Economical Design of Concrete Buildings
Lawrence Novak
General Considerations
• Three major costs in concrete
construction:
– Concrete
25%
– Reinforcement 25%
– Formwork
50%
2
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 $
3
Economical Formwork
4
Lower Cost through Formwork Economies
• Minimizing material quantities guarantees
“inefficient” designs
Cost
Complexity
5
Structure Cost
Structure
Cost
No. of Stories
6
Flat Plate
7
Flat Plates
• Thickness controlled by two-way
(punching) shear
8
Flat Plate  Flexibility
9
Flat Plate Construction
10
Flat Slab
11
New in
2008
Shear Cap
Slab-Column Systems
Drop panels
and
column capital
12
One-Way Joist (Standard)
13
One-Way Joist
14
Waffle Slab
15
How is a Pan Job Built?
• Deck
• Endcaps & diaphragms
• Pans laid
16
Pan Construction - Deck
17
Pan Construction – Endcaps
18
Pan Construction - Diaphragms
19
Pan Construction – Lay Pans
20
Pan Construction – Ready for Rebar
21
System Selection
• Achieve lower
total cost by
balancing the
trade off between
formwork (labor)
and material
Comparative Costs
Concrete
Concrete
Rebar
Formwork
Pan Deck
Rebar
Formwork
Beam & Slab
22
Different Systems – Different Costs
High Cost
Beam & Slab
Unique Pans
Beam & Slab
Flat Pans with Drop Beams
Flat Slab with Drops
Low Cost
Flat Pans
Flat Plate
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
35
30
Construction of
Superstructure Frame
25
20
Months
15
10
5
CONCRETE
STEEL
50 Stories
CONCRETE
STEEL
5 Stories
27
Myths and Misperceptions
• Account For Material Order Duration
Add Time for
Material Order
35
30
w14x90 or w21x44
Next Roll Dates
Sept. / Oct.
25
20
Ref: http://www.nucoryamato.com/
Months
15
10
5
CONCRETE
STEEL
50 Stories
CONCRETE
STEEL
5 Stories
Material
Order
time
Myths and Misperceptions
• Account For Architectural Finishes + Cladding
(Time to Occupancy)
Add Time to Finish Building
35
(i.e.: Time to Occupancy)
30
25
20
Months
Completion to Occupancy
15
10
CONCRETE
STEEL
50 Stories
CONCRETE
STEEL
5 Stories
Myths and Misperceptions
• Concrete takes longer to build than steel
30
Price Stability = Cost Effective Construction
Base Year 2002 = 100
• Do not underestimate the importance of
price stability on decision making
31
Formwork Considerations
• Use available standard form sizes
• Repeat sizes
• Strive for simple formwork
32
Slab Systems
33
Forming of Drop Panel
34
Flat Slab
35
Drop Panel Depth
Nominal lumber
size
Actual lumber
Plyform
size (in.)
thickness (in.)
h1
(in.)
2X
4X
1-1/2
3-1/2
¾
¾
2-1/4
4-1/4
6X
8X
5-1/2
7-1/4
¾
¾
6-1/4
8
36
Standard Form Dimensions
37
Spandrel Beams
38
Drop Spandrels Increase Cost
• A flat spandrel with imbeds for steel
framing will usually beat a drop
spandrel with imbeds.
Beam/Column Intersections
40
Vertical Elements
• Walls
• Columns
41
Column Economics
• More Economical to:
– Use Larger Column Sizes
• (1 – 2%) Steel
– Use Larger Bars
– Minimize Column Changes
– Reduce Number of Splices
42
Nonslender Tied Columns
43
Short Tied Column Axial Design
Simplified Column Capacity
(short tied column, fy = 60 ksi)
3.5
f’c = 6,000 psi
Pu / Ag
(ksi)
3.0
f’c = 5,000 psi
2.5
2.0
f’c = 4,000 psi concrete
1.5
1.0
0.5
0.0
0.5%
Per ACI 318-14
Section 10.3.1.2
Not allowed for special
moment frames for
seismic
0.6%
0.7%
0.8%
0.9%
1.0%
1.1%
r = As / Ag
1.2%
1.3%
1.4%
1.5%
44
Short Tied Column Axial Design
Simplified Column Capacity
Column Cost Comparision
(short tied column, fy = 60 ksi)
(short tied column, fy = 60 ksi)
Pu / Ag (ksi)
Relative Cost Per Capacity
3.5 1.6
f’c = 6 ksi
3.0
1.4
f’c = 5 ksi
2.5
2.0 1.2
f’c = 4 ksi concrete
f’c = 4 ksi concrete
1.5
1.0
f’c = 6 ksi
1.0
0.5 0.8
0.5%
0.0
0.5%
0.6%
0.7%
0.8%
f’c = 5 ksi
0.9%
1.0%
1.1%
1.2%
1.3%
1.2%
1.3%
1.4%
1.5%
r = As / Ag
0.6%
0.7%
0.8%
0.9%
1.0%
1.1%
1.4%
 1% Steel
steel is
is generally
Generallyoptimum
Optimum r = As / Ag

Higher
Concrete
is General

At 1%Strength
steel, Column
Capacity
(Pu /Optimum
Ag) is approx. = f’c / 2
1.5%
45
46
Frame-Two-way Slab-Column
47
Beam and Slab
48
Site Cast (Tilt-up)
•
•
•
•
Fast
Quality
Simple
Energy efficient
49
Site Cast (Tilt-up)
• Built-in Veneer
– Embed brick
“facers”
– Reduces time
and costs vs.
brick and
mortar
construction
M Brick
50
Factory Cast (Precast)
•
•
•
•
Fast
Quality
Simple
Energy efficient
51
PT Slabs
52
What if your Shape is Not Standard?
How do you Find the Optimal Form?
53
Study in Topology
Function
Strength
Aesthetic
Hanger, Orbetello, Italy
Why optimize the structure’s
layout?
Optimized structures are
inherently elegant
Gatti Wool Mill, Rome
54
Simple Frames of Least Weight
Mitchell Theorem is satisfied if the bars in a frame are subjected
  Where V is the minimum Volume,
to stresses of the same sign:
Vf   Fi  ri
Bar under two opposed forces
ri is the vector location of vector Force Fi
and f is the allowable stress
Triangular and
tetrahedral frames
Catenaries
Arches
55
Conclusions
• System Selection
• Formwork Systems
• Constructability
• Balance Optimum Design and Constructability
– Lest Material is not always the most Economical
Design
56
Past - Present - Future
57