Design of a Concrete Terraced Structure
Transcript of Design of a Concrete Terraced Structure
Design of a Concrete Terraced Structure
Vipsanius Incorporated
“Designs to stand the test of time”
Meet the Team
Project Manager: Josh Prines
Structural Team: Roberto Cintron, Kwaku Boampong, Nisarg Thakkar, Hal
Hamilton Jr.
Lower Hill Redevelopment Proposal
Architectural rendering courtesy of Bjarke Ingles Group
(BIG)
28-acre masterplan; 1,200 residential units, 1 million SF of retail
and commerce space
An attempt to combine a green network of walking and
bicycle paths with the urban street grid of downtown
Pittsburgh
Promote a sense of sustainability and community; reversing a
trend of vacating property in the area
Project Overview
Site Overview
Project Scope
Complete design of all above-grade structural members. The following items will be cast-in-place
on site:
Beams
Columns
Stairwell Shear Walls
Elevator Shear Walls
Floor Slabs
Construction Management
Complete Construction Schedule
Above-grade construction estimate
Return on Investment Analysis
Geotechnical will be subcontracted
Advantages of Using Concrete
Fire Resistance
Affordability
Material Availability
Durability
Floor Layouts
Structural Framing Layouts
Dead Loads
Risk Category: II (ASCE 7-
10, Table 1.5-1)
Slab:
7.5” thickness
Lightweight Concrete
(115 pcf)
Cast-in-place
Dead Loads (Cont.)
ASCE 7-10, Chapter C-3
Interior non-load bearing walls
6” thickness
Exterior load bearing wall
8” thickness – 87 psf
Mechanical Allowance – 5 psf
Elevator Enclosure Wall
8” thickness -100 psf
Minimum required 3” for fire resistance (2 hours)
Dead Loads
Live loads
Base values (ASCE 7-10, Table 4-1)
“Ordinary flat, pitched, and curved roofs”: 20 lb/ft²
“Private rooms and corridors serving them”: 40 lb/ft²
“Roofs used for roof gardens” (patios): 100 lb/ft²
Determined for each column and girder
Reduced according to sections 4.7.2 and 4.8.2 of ASCE 7-10
Tributary Areas
Main Wind Force Resisting System
Consists of the stairwell and elevator shafts
Moment frames were considered but
they’re not the MWFRS
Stairwell needed 5” thickness for fire
protection
Non-seismic region so shear walls were
sufficient
Wind loads
ASCE 7-10 Directional Procedure
(Chapter 27)
Complex building geometry
Conservative methodology to assign
appropriate wind pressures
Biggest Challenge: Torsion
Wind Load Cases (ASCE 7-10)
Four different load cases
Only considering the cases with torsion
(Case 2 and Case 4)
Case 2: Uses 75% of the design wind
pressure acting on the principal axis with the
torsional moment as shown.
Wind Load Cases (ASCE 7-10)
Case 4: 56% of the design wind pressure
acting on both faces of the building
simultaneously.
Underestimated eccentricity from this load
case
LATERAL LOAD ANALYSIS FOR BUILDINGS
WITH SETBACK By Victor W.-T. Cheung and
W. K. Tso, M. ASCE
Wind Design
Used tributary areas for each floor to
determine a resultant force per floor
Used the lever arm to determine the torsion
on the MWFRS
Torsion results
El. (ft) Torsion(kip-ft)
12 1262.59425
24 1145.19375
36 1046.440125
48 952.9305
60 880.228125
72 420.948
Total 5708.33475
Wind Load Design
We also used an article LATERAL LOAD ANALYSIS FOR BUILDINGS WITH
SETBACK By Victor W.-T. Cheung and W. K. Tso, M. ASCE
Used to properly calculate the eccentricities for both faces of the building
Also to apply shear per floor to analyze the shear walls
Snow Load Overview
Acts vertically on the structure
Supported by the columns
Each floor experiences a portion of the load due to stepped patios
Wind forces cause a drift on stepped side of building
Snow Load Analysis (ASCE 7-10;
Chapter 7)
Seismic Load Analysis (ASCE 7-10;
Chapters 11 & 12)
Seismic Load creates a gravity load and lateral load on structure
Torsion & Deflection
Seismic Design Category A
Determine base shear of structure using the equivalent lateral force
analysis
Load generated by the inertia of the mass of the structure
Redistributed to each level as a point load at the center of mass of the
structure
Broad Design
• Two-way and One-way slab (only corridor)
• Don’t need East-West horizontal members in corridor area
• Beam & Girder will be in same plane
• They will share tributary area
• Both will transfer load to column
• Lateral force resisting system
• Elevator shaft and shear walls
Slab Design
• Lightweight Concrete (115 pcf)
• Designed for Deflection (Service limit state)
• Check for Shear and Flexure
•Simple
•Useful where spans, loads, load distributions, and member sizes and proportions fall in usual range
Setting Upper
Limits to Span-Depth Ratio
• Complex
• Compared to predicted value imposed by codes
• Approximate
Calculate Deflection
Slab Design
One-Way
Short Direction
Flexural Reinforcement to resist entire load
Long Direction
Minimum steel for temperature and shrinkage effect
Two-Way
Flexural Reinforcement in
both direction
Slab Design
Effect of Edge beam on slab thickness
α-factor (Beam to slab stiffness ratio)
Slab thickness is uniform throughout irrespective of loading.
