FDOT Exceptions to LRFR | July, 2006
Appendix F - Posting Avoidance |

F.6 POSTING AVOIDANCE (NEW)

The following methods of posting avoidance are presented in an approximate hierarchy judged to return the greatest benefit for the least cost or effort for Florida bridges. This hierarchy is not absolute and may change depending on the particular bridge being load rated.

Under no circumstance shall a posting avoidance technique be used when load rating a newly designed bridge or when calculating permit capacity.

Posting avoidance techniques require either a Variance or an Exception. A Variance must be approved by the FDOT District Structures Engineer with concurrence of the District Structures and Facilities Engineer with a copy sent to the State Structures Design Engineer. An Exception requires the approval of the State Structures Design Engineer and may require notification of the Federal Highway Administration. For bridges where the owner is a local government, concurrence from the bridge owner is required before variance or exceptions are processed by FDOT.

F.6.1 DYNAMIC LOAD ALLOWANCE (IM) FOR IMPROVED SURFACE CONDITIONS (VARIANCE)

Using field observations and engineering judgment for spans greater than 40 feet, the Dynamic Load Allowance may be reduced if the following conditions exist:

• Where the bridge approach and the bridge have a smooth transition and where there are minor surface imperfections or depressions, the Dynamic Load Allowance (IM) may be reduced to 20%.

• Where there is a smooth riding surface on the bridge and where the transitions from the bridge approaches to the bridge deck across the expansion joints are smooth, the Dynamic Load Allowance (IM) may be reduced to 10%. (An example of this would be a deck slab finished by grinding and grooving to remove irregularities with no bumps or steps at expansion joints).

F.6.2 APPROXIMATE AND REFINED METHODS OF ANALYSES (VARIANCE)

When using an approximate method of structural analysis (code defined live load distribution LRFD 4.6.2), a rating factor as low as 0.95 can be rounded up to 1.0.

Refined methods of structural analyses (e.g. using finite elements) may be performed in order to establish an enhanced live load distribution and improved load rating. For fully continuous structures, a more sophisticated analysis of this type does not eliminate the need for a time-dependent construction analysis to determine overall longitudinal effects from permanent loads (e.g. BD 2 analysis).

F.6.3 SHEAR CAPACITY BY AASHTO LRFD FOR SEGMENTAL BOX GIRDER BRIDGES (VARIANCE)

When calculated in accordance with the AASHTO LRFD 5.8.6, the shear capacity, at the strength limit state, is based upon an assumed crack angle of 45 degrees, and may lead to an unsatisfactory load rating. The assumed angle of crack may be reconsidered and the capacity recalculated according to the procedure in this Appendix B of "Volume 10 A Load Rating Post-Tensioned Concrete Segmental Bridges" (Dated Oct. 8, 2004).

F.6.4 EXISTING BRIDGE INVENTORY BEFORE JANUARY 2005 (VARIANCE)

If the bridge load carrying capacity as determined by Service III Limit State is causing unusual hardship and the current bridge inspection is showing no signs of either shear or flexural cracking, the capacity established for load posting and overweight vehicle permitting can be established using Strength Limit State.

F.6.5 SHEAR CAPACITY – SEGMENTAL CONCRETE BRIDGES (BOX GIRDER) - CRACK ANGLE AND PRINCIPAL TENSION (VARIANCE)

To calculate a crack angle more exactly than the assumed 45 degree angle use the specifications, use the procedure found in Appendix B of "Volume 10 A Load Rating Post-Tensioned Concrete Segmental Bridges" (dated Oct. 8, 2004) found on the Structures Design Office internet web site.

F.6.6 STIFFNESS OF TRAFFIC BARRIER (EXCEPTION)

Barrier stiffness should be considered and appropriately included if necessary. Inclusion of the barriers acting compositely with the deck slab and beams should improve longitudinal load ratings. When barriers are considered in this manner, the difference in the modulus of elasticity of the lower strength barrier concrete relative to that of the deck slab and to that of the beams should be taken into account. The presence of joints in a barrier reduces the overall effective section at the joint to that of the deck slab plus beam. This may result in a local concentration of longitudinal stress that should be appropriately considered.

