6 LOAD RATING ANALYSIS

Was previously titled Section 6 - Load and Resistance Factor Rating

6.0  OVERVIEW OF LOAD RATING METHODS AND PROCEDURES

The load rating of existing structures shall be in accordance with Table 2-1.  The order of preference in rating methodologies is: (1) load and resistance factor rating (LRFR), (2) load factor rating (LFR) and (3) allowable stress rating.

C6.0

      Add the following:

In 1993 an agreement was reached between the FHWA and the FDOT concerning the use of allowable stress method for load rating bridges. In summary, the agreement states allowable stress rating is not permitted for bridges on the National Highway System if the bridge is either structurally deficient or functionally obsolete.

 

Table 2-1  Acceptable Load Rating Methodologies

DESIGN

METHODOLOGY

LOAD-RATING METHODOLOGY

Allowable Stress Rating - ASR (Appendix D.6)

Load Factor Rating LFR (Appendix D.6)

Load & Resistance

Factor Rating - LRFR

(Section 6)

Allowable Stress Design (ASD)

(1)

Load Factor Design (LFD)

 

Load & Resistance Factor Design (LRFD)

 

 

(2)

1.) Allowable stress rating is not permitted for bridges on the National Highway System if the bridge is either structurally deficient or functionally obsolete.

2.) Bridges designed using the LRFD methodology before January 7, 2005 may be load rated using either the LFR or LRFR methodologies. For new designs (January 7, 2005 and after), the Department will not allow the use of an alternative load rating methodology (Appendix D.6) or posting avoidance techniques, with the exception of curved steel bridges (see 6.6.1).

The analysis shall include reference to the dated Structures Manual.

6.1  INTRODUCTION

6.1.6 Evaluation Methods (01/07) 2007 Jan: Added new section 6.1.6 Evaluation Methods.

Add the following:

The FDOT preferred load rating program is VIRTIS. VIRTIS should be used if the program is capable of performing the load rating analysis of the bridge. For LRFR load ratings Conspan and Smart Bridge is recommended.

 

 

C6.1.6

Add the following:

At present, VIRTIS  is not capable of performing a LRFR load rating.

 

6.1.7 Load Rating

Delete the last two sentences  and add the following:

The routine FDOT rating process is shown in FDOT Figure 6-1.  Rate bridges designed January 2005 and after using LRFR. For bridges other than prestressed concrete segmental box girders,  designed before January 2005, use Appendix D.6 for rating.  For bridges designed using the LFD methodology before January 2005, LRFR may be used as an alternative.

 

Replace Figure 6-1, Flowchart for Load Rating, with FDOT Figure 6-1.

C6.1.7

Add the following:

The rating process of AASHTO LRFR suggests that each permit vehicle be evaluated individually.  Such is not the case with FDOT or with most other States.  Traditionally, annual blanket permits were issued based upon a comparison of force effects of the permit vehicle in question to that of the HS20 operating rating.  To continue the practice of having information available to easily judge permit applications, FDOTs rating process includes an FL120 permit-load rating as part of the routine rating of bridges.  Single-trip permit vehicles will be evaluated outside of the routine FDOT rating process.

Since Appendix D.6 does not specifically address prestressed concrete segmental box girders, perform all rating analysis  for this bridge type, using LRFR procedures. For this bridge type, a minimum acceptable rating factor of  1.0 is required for all legal loads and the FL120 Permit load.

 

 

6.1.7.1 Design Load Rating

Replace the 3rd sentence of the 1st paragraph with the following:

Under this check, bridges are screened for both the strength and service limit states.

Delete the 4th and 5th sentences of the 1st paragraph.

Replace the 2nd sentence of the second paragraph with the following:

Bridges that have a design load rating factor equal to or greater than 1.0 at the operating level will have satisfactory load rating for all three Florida legal loads.

 

 

6.1.7.2 Legal Load Rating

Replace the 3rd sentence of the 1st paragraph with the following:

Using this check, bridges are screened for both the strength and service limit states as noted in Table 6-1.

Delete the 4th and 5th sentences of the 1st paragraph.

