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.) 2X1-1/2¾2-1/4 4X3-1/2¾4-1/4 6X5-1/2¾6-1/4 8X7-1/4¾8 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 structure’s 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