Economical Design of Concrete Buildings Lawrence Novak 2 General Considerations Three major costs in concrete construction: Concrete 25% Reinforcement 25% Formwork 50% 3 Cost Efficiencies Optimized designs unwarranted for structures of moderate size & height Example: floor system thickness Simplify concrete formwork Expediency of construction versus efficiency of structural design $ $ $ Cost savings $ 4 Economical Formwork 5 Lower Cost through Formwork Economies Minimizing material quantities guarantees inefficient designs Complexity Cost 6 Structure Cost No. of Stories 7 Flat Plate 8 Thickness controlled by two-way (punching) shear Flat Plates 9 Flat Plate Flexibility 10 Flat Plate Construction 11 Flat Slab 12 Slab-Column Systems Drop panels and column capital New in 2008 Shear Cap 13 One-Way Joist (Standard) 14 One-Way Joist 15 Waffle Slab How is a Pan Job Built? Deck Endcaps & diaphragms Pans laid 16 17 Pan Construction - Deck 18 Pan Construction Endcaps 19 Pan Construction - Diaphragms 20 Pan Construction Lay Pans 21 Pan Construction Ready for Rebar System Selection Achieve lower total cost by balancing the trade off between formwork (labor) and material 22 Different Systems Different Costs 23 Paper - Scissors - Rock Labor beats Material 1 CY Concrete = 2 Hours of Labor 24 Trump Mat Foundation Solid vs. Shaped 25 Myths and Misperceptions Concrete takes Longer to Build than Steel 26 Myths and Misperceptions Concrete takes Longer to Build than Steel Construction of Superstructure Frame Months 5 Stories50 Stories CONCRETE STEEL 20 CONCRETE STEEL Myths and Misperceptions Account For Material Order Duration CONCRETE STEEL Add Time for Material Order Months 5 Stories50 Stories CONCRETE STEEL 25 Material Order time Ref:w14x90 or w21x44 Next Roll Dates Sept. / Oct. Myths and Misperceptions Account For Architectural Finishes + Cladding (Time to Occupancy) Add Time to Finish Building (i.e.: Time to Occupancy) Completion to Occupancy Months 5 Stories50 Stories CONCRETE STEEL 20 CONCRETE STEEL Myths and Misperceptions Concrete takes longer to build than steel False 30 Price Stability = Cost Effective Construction 31 Do not underestimate the importance of price stability on decision making Base Year 2002 = 100 32 Formwork Considerations Use available standard form sizes Repeat sizes Strive for simple formwork 33 Slab Systems 34 Forming of Drop Panel 35 Flat Slab 36 Nominal lumber size Actual lumber size (in.) Plyform thickness (in.) h 1 (in.) 2X1-1/22-1/4 4X3-1/24-1/4 6X5-1/26-1/4 8X7-1/48 Drop Panel Depth 37 Standard Form Dimensions 38 Spandrel Beams Drop Spandrels Increase Cost A flat spandrel with imbeds for steel framing will usually beat a drop spandrel with imbeds. 40 Beam/Column Intersections 41 Walls Columns Vertical Elements 42 Column Economics More Economical to: Use Larger Column Sizes (1 2%) Steel Use Larger Bars Minimize Column Changes Reduce Number of Splices 43 Nonslender Tied Columns 44 Short Tied Column Axial Design Per ACI Section Not allowed for special moment frames for seismic 45 Short Tied Column Axial Design 1% steel is generally optimum At 1% steel, Column Capacity (Pu / Ag) is approx. = f c / 2 45 1% Steel is Generally Optimum Higher Strength Concrete is General Optimum 46 47 Frame-Two-way Slab-Column 48 Beam and Slab 49 Site Cast (Tilt-up) Fast Quality Simple Energy efficient 50 Built-in Veneer Embed brick facers Reduces time and costs vs. brick and mortar construction M Brick Site Cast (Tilt-up) 51 Factory Cast (Precast) Fast Quality Simple Energy efficient 52 PT Slabs 53 What if your Shape is Not Standard? How do you Find the Optimal Form? Study in Topology Optimized structures are inherently elegant Why optimize the structures layout? Aesthetic Function Strength Hanger, Orbetello, Italy Gatti Wool Mill, Rome 54 Simple Frames of Least Weight Mitchell Theorem is satisfied if the bars in a frame are subjected to stresses of the same sign: Bar under two opposed forces Triangular and tetrahedral frames Catenaries Where V is the minimum Volume, r i is the vector location of vector Force F i and f is the allowable stress Arches 55 Conclusions System Selection Formwork Systems Constructability Balance Optimum Design and Constructability Lest Material is not always the most Economical Design 56 57 Past - Present - Future