 

6.1.8 Component-Specific Evaluation

Add the following:

Bridges may contain local details that must be appropriately designed to carry local loads or distribute forces to the main bridge components (beams). Although forces in these details can vary as a function of the applied live loads (with the exception of in-span beam splices), it is recommended that they not be included in the load rating. Rather, the capacities of such details should be check only for critical loads or ratings and then only if there is evidence of distress (e.g. cracks).

C6.1.8

Add the following:

Important local details in concrete bridges include diaphragms and details, such as corbels, that support expansion joint devices and anchorages for post-tensioning tendons.  The behavior of these details and the forces to which they are subjected may be determined by appropriate models or hand calculations. Analysis methods and design procedures are available in LRFD (e.g. strut and tie analysis).

6.1.8.3 Diaphragms

The main purpose of transverse diaphragms is to provide lateral stability to girders during construction and wind loading.  

Transverse diaphragms themselves need not be analyzed as part of a routine load rating. Only if there is evidence of distress (e.g. efflorescence, rust stains or buckling), or at the discretion of the engineer, should it be necessary to more closely consider the forces and stresses in a diaphragm.

The stiffness of any transverse diaphragms should be included, if significant and appropriate, in any finite element analysis program used to establish Live Load Distribution Factors.

 

6.1.8.4 Support for Expansion Joint Devices

Expansion joint devices are usually contained in a recess formed in the top of the end of the top slab and transverse diaphragm. Occasionally, depending upon the need to accommodate other details, such as drainage systems, this may involve a corbel - usually as a contiguous part of the expansion joint diaphragm.  It is not necessary to analyze such a detail for routine load rating.  Only if there is evidence of distress (e.g. cracks, efflorescence or rust stains), or at the discretion of the engineer, should it be necessary to more closely consider the forces and stresses in such a detail.

 

6.1.8.5 Anchorages for Post-Tensioning Tendons

Anchorages are normally contained in a widened portion of the web at the ends of a beam. It is not necessary to analyze anchorage details for routine load rating.  Only if there is evidence of distress (e.g. cracks, efflorescence or rust stains) should it be necessary to more closely consider the forces and stresses in such a detail itself.  

Changes in the gross section properties at anchor block zones should be properly accounted for in any finite element analysis program used to establish principal tension/bursting.

 

6.1.8.6 Post Tensioned Concrete Beam Splices within a Span

Beam splices within a span are frequently used to connect portions of continuous girders. Such splices usually require reinforcing bars projecting from the ends of the precast beams and into a reinforced, cast-in-place transverse diaphragm. Longitudinal post-tensioning ducts are connected and tendons pass through the splice.

Beam splices are typically near inflection points; consequently, live load effects may induce longitudinal tensile stress in the top or bottom. Therefore, the longitudinal tendons are approximately concentric, i.e. at mid-depth of the composite section. It is necessary to check longitudinal flexure and shear effects at in-span beam splices.

 

6.1.8.7 Post Tensioned Concrete Beam Dapped Hinges within a Span

Dapped hinges are rarely used in beam bridges in Florida. Forces acting through dapped hinges within a span should be calculated for statically determinate structures or be determined as a part of the time-dependent construction analysis for indeterminate structures.  Maximum live load reactions should also be calculated. Once all reaction forces are known, local analyses should be performed to develop the hinge forces into the main beam components using suitable strut-and-tie techniques. An alternate approach would be to develop three-dimensional finite element models to analyze the flow of forces.

 

6.1.8.8 Bascule Bridges (01/09) 2009 Jan: Added section.

When evaluating Bascule Bridges, assume the main girders act as a cantilever without any benefit from the span locking device.

 

6.1.8.9 Gusset Plates on Truss Bridges (01/09) 2009 Jan: Added section.

When evaluating new and existing truss bridges with gusset plates, follow FHWA Technical Advisory T 5140.29 "Load-carrying Capacity Considerations of Gusset Plates in Non-Load-path-redundant Steel Truss Bridges."

 

6.2  LOADS FOR EVALUATION

6.2.3  Transient Loads

6.2.3.1  Vehicular Live Loads (Gravity Loads): LL

Replace the vehicles given after Legal Loads: with the  following:

Florida Legal Loads (SU4, C5, and ST5, see 6.4.4.2.1 for vehicle configurations).

