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ECONOMICS OF POST-TENSIONED BUILDINGS

ECONOMICS OF POST-TENSIONED BUILDINGS. Bijan O Aalami Professor Emeritus, San Francisco State University Principal, ADAPT Corporation. Cost Analysis of Concrete Skeleton. Structural Frame Floors Walls Columns Foundations. Assumptions of Economic Model.

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ECONOMICS OF POST-TENSIONED BUILDINGS

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  1. ECONOMICS OF POST-TENSIONED BUILDINGS Bijan O Aalami Professor Emeritus, San Francisco State University Principal, ADAPT Corporation

  2. Cost Analysis of Concrete Skeleton • Structural Frame • Floors • Walls • Columns • Foundations

  3. Assumptions of Economic Model • Parking structures will serve as basis of comparison due to minimal architectural • Features and/or • Function • To simplify model, considerations due to location have been specifically excluded; for example: • Availability and/or • Construction practice • Focus of analysis is • the relationship of • Layout • Material costs • relative costs of different layouts

  4. Reference Codes • The quantities quoted reflect practice in the United States • Governing Codes • ACI Building Code • Unified Building Code (UBC)

  5. Benchmark Building Lakeshore Tower, Irvine, California

  6. Project Data Total Gross Sq. Meter (GSF) = 55,101m2 (593,102 ft2) Total Elevated Sq. Meter (ESF) = 48,060 m2 (517,314 ft2) Total Cost of Parking Structure = $11,700,000 Total Number of Cars = 1,837 Number of Levels = 8 Area (SF) per Car = 30 m2/car (323 ft2/car) Cost per Car = $6,369/car Cost per Gross Sq Meter (/GSF) = $212.34/ m2 ($19.73/ft2) Cost per Elevated Sq Meter (ESF) = $243.46/m2 ($22.62/ft2) Owner/Developer : Birtcher Development Contractor : Arciero Brothers, Inc. Architect : Wayne Banks & Assoc. Structural Engineer : Bijan, Florian & Assoc., Inc.

  7. Cost Reference • Current construction practice for development in California require multilevel parking structure to be: • Integral within building proper • Or alongside the primary building • Lakeshore Towers parking Structure is • Eight level concrete frame • One-half level below grade • Rectangular in layout • Unobstructed by site constraints • Therefore, selection of frame and other items was governed primarily by cost considerations

  8. Future Cost Considerations • Anticipated Service Life • Replacement • Maintenance • Operations

  9. Cost Analysis Practice • Cost Per Car: • From and owner’s and/or developer’s standpoint, the cost per car stall is of primary interest. This is the entire cost of construction divided by the total number of parking stalls created. • Cost Per Elevated Square Meter (ESF): • A concrete frame contractor or subcontractor generally expresses its expense items in terms of cost per square meter (cost per square foot) of suspended floors, in order to arrive at a comparative cost value. The floor slab area constructed is used as a common denominator for all items, such as façade cost, even though some may not directly relate to the floor areas.  • Cost Per Gross Square Meter (GSF): • Mechanical, electrical and some other installations are generally given in terms of their unit cost per total floor area, including the slab-on-ground.

  10. Cost Analysis of the Benchmark Building Statistical Values • Framing • Parallel beams (One-way system) • 16 inches wide • 36 inches deep • 18 feet apart • Span length = 64 feet over two bays • Columns • 24 inches square • Façade (precast panels) • 60 inches high • 5 inches thick • Lateral System • Six shear walls around perimeter and interior form • Seismic Zone IV • Foundation (5 ft below grade) • Continuous strip footings • Grade beams • Dry, Non-Corrosive Environment

  11. Cost Components of Reference Parking Structure

  12. Cost Items of Reference Parking Structure

  13. Specialized Consultants • Geotechnical • Mechanical Engineers • Electrical Engineers • Civil Engineers • Architectural • Structural Engineers