For Fire and Heat – 5”
One-way portion - 4.6”
Two-way portion - 7.5”
Minimum thickness required (International Building Code) - 3.5”
Selected thickness - 7.5”
Sufficient for seismic
One-Way Slab
Critical Section
Minimum thickness- l/28 = 4.3”
Correction factor when using
light weight concrete : 1.65 – 0.005w >= 1.09
Minimum thickness – 4.6”
One-Way Slab
Chosen thickness- 7.5”
Load Combination:
1.2 w(d) + 1.6 w(l) = 271.45 psf
Maximum moment based on moment coefficients and clear span
Mu As reinforcement
As is so small so Min As governs
Check for shear and crack control
Calculation for One-Way
One-Way Slab Cross Section
# 4 bar at spacing of 8” (spacing governed by crack control) (short direction)
# 4 bar at 15” o.c. (min reinforcement for temp. & shrinkage) (transverse direction)
Two-Way Slab
Critical Section
Biggest span
25’*25’
Minimum thickness (ACI):
ln/33 (interior) (without beams)
ln/30 (corner, exterior) (without beams)
When beams are present thickness reduces by 15%
Effect of edge beam (αf-factor) (Beam to slab stiffness ratio)
Light weight concrete correction factor
7.5”
Two-Way Slab
Chosen thickness- 7.5”
Direct-Design Method for
Moment distribution
Statical Moment (Mo)
Two-Way Slab Calculation
East-west Slab strip 5
A5 B5 C5
l1 25 20
ln23.3333333
318.3333
3
l2 25 25
A_inf 625 500
reduced live load (ksf) 0.034 0.034
Mo 279.6173.097
2
end mid end mid end
coeff -0.16 0.57 -0.7 -0.7 0.57 -0.16
neg & pos moment (kip-ft) -44.736 159.372 -195.72 -121.16898.6654
2 -27.6956
Sum of column moments (kip-ft) 44.736 70
27.69556
Two-Way Slab Cross Section
Moment Distribution to slab and beam
Mu As reinforcement
Column Strip – 5 #5 bars
Middle Strip – 10 #5 bars
Horizontal members: Design
Microsoft Excel spreadsheet
Base assumptions
4-k/in2 concrete
60-k/in2 steel
Equal reinforcement at top and bottom of beam
Includes formulas from ACI manual
Also includes references in ACI manual for easy corroboration
Optimized with Solver
Then manually rounded up to standard bar sizes
Some integrated integer constraints are included
Spreadsheet Workings
IF(input,output_if_true,output_if_false), OR(input_1,input_2,...)
Strength reduction factor φ=IF(εc>0.005,0.9,IF(εc<0.002,0.65,0.65+(εc-0.002)*250/3))
MAX(input_1,input_2,...), MIN(input_1,input_2,...)