Nevertheless, load ratings should benefit from reasonable consideration of barrier stiffness.

F.6.7 CONCRETE BOX GIRDER - LONGITUDINAL TENSION IN EPOXY JOINTS (EXCEPTION)

The AASHTO Guide Specification for Segmental Bridges and LRFD limit longitudinal tensile stresses to zero at epoxied match-cast joints under Service level conditions. The ability of the epoxy joint to accept tension is not considered. However, in properly prepared epoxy joints the bond usually exceeds the tensile strength of the concrete. Consequently, for posting avoidance, tensile stresses may be accepted as a function of the location and quality of the epoxy joint:

• For top fiber stresses on the roadway surface – no tension is permitted for all load rating calculations.

• For bottom fiber stresses –

(a) Allow 200 psi tension at good quality epoxy joints (i.e. no leaks and fully sealed).

(b) No tension allowed for poor quality epoxy joints (i.e. leaky or not filled, gaps).

F.6.8 CONCRETE BOX GIRDER - TRANSVERSE TENSILE STRESS LIMIT IN TOP SLAB (EXCEPTION)

For Legal and Permit loads, the permissible tensile stress in a transversely post-tensioned slab is set at 6.0√f’c, regardless of the environment (FDOT Table 6-9B). For posting avoidance, up to 7.5√f’c may be allowed providing that:

a) There is sufficient bonded reinforcement to carry the calculated tensile force in the concrete computed on the assumption of an uncracked section at a stress of 0.5fy, and,

b) It is verified by field inspection that there are no cracks in the bridge deck as a consequence of routine or historically heavy vehicular traffic.

F.6.9 CONCRETE BOX GIRDER - PRINCIPAL TENSILE STRESS (EXCEPTION)

If the load rating based upon the limiting principal tensile stress at the neutral axis of the basic beam or composite section is not satisfactory, the rating factor with regard to principal tension may be taken as 1.00 providing that:

a) There is no visible evidence of any representative cracking in the webs.

b) The capacity is satisfactory under the required Strength Limit State.

However, if during field inspection, cracks are discovered at or near a critical section where, by calculation, the principal tensile stress is found to be less than the allowable, then further study is recommended to determine the origin of the cracks and their significance to normal use of the structure. If possible, a check should be made of construction records to determine if there was any change of construction, temporary loads or support reactions that may have induced a significant but temporary local affect.

F.6.10 REDUCED STRUCTURAL (DC) DEAD LOAD (EXCEPTION)

A lower dead load factor may be considered in accordance with the following criteria. Under no circumstance should this load factor be less than 1.10. For the self weight determined by:

a) Design Plan or Shop Drawing dimensions and assumed average density for concrete, reinforcement and embedded

items: γDC = 1.25.

b) As-built dimensions, deck slab thickness and build-up using concrete density determined from construction records, adjusted for weight of embedded reinforcing: γDC = 1.15.

c) Actual beam weights measured during construction: γDC = 1.10.

Cases (b) and (c) may only be used providing that neither additional structural component (DC) nor superimposed dead loads (DW) has been added whose weight cannot be accurately ascertained.

In using either (a) or (b) above, and when it is known that the original design was based on an assumed density for normal concrete and that a check or investigation can verify that a bridge has been constructed with Florida Limerock, then the unit weight may be reduced to 138 lbs per cubic foot for the concrete plus an allowance for the weight of steel.

F.6.11 REDUCED SUPERIMPOSED (DW) DEAD LOAD (EXCEPTION)

The load factor for superimposed dead loads including wearing surface and utilities is normally γDW = 1.50. A lower factor may be considered if weights are determined from an accurate survey. Under no circumstance should this be taken less than γDW = 1.25.