  14. Formwork and Material Cost with Respect to Total Cost of Concrete Frame

  15. Concrete Frame Cost of Concrete Frame Components with Respect to Concrete Frame Cost

  16. Concrete Frame Formwork Cost with Respect to Concrete Frame

  17. Concrete Frame Rebar Requirement in Structural Members Post-Tensioning Requirement in Structural Members

  18. Concrete Frame Reinforcing Requirements of the Framing Systems (kilograms per elevated sqm)

  19. Concrete Frame Cost Comparison of the Framing Systems with Respect to Material Used ($ per elevated sqm)

  20. Concrete Frame

  21. Vertical Pedestrian Access • Hydraulic Driven Elevators (Up to seven levels) • $10,000/stop • Minimum $40,000/elevator • Gear Driven Elevators • $18,000/stop • Minimum of $100,000/elevator

  22. Plumbing Minimal drain-off plumbing required due to sloped slab; therefore, low-cost item. During the design stage, drainage of floor levels does not typically receive due attention Adequate slope of not less than 21 mm per meter (.25 inches per foot), to drain the floor slabs at all locations is recommended. The slab drainage of the Reference Structure amounted to $2.15/GSM ($/square meter of gross area of the structure) ($0.20/GSF). De-watering of below grade areas and storm drains were excluded from this work.

  23. Ventilation • Ventilation is required for enclosed or partially enclosed areas. • The cost for ventilating a level of a parking structure is measured in $/cubic meter ($/cubic feet) of the structure. • Costs ranging from $3.50/m3 ($123.60/ft3) to $5.30 m3 ($187.17/ft3) of volume of garage are common. • An average of $4.60/ m3 ($162.45/ft3) of volume of the structure is typically used for budgeting of a project. • Overall ventilation cost for below grade and enclosed areas of the Lakeshore Structure amounted to $2.15/GSM ($75.93/ft3).

  24. Electrical • It is not uncommon for electrical material and installation cost to reach $6.50/GSM ($69.97/GSF). This includes lighting on all levels, temporary power and traffic control. • Parking facilities with interior enclosed spaces or below grade parking demand higher electrical cost. • The electrical costs for the Reference Parking Structure are recorded at $8.50/GSM ($91.49/GSF), due to electrical supply to auxiliary utility rooms.

  25. Fire Protection • The installation of a continuous water supply to provide a fully sprinkled parking structure including standpipes, adds approximately $8.60/GSM ($92.57/GSF) to $9.70/GSM ($104.41/GSF) to the cost of the structure.

  26. Testing and Inspection • Material testing is performed on a regular basis to confirm minimum material strength and performance criteria. • Independent testing agency • Tasks • Record construction execution • record whether or not construction progress is in compliance with design drawings and specifications. • Fee (2-3% of Total Construction Costs) • Testing • Inspection

  27. Finish • “Finish” category relates to final work of concrete frame contractor • Executed after completion of frame • General items included • Grouting of Stressing Pockets • Pour-strip closure • Grouting of temporary releases • Expansion joint installation • Joint caulking • Sag and patch • Grinding • Clean-up • Strict quality control can reduce costs significantly

  28. Miscellaneous • Delivery and Installation of: • Guard posts • Wheel stops • Doors • Finish Hardware • Glass • Etc.

  29. Paint • The following elements at interior locations are painted white to improve visibility ($1.72/GSM) • Columns • Walls • Beams • Striping of car stalls ($1.07/GSM)

  30. Durability • Some durability improvements • Water proofing • Crack mitigation and control • Restricted concrete mix designs • Use of concrete additives • Restricted criteria for concrete curing • Increased concrete cover to reinforcing steel • Epoxy coated mild reinforcement • Encapsulated post-tensioning

