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FDOT Exceptions to LRFR |
July, 2006 |
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Appendix E.6 Rating Segmental Concrete Bridges |
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E.6 RATING OF SEGMENTAL CONCRETE BRIDGES
E.6.8 STEP-BY-STEP SUPPLEMENT (NEW)
E.6.8.1 Load Factors and Load Combinations
Load factors and load combinations for the Strength and Service Limit States shall be made in accordance with FDOT Table 6-1. Load factors for permanent (e.g. dead) loads and transient (e.g. temperature) loads are provided. Note: one-half thermal gradient (0.5TG) is used only for longitudinal Service Inventory conditions.
STRENGTH I and II and SERVICE I and III limit states are used in the context of their definitions as given in FDOT Table 6-1 summarizing:
STRENGTH I - applies to Design Load Rating (Inventory and Operating) and Legal Load Rating.
STRENGTH II - applies only to Permit Loads.
SERVICE I - applies primarily for concrete in compression but is also to prevent yield of tension face reinforcement or prestress under overloads (permits). This limit state is extended to concrete tension in transversely prestressed deck slabs, typical of most segmental bridges.
SERVICE III - applies to concrete in longitudinal tension and principal tension. Load factors for SERVICE III for Design Operating, Legal, and Permit ratings have been selected in conjunction with either higher allowable tensile stress or, in the case of joints in segmental bridges that cannot carry tension, use of the number of striped lanes.
The following is a detailed checklist of the load applications, combinations and circumstances necessary to satisfy FDOT and AASHTO LRFR ratings.
E.6.8.2 Design Load Rating - Inventory
Transverse:
Apply HL93 Truck or Tandem (FDOT Table 6-1).
Do not apply uniform lane load.
Apply same axle loads in each lane if multiple lane loading apples.
Apply Dynamic Load Allowance, IM = 1.33 on Truck or Tandem.
For both Strength and Service Limit States, use number of load lanes per LRFD.
Apply multi-presence factor: one lane, m =1.20; two lanes, m = 1.00; three, m = 0.85; four or more, m = 0.65. (Maximum value of m = 1.20 is the appropriate AASHTO LRFD / LRFR current criteria to allow for rogue vehicles).
Place loads in full available width as necessary to create maximum effects.
Apply pedestrian live load as necessary (counts as one lane for m).
Apply no Thermal Gradient transversely.
Use SERVICE I Limit State with live load factor, γL = 1.00 and limit concrete transverse flexural stresses to values in FDOT Table 6-9B. (Note: = 1.00 as AASHTO LRFR).
For STRENGTH I Limit State use live load factor, γL = 1.75.
Longitudinal:
Apply HL93 Truck or Tandem, including 0.64 kip/ft uniform lane load (FDOT Table 6-1).
Apply same load in each lane.
Apply Dynamic Load Allowance, IM = 1.33 on Truck or Tandem only.
For both Strength and Service Limit States, use number of load lanes per LRFD.
Apply multi-presence factor: one lane, m =1.2; two lanes, m = 1.00; three, m = 0.85; four or more, m = 0.65. (Maximum value of m = 1.20 is the appropriate AASHTO LRFD / LRFR current criteria for notional loads and rogue vehicles).
For negative moment regions: apply 90% of the effect of two Design Trucks of 72 kip GVW spaced a minimum of 50 feet apart between the leading axle of one and the trailing axle of the other, plus 90% of uniform lane load.
Place loads in full available width as necessary to create maximum effects.
Apply pedestrian live load as necessary (counts as one lane for m).
For Thermal Gradient, apply 0.50TG with live load for Service but zero TG for Strength.
Use SERVICE III Limit State with live load from striped lanes and limit longitudinal tensile stress to values in FDOT Table 6-9B as appropriate.
For STRENGTH I Limit State use live load factor, γL = 1.75.
E.6.8.3 Design Load Rating - Operating
Transverse:
Apply one HL93 Truck or Tandem per lane (FDOT Table 6-1).
Do not apply uniform lane load.
Apply same axle loads in each lane if multiple lane loading apples.
Apply Dynamic Load Allowance, IM = 1.33 on Truck or Tandem.
For both Strength and Service Limit States, use number of load lanes per LRFD.
Apply multi-presence factor: one and two lanes, m =1.0; three, m = 0.85; four or more, m = 0.65. (Maximum limit of 1.0 applies because this is a rating for specific (defined) axle loads, not notional loads or rogue vehicles).
Place loads in full available width as necessary to create maximum effects.
Apply pedestrian live load as necessary (counts as one lane for m).
Apply no Thermal Gradient transversely.
Use SERVICE I Limit State with live load factor, γL = 1.00 and limit concrete transverse flexural stresses to values in FDOT Table 6-9B
For STRENGTH I Limit State use live load factor, γL = 1.35.
