Post-Tensioning Manual 6thEd

8/12/2019 Post-Tensioning Manual 6thEd

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POST-TENSIONING MANUAL

This chapter provides basic information on cable stays. It isnot intended as a comprehensive design resource. Fordesign requirements and additional information see thePTI Recommendations for Stay Cable Design, Testing and

Installation. - 1

13.2 ENGINEEREVG OF STAY CABLE STRUCTURES

13.2.1 Design Elements and Responsibility

The design of cable stays covers five essential elements.1. Design of cable stays for static loads and fatigue2. Design against stay vibration3. Design of anchorage details4. Design of corrosin protection features5. Design of cable erection procedure

The design responsibility for elements 1 to 4 is typicallyshared between the bridge Design Engineer and the staysupplier. The responsibility for element 5 is normally alsoshared with the contractor.The licensed design professional is normally expected to perform and be responsible for all design-related aspectsof the overall structure. Included are such stay-relatedtems as: Specifying design and performance requirements for

the cables which are applicable for the particularstructure; often the requirements follow the PTI Rec-ommendations for Stay Cable Design, Testing and

Installation ]3A

Detailing cable arrangement and basic anchorage provisions; this includes such constructability con-siderations as space requirements for anchorages,installation and stressing equipment; it also includesstructural feasibility to remove and replace cables ifthis is a design objective

Determining cable sizes and forces Specifying anchorage placement and assembly tolerances Design of connection details to support the stay

anchorage and toensure the force transfer from the anchorage into the

main structure; for steel structures this includes thedesign of force transfer members into the main

structure; for concrete structures this includes thedesign of confinement and bursting reinforcement inthe anchorage zone

The supplier of stay cables is typically responsible for thecable hardware, including: Design and testing of the cable system and its com-

ponents to meet the design and performance require-ments of the contract documents

Quality control and quality assurance of hardwarecomponents

Furnishing to the licensed design professionaldetailed shop drawings for all cable components,testing records and quality control documentation

Cable hardware performance in accordance with thecontract documents

The contractor normally carnes the prime responsibilityfor the cable installation. However, cable installation andstressing require careful planning and monitoring, andinvolve all parties. The contractor may subcontract thecable installation to a specialty contractor, who often isalso the supplier.

13.2.2 Construction Engineering for Cable-Stayed Structures

Stay cable erection engineering for construction stages anassociated geometry control is an essential task for successful construction. The requirements for an engineerederection program are described in the PTI Recommendations.13' The basic elements of an engineered construction program include:

Establishing permissible construction equipmentloads

Developing a system of temporary works for instal-lation of stays and deck elements

Developing an erection cycle for girder and stayswhich controls strength requirements for intermedateconstruction stages

Developing a program for stay erection whichensures that the completed structure achieves proper

deck profile and acceptable cable forces Developing procedures to adjust cable forces if nec-essary to obtain the correct geometry profile of the bridge deck at the end of construction

13.3 STAY CABLE DESIGN

13.3.1 Design Methodology

The Third Edition of the PTI Recommendations for StayCable Design, Testing and Installation 2 uses the conven-tional allowable stress design approach for designing sta

cables. Fatigue was considered by providing allowablestress ranges for various numbers of load cycles. Section13.3.2 outlines the allowable stress requirements.The Fourth Edition of the PTI Recommendations for StayCable Design, Testing and Installation nA is based on the

AASHTO LRFD Bridge Design Specification. ]3A It uses thestrength design methodology and applies it to the design of

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POST-TENSIONING MANUAL

I Cost per Linear Foot ofSpandrel

I Cost per Square Foot ofSpandrel

Barrier Cable

Masonry

Cast-m-place

Precast at Site

|$125Precast

$80$0 $20

Fig. 16.2 Cost of Exterior Barrier Systems

This chart uses the following configuration in its cost com- parisons:

Barrier cable system consists of 11 cables Masonry and cast-in-place spandrels are 42 in. high and

built on the slab

Precast spandrels are 60 in. high and extend over the

edge of the slab16.2 BUILDING CODE REQUIREMENTS

IBC outlines requirements for parking garage barrier sys-tems in Section 406.2. This section includes requirementsfor the barrier system to meet two (2) distinct objectives: Pedestrian protection (Section 406.2.3) Automobile restraint (Section 406.2.4)

While the Licensed Design Professional will typically only be concerned with the barriers that are necessary for auto-mobile restraint, the locations requiring vehicle barriers

will, in most cases, require pedestrian protection as well.Given this condition, it is logical to design a single barriersystem that meets both requirements, as discussed in thefollowing sections.16.2.1 Pedestrian ProtectionBarrier systems for pedestrian protection are required atexterior and interior vertical openings where vehicles are parked or moved, and along open-sided walking reas orramps, when the vertical distance to the ground or surface below exceeds 30 in. (762 mm). The pedestrian barrier system(guard) must meet the physical requirements of IBCSection 1003.2.12.

This section states that the guard must form a protective barrier not less than 42 in. (1067 mm) high, "measuredvertically from the leading edge of the tread or adjacentwalking surface." Openings in the guard must be limitedsuch that a 4 in. (102 mm) diameter sphere cannot passthrough any opening up to a height of 34 in. (864 mm)Above a height of 34 in. (864 mm) a sphere of 8 in. (203mm) cannot pass through the opening(s).This section also outlines the minimum loading requirements for guards for pedestrian protection; however thesloads are not discussed herein as they represent only a smafraction of the load capacity required for vehicle barriers.

16.2.2 Automobile Restraint

IBC Section 406.2.4 requires vehicle barriers not less tha24 in. (607 mm) in height to be placed at the ends of drivlaes and at the end of parking spaces where the differencin adjacent floor elevation is greater than 12 in. (305 mm)Vehicle barriers of all types must meet the physicalrequirements of IBC Section 1607.7.This section states that barriers for garages designed fo passenger cars are to be designed to resist a single (unfactored) load of 6000 Ibs (26.70 kN) applied horizontally iany direction to the system. For design purposes, the codassumes the load to act at a minimum height of 18 in. (45mm) above the floor surface on an rea not to exceed 1 sq (0.09 m2).Barriers for garages that accommodate trucks and buseare to be designed in accordance with an approved methothat contains provisions for larger vehicles. Depending on

$100 $120 $140

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Post-Tensioning Manual 6thEd

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Transcript of Post-Tensioning Manual 6thEd