PSC box girders

4.7.6.1 Segmental superstructure construction

a) The preferred option involving precast segmental sections is that all joints between precast sections must have an insitu concrete connection and must be a minimum of 400 mm wide with lapped reinforcement.

b) Match-cast segmental superstructure construction may be used only in exceptional

circumstances, subject to departmental acceptance by Deputy Chief Engineer (Structures). The criteria listed in Clause 4.11.6.2 shall be used as the basis of a submission to use match cast precast concrete boxes.

4.7.6.2 Design criteria 4.7.6.2.1 General

a) All permanent prestressing shall be internal.

b) Grouting of internal prestressing ducts shall be in accordance with MRTS89.

c) The dimensional tolerance of wide units is critical, as large differences in thickness can result in cracking. As large dimension units can cause instability, the maximum dimension of a precast box shall be 14 m.

4.7.6.2.2 Durability

a) A multi-redundant system shall be used to ensure that there is no water ingress resulting from the use of match cast systems.

b) All concrete shall be cured in accordance with Standard Specification MRTS70. No reduction in curing standard will be permitted.

c) All faces between match cast units shall be coated with wet-to-dry epoxy.

d) All prestressing ducts shall be in accordance with Clause 7.1.6 of Standard Specification MRTS89. (Steel ducts are not permitted).

e) A proprietary seal shall be used on all ducts at each match cast joint.

f) The entire width of the top surface of the deck shall have a 20 mm x 20 mm recess joint between the match cast boxes and sealed with a waterproof joint after stressing.

g) The entire top surface shall be sealed with a high performance waterproof membrane with a grit to ensure grip of the DWS. Bituminous waterproofing to Standard Specification MRTS84 is not permitted.

Chapter 4: Bridge component design requirements

4.7.6.2.3 Design

a) The standards which shall be used in the design of PSC box girders are listed in Table 4.7.6.2.3(a).

b) There will be a residual compression of 1 MPa for ALL serviceability load cases. c) Tapered shear keys shall be used on each match cast joint.

Table 4.7.6.2.3(a) - PSC box girders design standards

Design Standard Section Description

AS 5100.2 All sections Design loads and combinations

AS 5100.5 All sections Concrete Design

AS 5100.5 Appendix B Design of Segmental Bridges

d) The standards which shall be used during the erection stage of match cast concrete box girders are listed in Table 4.7.6.2.3(b)

Table 4.7.6.2.3(b) - PSC box girders loading criteria

Loading Criteria

Dead load As per AS 5100.2

Erection live load 0.5 kPa on all deck surfaces (minimum)

Differential temperature 70% of AS 5100.2 values

Wind load 70% of AS 5100.2 values, with no launching to

be carried out during strong winds. Differential settlement and construction tolerance

allowances between bearing levels

As specified by designer (must be monitored and controlled during construction)

Load factors, limit states As per AS 5100.2

e) The criteria for other types of incrementally launched bridges shall be subject to written agreement by Transport and Main Roads Deputy Chief Engineer (Structures).

4.7.6.3 Additional segmental bridge design standards not covered in AS 5100.

a) AS 5100 and the previous criteria shall be the main reference for design. However, where this standard does not specifically address match cast box girders, AASHTO LRFD may be used. b) The standards which shall be used for segmental bridge design and construction of

prestressed concrete segmental bridges are listed in Table 4.7.6.3

Table 4.7.6.3 - Segmental bridge design standards

Design Standard Section Description

AASHTO LRFD Section 4.6.2.9 Analysis of Segmental Bridges

AASHTO LRFD Section 5.14.2 Segmental Construction

i) AASHTO LRFD Section 4.6.2.9

Chapter 4: Bridge component design requirements

• Specific requirements for transverse analysis of box sections

• Specific requirements for erection analysis ii) AASHTO LRFD Section 5.14.2

• Specific requirements for construction analysis

• Definition of minimum construction loads (out of balance segments, erection equipment, and so on)

• Specific requirements for limiting stresses during construction

• Construction load combinations

• Considerations for provisional post-tensioning ducts and future deflection adjustment

• Specific requirements for minimum section dimensions

• Specific requirements for seismic design

• Specific requirements for segmental joints (precast and insitu joints)

• Specific requirements for balanced cantilever construction

• Specific requirements for force effects due to construction tolerances

• Segmental bridge design detailing requirements

• Specific requirements for segmental bridge substructures.

4.7.6.4 Precast segmental bridge deck analysis

The method of analysis for precast segmental bridges is specifically covered in AASHTO LRFD Section 4.6.2.9. The following issues are to be considered in addition to those required for standard precast girder decks:

• Shear lag (effective flange widths)

• Construction stage analysis

• Prestress stage analysis

• Analysis for time-dependent effects, in particular the concrete segment creep and shrinkage properties which vary with time

• Residual creep effects

• Torsional and distortional warping for box sections and so on

• Construction.

4.7.6.5 Construction

All construction shall be in accordance with Standard Specifications MRTS70, MRTS71, MRTS72, MRTS73 and MRTS89.

4.7.6.6 Lifting of PSC box girders

Lifting points for PSC box girders where applicable shall be designed and RPEQ certified in accordance with either MRTS72 or MRTS73 as applicable by the designer.

Chapter 4: Bridge component design requirements