Replace the vehicle given after Permit Loads: with the  following:

Florida Permit Load (FL120, see 6.4.5.4.2.1 for vehicle configurations). For new bridges the minimum rating factor for the FL120 is 1.0.

 

C6.2.3.1

        Add the following:

For simple span bridges, see figure C6-4 for a comparison of legal loads and HL-93.  

 

6.3 STRUCTURAL ANALYSIS

 

Add the following:

Transverse and longitudinal ratings shall be reported for post-tensioned concrete segmental bridges.  All bridge decks designed with transverse prestressing require transverse ratings. For all other bridges, only longitudinal ratings are typically required.

 

6.3.2  Approximate Methods of Structural Analysis

Add the following:

Approximate methods include  one-dimensional line-girder analysis using LRFD distribution factors.

For bridge superstructures meeting the requirements of LRFD 4.6.2.2, use the approximate live load distribution factors in the initial load rating.

For bridges constructed with composite prestressed deck panels, the live load distribution factors will be increased by a factor of 1.1 thus increasing the load and reducing the capacity.

C6.3.2

Add the following:

Deck superstructures, utilizing composite prestressed deck panels have performed poorly. The deck cracked around the perimeter of the panel and the deck stiffness is softened therefore, a reduction in stiffness occurs. If conditions are severe, the live load distribution can be calculated as if the deck panels are simple supported on the girders.

6.3.3 Refined Methods of  Analysis (01/09) 2009 Jan: Added documentation requirements when using refined methods.

Add the following:

Refined methods of analysis include two or three dimensional models using grid or finite-element analysis.

All analyses will be performed assuming no benefit from the stiffening effects of any traffic railing barrier or other appurtenances.

When a refined method of analysis is used, indicate the name, version, and date of the software used on the FDOT Load Rating Summary Tables.

C6.3.3

Delete the second paragraph of the commentary in its entirety

6.4  LOAD RATING PROCEDURES

 6.4.2  General Load Rating Equation (07/07) 2007 July:  Reworded section to clarify the use of transformed section properties before and after losses.  2007 Jan: Deleted the requirement for refined estimates of time-dependent losses.

Add the following:

When calculating the Service Limit State capacity for prestressed concrete flat slabs and girders with bonded tendons/strands, use  transformed section properties when calculating stresses before losses (at transfer) and after losses (including loss of prestress.)

 

C6.4.2

Add the following:

For a detailed explanation of stress calculations in prestressed concrete girders, see NCHRP 496. The correct use of transformed section properties for calculation of prestress losses is essential for the precise calculation of stresses at service limit state.

6.4.2.2  Limit States

Replace Table 6-1 with FDOT Table 6-1

 

6.4.2.3 Condition Factor

Delete the first sentence.

Add the following after Table 6-2:

The Florida DOT prefers load ratings be performed taking account of field measured deterioration. However, in the absence of measurements, global condition factors shall be used.

 

6.4.2.4 System Factor

Delete the third paragraph.

Replace Table 6-3 with FDOT Tables 6-3A, B, C and D.

Replace the second paragraph with the following:

The system factors of FDOT Tables 6-3A, 6-3B, 6-3C, and 6-3D shall apply for flexural and axial effects at the Strength limit states.  Higher values than those tabulated may be considered on a case-by-case basis with the approval of the Department.  System factors need not be less than 0.85.  In no case shall the system factor exceed 1.3.

 

6.4.4  Legal Load Ratings

6.4.4.1 Purpose

Replace the 1st sentence of the 1st paragraph with the following:

Bridges that do not have sufficient capacity under the design-load rating operating level (i.e. RF  1.0 or less) shall be load rated for the SU4, C5, and ST5 legal loads to establish the potential need for load posting or strengthening.

 

6.4.4.2.1 Live Loads (01/09) 2009 Jan: Replaced entire section.

Replace this article with the following:

For all span lengths, the critical load effects shall be created by:

  •  For all load effects, SU4, C5, and ST5 Florida legal loads defined in Figure 6-3.  Assume the SU4, C5, and ST5 trucks are in each loaded lane; do not mix trucks.

