 035000 Cast Decks and U nd erlayment. Metal decking is part of Division 05—Metals subdivisions
of the metal deck, the average nominal thickness of the slab must be determined. For the metal deck shown in Figure 2, the average nominal thickness is determined by the following
equation: TAve = T + (W1 + W2)D /( 2S) Eq. 1, where: T = Distance from Top of Slab to Top o f Metal D eck W1 = Width of the Metal Deck Cell at the Bottom W2 = Width of the Metal Deck Cell at t he Top Top D = Depth of th e Metal Metal Deck Cell Cell S = Spa cing of th e Metal Deck Cells Cells The amoun t of concrete needed to account for the d efle eflecti ction on of the beam is a function of how much a structure deflects under the weight of the concrete as the concrete is being poured. The amount of deflection is dependent on the density of the concrete, the thickness of the slab, the dimensions of the metal deck, the distance between the supports, and the size of the beams supporting the metal deck. For a deflection that is limited to 1/360 1/3 60 of th e span span of the b eam ( a common li limit mit placed placed on the deflection deflection o f a beam), the deflection typically adds 10 to 15 15 percent to the amoun t of concrete needed to pour the sl slab ab ( JS JSE E 2002). The deflection may be offset by cambering the steel beams, as shown in Figure Figure 3, such such th at th e weight of th e concrete causes the beam to deflect to a po int where the slab is the same thickness at the center of the span as it is near the supports. The calculation of quantity of concrete needed for a composite slab is shown in the following example: Example 1: A 10- foot by 5- foot by 3-inch thick slab is poured over the metal deck shown shown in Figure 4. 4. The dept h of the slab is measured from the top of the slab to the top of the metal deck. Determine the number of yards of (Conti nu ed on page 14)
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ESTIM ATING TO D AY
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Est im at ing Co ncret e Volu m es f or Decks (Conti nu ed from page 12)
concrete needed to pour the slab. Allow 12 percent for deflection and 5
ting the cells as shown in Figure 5: The following formula needs to be entered into Cell B10 =B1*B2*((B3+(B5+B6)*B4/(2*B7))/ 12)/27*(1+ B8)*(1+ B9) The data for the slab is entered in Cells B1 thro ugh B9. The data shown in Figure 5 is from Exam ple 1.
Ratios and Analysis
percent for waste. The average nominal thickness is determin ed using Eq . 1 as follows: TAve = 3 in + (4-1/2 in + 2-1 /2 in)(1-1/2 in)/(2 x 6 in) = 3.875 in The volume of co ncrete is calculated as follows: V = ( Length )(Width) (Thickness) V = (100 ft)(50 ft)(3.875 in)(1 ft/12 in)(1 yd3/27 ft3) = 59.8 yd3 Add 12% for deflection: V = 59.8 yd3(1 + 12/100) = 67.0 yd3
Add 5% for waste: V = 67.0 yd3(1 + 5/100) = 70.3 yd3 The volume of con crete for a co mposite slab may be set up in a spreadsheet by entering the data and format-
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The amoun t of concrete needed for deflection may be compared between projects, provided they have similar design characteristics, na mely: the density of the concrete, the thickness of the slab, the dimensions of the metal deck, the distance between the supports, and size of the beams supporting the metal deck. When th ese factors are different they may affect the amount of concrete needed to account for the deflection.
Glossary A slightly convex curve placed in th e top surface of a beam or girder. The amount a beam or girder bends as a result of a load placed on the beam or girder. Forces placed upon structural members due to the weight of the structure, the occupants, wind, earthqua ke, and so forth. The distance between supports for a beam or girder.
References JSE (2002). “Co nstruction Considerations for Composite Steeland -Con crete Floor Systems,” Journ al of Structural Engineering, Vol. 128, No. 9, p. 1101.
Conclusion Composite decks consist of a concrete slab poured over metal decking supported by structural steel beams. When calculating the quantity of concrete, the estimator must take i n t o account the unevenness of the metal deck using Eq. 1 and the deflection of the supporting beams as the concrete is pou red . Typically, deflection adds 10 to 15 percent to the amount of concrete needed for beams limited to a deflection of 1/360 of their span. The add itional concrete needed to account for the deflection may be eliminated by proper cambering of th e beams.
Steven Peterson i s an associ ate professor i n t he Parson Constru ction M anagement Technology program at Weber State Un i versi ty. H e teaches courses i n estimat- i ng, schedul i ng, an d constru cti on fi nan ce. Pri or to teaching at Weber State, Steve worked as a proj ect manager and estimator i n the constructi on i ndu str y. H e recei ved an M BA an d a BS i n engin eeri ng from the Uni versity of Ut ah. Steven i s the author of Construction Accounting and Financial M anagement and i s worki ng on a book entitled Construction Estimating with Excel. H e can be reached at (801) 626- 7556, or
[email protected].
ESTIMATING TOD AY