Longitudinal:
Apply HL93 Truck or Tandem, including 0.64 kip/ft uniform lane load (FDOT Table 6-1).
Apply same load in each lane.
Apply Dynamic Load Allowance, IM = 1.33 on Truck or Tandem only.
For the Strength Limit State, use number of load lanes per LRFD.
For the Service Limit State use the number of striped lanes.
Place loads in full available width as necessary to create maximum effects (for example, in shoulders).
Multi-presence factor: HL93 Design Load (including uniform lane load) one lane, m =1.20; two lanes, m = 1.00; three, m = 0.85; four or more, m = 0.65. (The maximum value of 1.20 for one lane is necessary because the load is a notional load with a uniform lane load component).
For negative moment regions, apply 90% of the effect of two Design Trucks of 72 kip GVW each spaced a minimum of 50 feet apart between the leading axle of one and the trailing axle of the other, plus 90% of 0.64 kip/LF uniform lane load.
Apply pedestrian live load as necessary (counts as one lane for m).
Apply no Thermal Gradient.
Use SERVICE III Limit State with live load factor developed from striped lanes and limit concrete longitudinal flexural tensile and principal tensile stresses to values in FDOT Table 6-9B.
For STRENGTH I Limit State use live load factor, γL = 1.35.
E.6.8.4 Legal Load Rating
Longitudinal:
Apply FDOT Legal Load Trucks SU4, C5 and ST5 (FDOT Table 6-1).
Apply same truck load in each lane using only one truck per lane (i.e. do not mix Trucks).
Apply no uniform lane load.
Apply Dynamic Load Allowance, IM = 1.33 on Legal, HL93 Truck or Tandem.
For the Strength Limit State, use number of load lanes per LRFD.
For Service Limit States, use number of striped lanes.
Place loads in full available width as necessary to create maximum effects (i.e., in shoulders).
Use multi-presence factor: one and two lanes, m = 1.00; three, m = 0.85; four or more, m = 0.65.
Apply no pedestrian live load (unless very specifically necessary for the site - in which case it counts as one lane for establishing m).
Apply no Thermal Gradient.
Use SERVICE III Limit State with live load factor developed from striped lanes and limit concrete longitudinal flexural tensile and principal tensile stresses to values in FDOT Table 6-9B.
For STRENGTH I Limit State, use live load factor, γL = 1.35.
Negative moments load ratings may be limited by AASHTO LRFR 6.4.4.2.1. If the value of the Rating Factor for the AASHTO Limiting Critical Load is less than 1.00, then the basic rating factor for all FDOT Legal Loads shall be reduced by multiplying by this value. See Appendix B.6.2(c) for load model.
E.6.8.5 Permit Load Rating
Longitudinal, annual blanket permits:
Apply ONE Permit Vehicle (FL120) in all lanes (FDOT Table 6-1).
For spans over 200 feet, apply a uniform lane load of 0.20 kip / LF in the lane with the permit vehicle. This uniform lane load should be applied beyond the footprint of the vehicle to create the maximum effects. However, for convenience, it may be applied coincident with the vehicle.
For the Strength Limit State, use number of load lanes per LRFD.
For Service Limit States, use a reduced load factor or see FDOT Table 6-1.
Place loads in full available width as necessary to create maximum effects (for example, in shoulders).
Use multi-presence factor: one and two lanes, m = 1.00; three, m = 0.85; four or more, m = 0.65.
Dynamic Load Allowance, IM = 1.33 on Permit Trucks.
Apply no pedestrian live load (unless very specifically necessary for the site - in which case it counts as one lane for establishing m).
Apply no Thermal Gradient.
Use SERVICE III Limit State with live load developed from striped lanes and limit concrete longitudinal flexural tensile and principal tensile stresses to values in FDOT Table 6-9B as appropriate.
For STRENGTH II Limit State, use live load factor, γL = 1.35.
Reduced Dynamic Load Allowance (IM) or live load factor (γL) may be considered only to avoid restrictions.
E.6.8.6 Capacity Strength Limit State
The capacity of a section in transverse and longitudinal flexure may be determined using any of the relevant formulae or methods in the LRFD Specifications, or AASHTO Guide Specification for Segmental Bridges dated 1999, including more rigorous analysis techniques involving strain compatibility. The latter should be used in particular where the capacity depends upon a combination of both internal (bonded) and external (unbonded) tendons.
For Load Rating, the capacity should be determined based upon actual rather than specified or assumed material strengths and characteristics. Concrete strength should be found from records or verified by suitable tests. If no data is available, the specified design strength may be assumed, appropriately increased for maturity. All new designs will assume the plan specified concrete properties. Post construction will include updated concrete properties.