  •  For negative moments and reactions at interior supports, a lane load of 0.2 klf combined with two Type SU4, two Type C5, and two Type ST5 (three load cases), multiplied by 0.75 heading in the same direction separated by 30 ft.

 In addition, for span lengths greater than 200 ft., critical load effects shall be created by:

  •  SU4, C5, and ST5 Florida legal loads, applied separately, multiplied by 0.75 and combined with a lane load of 0.2 klf.

Dynamic load allowance shall be applied to the legal vehicles and not the lane loads.

 

6.4.4.2.3 Generalized Live Load Factors

Revise Table 6-5 as follows:

For all Traffic Volumes, revise all Load Factors to 1.35.

C6.4.4.2.3

       Add the following:

The LRFD HL-93 live-load model envelopes FDOT legal loads.  As such, if the live load factor of 1.35 for the design-load operating rating yields a reliability index consistent with traditional operating ratings, this live load factor can be used for legal-load rating of the FDOT legal loads.

        Live load factors for FDOT legal loads are not specified as a function of ADTT.

 

Figure 6-3

 

FDOT Table 6-1 (01/08) 2008 Jan: Deleted SL(4 ) from Post-tensioned Concrete (Longitudinal) Design Load (Inventory) Load Factor; Changed footnote (1) to refer to FDOT Table 6-9B.

Bridge Type

Direction

Limit State

Load Factors

Permanent Load

Transient Load

Design Load

Legal Load

Permit Load

DC

DW

EL

FR

TU(2)

CR

SH

TG(2)

Inventory

Operating

LL

LL

LL

LL

Steel

Longitudinal

Strength  I

1.25

1.50

n/a

n/a

n/a

n/a

1.75

1.35

1.35

n/a

Strength II

1.25

1.50

n/a

n/a

n/a

n/a

n/a

n/a

n/a

1.35

Service

II (3)

1.00

1.00

n/a

n/a

n/a

n/a

1.30

1.00

1.30

0.90

Reinforced Concrete

Longitudinal

Strength I

1.25

1.5

n/a

n/a

n/a

n/a

1.75

1.35

1.35

n/a

Strength II

1.25

1.5

n/a

n/a

n/a

n/a

n/a

n/a

n/a

1.35

Prestressed Concrete (Flat Slab and Deck/Girder)

Longitudinal

Strength I

1.25

1.5

n/a

n/a

n/a

n/a

1.75

1.35

1.35

n/a

Strength II

1.25

1.5

n/a

n/a

n/a

n/a

n/a

n/a

n/a

1.35

Service III (1)

1.00

1.00

n/a

n/a

n/a

n/a

0.80

0.80

0.80

0.70

Wood

Longitudinal

Strength I

1.25

1.5

n/a

n/a

n/a

n/a

1.75

1.35

1.35

n/a

Strength II

1.25

1.5

n/a

n/a

n/a

n/a

n/a

n/a

n/a

1.35

Post-tensioned Concrete

Longitudinal

Strength I

1.25

1.5

1.00

1.00

0.50

n/a

1.75

1.35

1.35

n/a

Strength II

1.25

1.5

1.00

1.00

0.50

n/a

n/a

n/a

n/a

1.35

Service III (1)

1.00

1.00

1.00

1.00

1.00

0.50

0.80

0.80 or

1.0 SL (4)

0.80 or 1.0  SL (4)

0.70 or 0.90 SL (4)

Transverse

Strength I

1.25

1.50

1.00

n/a

n/a

n/a

1.75

1.35

1.35

n/a

Strength II

1.25

1.50

1.00

n/a

n/a

n/a

n/a

n/a

n/a

1.35

Service I

1.00

1.00

1.00

n/a

n/a

n/a

1.00

1.00

1.00

1.00

(1)  For Service III tensile stress limits, see FDOT Table 6-9B.

(2)  TU and TG is considered for Service I and Service III Design Inventory only.

(3) The Service II limit state need only be checked for compact steel girders.  For all other steel girders, the Strength limit states will govern.

