Post on 31-Mar-2015
D&C PT Contractor’s Role & Responsibilities
Chris M. Harris
Contracts Manager, Structural Systems
& PTIA Technical Committee
Introduction
• Why do PT contractors get involved in the design of the project to start with?
• What are the common efficiencies introduced to the structure by a PT contractor?
• Integration of the PT contractor into the design team.• What aspects of the existing design will the PT
contractor review/change• Examples• Conclusion
Why do PT contractors get involved in the design of the project to start
with ?
• Commercial desire
• Specialist engineers, design draftspersons, & design software
• Risk & reward allocation
• Time
• Constructability
What are the common efficiencies introduced to the structure by a PT
contractor ?
• Element thicknesses
• Support optimisation
• Slab structure type optimisation & formwork
• Reinforcement rationalisation
Integration of the PT contractor into the design team.
• Definition of structure performance
• Documentation status
• Coordination
• Design Program
• Certification
What aspects of the existing design will the PT contractor
review/change?
• Depends on the type of project
• Ieff / Igross (for buildings)
• Concrete grade / volume / thickness
• Structure type (flat plate / flat slab / banded)
• Restraint design
• Rebar content
Eureka Tower
• Conforming design
• Reinforced slabs
Eureka Tower
• Non conforming design• PT bands and slabs• Driving force TIME
Conclusions
• Project Engineers to coordinate with contractors
• Involve a PT contractor early
PT in buildingsPT is likely to be considered for most projects.
If the consultant doesn’t consider it, then the contractors will
Why PT?• Savings in materials• Savings in construction
time
• Improved durability and robustness
• Cost savings
Some perceived down side issues
• Time consuming to design by the occasional designer ?
• PT applies real force into a structure hence analysis (incl. secondary effects) needs to be realistic and specific.
Some perceived down side issues
• System and process needs to be understood and details coordinated sufficiently
• Finding most economical proportions of concrete, PT & reinforcement can be a lengthy iterative process, but usually yields substantial benefits. Software & experience is key.
Other effects to consider• Secondary effects in all
indeterminate PT structures, but can be beneficial
• Structural movements (creep, shrinkage, thermal)
• Structural restraints (walls, stability elements)
• Dynamic behaviour (may be a lighter, livelier structure)
If doubt exists, talk to a specialist PT designer (or the PTIA)
PT design is not different however...
• Effective & efficient design does require experience
• With sound engineering knowledge and practice this can be done successfully.
• Assistance and guidelines can normally be obtained from the specialist contractors
Design philosophy
• Consider the general concept of partially prestressed concrete
• Appropriate proportions of reinforcement and prestressing steel depends on the application (restrained v. unrestrained concrete)
The D&C route for PT delivery
The usual variants here are:
• D&C by tender invitation
• D&C by alternative design
If D&C is expected and planned for, time efficiencies and
innovations can occur
D&C Design BriefA Post Tensioning Design Brief is
critical for a successful D&C arrangement.
It should focus on the performance requirements on the design The extent of D&C area required
The loads and load paths
Member sizes with acceptance tolerances
Serviceability and durability requirements
Movements and Jointing strategy
Define the Design, certification & approval process
Performance requirementsAs far as possible indicate:Architectural requirements (tolerances, finishes etc)Site constraints and constructability issuesPour sequence & any limitations on edge stressing accessLimitations of the supporting structure.Any frame actions that need to be considered in floor design.Expected areas of restraint and strategy for its management incl. location & types of pour strips &/or temporary movement joints &/or permanent building joints.Roles and responsibilities in the design processInspections and installation signoffsQuality and safety requirements.
Types of PT Contract
•Supply and Install (S&I)–Structural design, documentation, inspection & certification by engineer–Shop drawings, install, QA by specialist contractor
•Design and Construct (D&C)–structural floor design, documentation by specialist contractor–Install, inspection & certification by specialist contractor–Engineer remains responsible for vertical elements, stairs, wind & earthquake design, etc.
D & C Tender Information•Post Tensioning Price
•Conventional Reinforcement Quantities
•Concrete Properties and Profiles
•Structural Design Criteria
•Detailed Scope of Work
•Client Responsibilities
•Contract Conditions
Design Considerations for PT Price
•Pour Size
•Strand Diameter
•Number of Strands per Tendon
•Couplers
•Efficient Approval Process
Concrete Properties and Profiles
• Concrete outlines and grade needs to be provided with PT price for client to determine total cost of solution.
• Be aware of concrete testing & strength requirements for initial & final stressing, these are essential for performance of concrete & safety at PT anchorages.
Structural Design Certificate
• Loads
• Durability Requirements
• Deflection Criteria
• Analysis Assumptions
Detailed Scope of Work
• Design Deliverables for D&C, usually:– Concrete outlines– PT drawing (design & shop in one)– bottom reinforcement drawing– top reinforcement drawing– sections & details
• Construction Services
Client / Builder Responsibilities
• Site Amenities• Supply of Power and Water• Lifting of our Materials on to the Decks• Supply and Install of Conventional Reinforcement
including Studrails where applicable• Concrete Infill of Top Surface Stressing Pans• Anything else (job specific)
Contract Conditions
• Reference to Standard Conditions of Contract
• Reference to EBA
• Variation Rates
• Payment Terms
• Retentions
• Liquidated Damages
• Insurances
Project RelationshipsDeveloper
Head Contractor
Formwork Sub-Contractor
ArchitectStructural Engineer
Post Tensioning D & C
Subcontractor
Reinforcement Supplier Steel Fixer Concreter
Timely Co-ordinationCo-ordination between:
•services consultants
•architect
•structural engineer
•builder’s services (alimaks etc)
all by others.
Co-ordination between:
•structural engineer
•pt design
shared by engineer & specialist pt designer.
Timeliness & information sharing is critical.
Late information will lead to re-work & additional costs.
Failure to co-ordinate will lead to re-work & additional costs.
Acknowledgements
John Fairweather
Frank Filippone
Jamie Mackenziewho are the authors of most of these slides.
However any errors are mine.