In particular, for shear or combined shear with torsion, the capacity at the Strength Limit State for segmental bridges should be calculated according to the AASHTO Guide Specification for Segmental Bridges. The Modified Compression Field Theory of LRFD may be used as an alternative, but only for structures with continuously bonded reinforcement (e.g. large boxes cast-in-place in cantilever or on falsework).
E.6.8.7 Allowable Stress Limits Service Limit State
Allowable stresses for the Service Limit State are given in FDOT Table 6-9B. The intent is to ensure a minimum level of durability for FDOT bridges that avoids the development or propagation of cracks or the potential breach of corrosion protection afforded to post-tensioning tendons. Also, these are recommended for the purpose of designing new bridges.
E.6.8.7.1 Longitudinal Tension in Joints
Type A Joints with Minimum Bonded Reinforcement
The Service level tensile stress is limited to 3√fc or 6√fc (psi) for cast-in-place joints with continuous longitudinal mild steel reinforcing for Design Inventory Rating. (Reference: AASHTO Guide Specification for Segmental Bridges and LRFD Table 5.9.4.2.2-1). Reduced reliability at Design Operating, Legal and Permit conditions is attained by using the number of striped lanes and by allowing an increase in tensile stress to 7.5√fc (psi) (FDOT Table 6-9B).
Type A Epoxy Joints with Discontinuous Reinforcement
The Service level tensile stress is limited to zero tension for epoxy joints for Design Inventory, Design Operating, Legal, and Permit ratings. (Reference: AASHTO Guide Specification for Segmental Bridges and LRFD Table 5.9.4.2.2-1). Reduced reliability is attained by using the number of striped lanes.
Type B Dry Joints
Early precast segmental bridges with external tendons and non-epoxy filled, Type-B (dry) joints were designed to zero longitudinal tensile stress. In 1989, a requirement for 200 psi residual compression was introduced with the first edition of the AASHTO Guide Specification for Segmental Bridges. This was subsequently revised in 1998 to 100 psi compression. Service Level Design Inventory Ratings shall be based on a residual compression of 100 psi for dry joints. For Design Operating, Legal, and Permit Ratings, the limit is zero tension. (Reference: AASHTO Guide Specification for Segmental Bridges and LRFD Table 5.9.4.2.2-1). Reduced reliability is attained by using the number of striped lanes.
E.6.8.7.2 Transverse Tensile Stress
For a transversely prestressed deck slab, the allowable flexural stresses for concrete tension are provided in FDOT Table 6-9B: namely, for Inventory 3√fc or 6√fc (psi) and for Operating 6√fc (psi).
E.6.8.7.3 Principal Tensile Stress Service Limit State
A check of the principal tensile stress has been introduced to verify the adequacy of webs for longitudinal shear at service. This is to be applied to both for the design of new bridges and Load Rating. The verification, made at the neutral axis, is the recommended minimum prescribed procedure, as follows:
Sections should be considered only at locations greater than H/2 from the edge of the bearing surface or face of diaphragm, where classical beam theory applies: i.e. away from discontinuity regions. In general, verification at the elevation of the neutral axis may be made without regard to any local transverse flexural stress in the web itself given that in most large, well proportioned boxes the maximum web shear force and local web flexure are mutually exclusive load cases. This is a convenient simplification. However, should the neutral axis lie in a part of the web locally thickened by fillets, then the check should be made at the most critical elevation, taking into account any coexistent longitudinal flexural stress. Also, if the neutral axis (or critical elevation) lies within 1 duct diameter of the top or bottom of an internal, grouted duct, the web width for calculating stresses should be reduced by half the duct diameter.
Calculate principle tension without the effect of thermal gradient.
Classical beam theory and Mohrs circle for stress should be used to determine shear and principal tensile stresses. At the Service Limit State, the shear stress and Principal Tensile Stress should be determined at the neutral axis (or critical elevation) under the long-term residual axial force, maximum shear and/or maximum shear force combined with shear from torsion in the highest loaded web, using a live load factor, γL = 1.00. The live load should then be increased in magnitude so the shear stress in the highest loaded web increases until the Principal Tensile Stress reaches its allowable maximum value (FDOT Table 6-9B).
The Service Limit State Rating Factor is the ratio between the live load shear stress required to induce the maximum Principal Tensile Stress to that induced by a live load factor of 1.00.
E.6.8.8 Local Details
Local Details (i.e. diaphragms, anchorage zones, blisters, deviation saddles, etc.) in concrete segmental bridges are discussed in Chapter 4 of Volume 10A Load Rating Post-tensioned Concrete Segmental Bridges. If a detail shows signs of distress (cracks), a structural evaluation should be performed for the Strength Limit State. The influence of anchorage zones shall be checked for principal tension in accordance with Structure Design Guidelines Section 4.5.11, Principal Tensile Stresses.