  31. (a) FLAT PLATE EXAMPLES OF TWO-WAY SLAB CONSTRUCTION Max. Span 8 m Limiting Criterion Punching Shear Rebar 1.08 kg/m 2 PT 2.84 kg/m 2 Max. Span 11.8 m Limiting Criterion Rebar Congestion Rebar 2.15 kg/m 2 5.09 kg/m PT 2 (b) FLAT SLAB WITH SQUARE COLUMN CAPITALS Max. Span 12.2 m Limiting Criterion Deflection Rebar 2.94 kg/m 2 3.87 kg/m PT 2 (c) FLAT SLAB WITH DROP PANELS Max. Span 13.4 m Limiting Criterion Rebar Congestion Rebar 2.01 kg/m 2 PT 4.16 kg/m 2 (d) FLOOR WITH SLAB BAND Max. Span 12.8 m Limiting Criterion Rebar Congestion Rebar 3.18 kg/m 2 PT 1.76 kg/m 2 (e) WAFFLE SLAB

  32. EXAMPLES OF ONE-WAY CONSTRUCTION Beam Spans 18-20 m Slab Spans 5.5-6.0 m Slab Thickness 125-150 mm Beam Depth 750-900 mm Beam Width 400-460 mm 2 Rebar 4.44 kg/m 2 PT 2.68 kg/m (a) ONE-WAY BEAM AND SLAB Beam Spans 13.7 m Limiting Criteria Rebar congestion in beam Joist Span 12.80 m 2 Rebar 5.38 kg/m 2 PT 2.69 kg/m 2-12 mm PT strands per joist (b) JOIST SLAB Beam spans comparable with joist slab. Joists may be spaced up to 1400 mm with two strands in each joist. (c) SKIP JOIST SLAB

  33. Lateral Load Resisting System • The design and cost of the lateral load resisting system for wind/seismic forces depend greatly on the geographic location. • Common systems are: • shear wall systems, • ductile moment resisting frames and • columns/slab or column and beam frames in moderate seismic regions.

  34. Facade • Two primary elements affect the cost of construction significantly • the construction method/material used • the total perimeter length of parking structure relative to the slab area.

  35. The safety barrier required at the building perimeter ranges in price from $6.40/m($1.98 /foot) to $40.00/m ($12.19/foot)..

  36. Configurations • Barrier Cables • (7 strands at 152.4mm oc (6.0 inches oc))   • Site Cast Spandrels • (Shape = continuous x 1,524mm x 127mm (60.0” x 5.0") • (Covers the elevated slab edge) • Masonry • (Shape = continuous x 1,066.8mm x 203.2mm) • (Above the elevated slab) • (Covers the elevated slab edge) • Pre-Cast Spandrels • (Shape = continuous x 1,524 mm (feet)x 127mm) (5.0 inches) • Cast-in-Place Spandrel • (Shape = continuous x 1,066.8mm(42.0 feet) x 127mm(5.0 feet)) • (Above the elevated slab)

  37. Equating Spandrel Cost to Cost/ESM, Using Constant Elevated Slab Area with Varying Perimeter Cost of Exterior Facade Or Barrier

  38. Quantities

  39. Typical Beam and Slab Parking Structure Framing 60' 60' 60' (18.3m) 30" (760mm) (a) ELEVATION BEAM COLUMN 18' OR 24' (5.5m OR 7.3m) (b) PLAN STUDY SPANS: 18ft & 24ft (5.5m & 7.3m) 16" TYPICAL (400mm) SLAB THICKNESS: 5" & 5.5" (18' SPAN) (125mm & 140mm) 6.5" & 7" (24'SPAN) (165mm & 180mm) (c) SECTION THROUGH SLAB

  40. Typical Flat Slab Parking Structure Framing (18.3m) 60' (5.0m) 16'6" 16'6" (5.0m) 27'0" (8.2m) 8" (200 mm) (a) ELEVATION: TRANSVERSE DIRECTION 28' TYP TRANSVERSE (8.5m) FRAME LONGITUDINAL (b) PLAN (c) ELEVATION: LONGITUDINAL SECTION

  41. Nonprestressed Reinforcement for Detailing

  42. Typical Quantities for Two-Way Slab Systems

  43. Typical Quantities for One-Way Slab Systems

  44. Thank you for listening

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