Used in combination
Maximum transverse-reinforcement spacing smax=MIN(d/2,24,ph/8,12)*IF(VS>4*√(fc')*b*d,0.5,1)
Solver
Number minimized: Total materials, weighted by tensile strength
Adding 1 in3 of 60-k/in2 steel = adding 15 in3 of 4-k/in2 concrete
First, use “Ignore integer constraints” checkbox to reach optimal solution without encountering
errors
Integers: Base, height, 2 × transverse-reinforcement spacing
Then, reënable integer constraints to ensure constructability
Manually round bar sizes up to next standard size
Horizontal member cross-sections
Uniform Section Unique Sections
Horizontal member design
Column Design
Columns are vertical structural members designed to support axial
compressive loads, with or without moments
Design to support axial compressive load, with moments
Supported Loads:
Dead & Live Load from floors above
Live Roof Load
Snow Load
Critical load determined using ASCE 7-10, LRFD load combinations
Columns are extended into foundation
Column Design
Alternatives
Tied Columns
Rectangular
Typically used in low seismic regions
Economical
Spiral Columns
Circular
Improved ductility and strength
Expensive
Column Design Procedure
(Tied Columns)
Determine critical load on column, Pu
Select trial size and trial reinforcement ratio
based on material properties
Properties:
f’c = 4 ksi
fy = 60 ksi
Determine steel ratio using interaction
diagram (use critical load and moment for
diagram)
Select reinforcement using new steel ratio
Check the max load capacity, ΦPn > Pu
Select ties (ACI Code Section 7.10.5)
Column Design Summary
Shear Walls
(Overview)
One shear wall resisting in lateral force wall
each stairwell
Height of wall above ground = 72’
Length of wall = 20’
Wall extends into foundation
Designed to resist shear & flexure, axial load,
and moment/torsional effects
Steel reinforcement in front face and back face
In both vertical and horizontal directions
Shear Wall
(Design Procedure)
Determine shear force on each floor
Select trial rebar and reinforcement ratio based on material properties
Minimum percentage of reinforcement in ACI Code Section 11.9.9.4
Check factored moment strength
Maximum moment is at the base
Accounts for factored axial load (ACI Code Eq. 9-6)
Check factored shear strength
Maximum shear strength is at the base
Shear Wall Design Summary
Shear Wall Reinforcement
(Cross Section)
Elevator Shear Wall
ACI 318-11
Reinforced Concrete Mechanics and Design
6th edition
Results
Vertical Reinforcement
2 rows of #8 rebar @ 18” O.C. & 2 #8 bar @ 4” O.C for support at the corners
Horizontal Reinforcement
2 rows of #5 rebar @ 18” O.C
Market Analysis and Net Operating
Income
*Number shown above is the Hard Cost only Pittsburgh (2012) = $178.23
*Both Soft Costs and Land Costs are excluded Pittsburgh (2016) = $184.86
*Rental Statistics from 2014*
Expected Budget for Construction
High-Rent Luxury Apartments in
Pittsburgh, PA
Aria Cultural District Lofts (7th Street Across from North Shore)
$2.2/sqft (1BR/1BA) – 684-972 sqft
The Encore on 7th Apartments (7th Street Across from North Shore)
$2.3/sqft (1BR/1BR) – 800-1000 sqft
Flats on Fifth (5th Ave – Crawford Roberts Hill)
$2.5/sqft (1BR/1BA) – 661-814 sqft
$2.3/sqft (2BR/2BA) – 1054 sqft
Source: Apartments.com
Construction Budget
Quantity Take-off
&
Estimate
Quantity Take-off
Quantities only include above-grade quantities
Subgrade work subcontracted
Superstructure Concrete = 5048 cy
Structural Framing, Exterior Walls, Shear Walls
Reinforcing = 126 tons; 161,500 LF
#3, #4, #5, #8, #9, #11 bars
C.S.I. Divisions 1-16 (Excluding 2-Site Work)
Source: Design Cost Data Archives
Concrete Labor Costs
Source: Construction Estimating 3rd Edition (David Pratt)
Construction Estimate
Lower Hill Redevelopment Project 48,000 SF
Return on Investment: 7.4%
1. Fees, Permits, Field Supervision, Insurance, Temporary Utilities
2. Pilings, Earthwork, Utilities
3. Formwork, Cast-in-place, Reinforcement
4. -
5. Miscellaneous
6. Rough Carpentry
7. Insulation, Damp proofing, Caulking
8. Doors, Frames, Hardware, Aluminum Windows & Doors
9. Metal Studs, Plaster, Drywall, Carpet, Painting, Staining
10. Signage, Fire extinguishers, Mailboxes
11. Appliances
14. Elevators (1)
15. Basic Materials, Fire Protection, Plumbing, HVAC
16. Basic Materials, Wire Conduit, Panel Boards, Fixtures, Fire Alarms
(Labor Included in Values)
Construction Schedule
Start Date: 2/18/16 Project Completion: 3/17/17 282 Work Days; Approximately 13 months
Questions?