(4)  For I-girders use a load factor of 0.8 (inventory, operating, legal) or 0.7 (permit); for segmental box girders use 0.8 (inventory) or 1.0 and striped lanes (SL) (operating and legal) or 0.9 and striped lanes (SL) (permit).

 

 

FDOT Table 6-3A  General System Factors (φs) (01/09) 2009 Jan: Added note for rolled members.

Superstructure Type

System Factors (φs)

Rolled/Welded Members in Two Truss/Arch Bridges*

0.85

Riveted Members in Two Truss/Arch Bridges*

0.90

Multiple Eye bar Members in Truss Bridges

0.90

Floor beams with Spacing > 12 feet and Non-continuous Stringers and Deck

0.85

Floor beams with Spacing > 12 feet and Non-continuous Stringers but with continuous deck

0.90

Redundant Stringer subsystems between Floor beams

1.00

All beams in non-spliced concrete girder bridges

1.00

Steel Straddle Bents

0.85

* Pertains to type of built-up or rolled members not type of connection.

 

FDOT Table 6-3B  System Factors (φs) for Post-Tensioned Concrete Beams

Number of Girders in Cross Section

Span Type

Number of Hinges Required for Mechanism

System Factors (φs)

Number of Tendons per Web

1

2

3

4

2

Interior

3

0.85

0.90

0.95

1.00

End

2

0.85

0.85

0.90

0.95

Simple

1

0.85

0.85

0.85

0.90

3 or 4

Interior

3

1.00

1.05

1.10

1.15

End

2

0.95

1.00

1.05

1.10

Simple

1

0.90

0.95

1.00

1.05

5 or more

Interior

3

1.05

1.10

1.15

1.20

End

2

1.00

1.05

1.10

1.15

Simple

1

0.95

1.00

1.05

1.10

The tabularized values above may be increased by 0.05 for spans containing more than three intermediate, evenly spaced, diaphragms in addition to the diaphragms at the end of each span.

 

FDOT Table 6-3C  System Factors (φs) for Steel Girder Bridges

Number of Girders in Cross Section

Span Type

# of Hinges required for Mechanism

System Factors (φs)

2

Interior

3

0.85

End

2

0.85

Simple

1

0.85

3 or 4

Interior

3

1.00

End

2

0.95

Simple

1

0.90

5 or more

Interior

3

1.05

End

2

1.00

Simple

1

0.95

The tabularized values above may be increased by 0.10 for spans containing more than three evenly spaced intermediate diaphragms in addition to the diaphragms at the end of each span.

The above tabularized values may be increased by 0.05 for riveted members

 

 

FDOT Table 6-3D  System Factors (φs) for Concrete Box Girder Bridges

Bridge Type

Span Type

# of Hinges to Failure

System Factors (φs)

No. of Tendons per Web

1/web

2/web

3/web

4/web

Precast Balanced Cantilever Type A Joints

Interior Span

End or Hinge Span

Statically Determinate

3

0.90

1.05

1.15

1.20

2

0.85

1.00

1.10

1.15

1

n/a

0.90

1.00

1.10

Precast Span-by-Span Type A Joints

Interior Span

End or Hinge Span

Statically Determinate

3

n/a

1.00

1.10

1.20

2

n/a

0.95

1.05

1.15

1

n/a

n/a

1.00

1.10

Precast Span-by-Span Type B Joints

Interior Span

End or Hinge Span

Statically Determinate

3

n/a

1.00

1.10

1.20

2

n/a

0.95

1.05

1.15

1

n/a

n/a

1.00

1.10

Cast-in-Place Balanced Cantilever

Interior Span

End or Hinge Span

Statically Determinate

3

0.90

1.05

1.15

1.20

2

0.85

1.00

1.10

1.15

1

n/a

0.90

1.00

1.10

For box girders with 3 or more webs, table values may be increased by 0.10.

 

6.4.5  Permit Load Ratings

6.4.5.1 Background

Add the following:

Calculate the capacity for permit trucks using one lane distribution factor for single trip permits and two or more lanes distribution factor for routine or annual permits as shown in Table 6-6. The two or more lanes distribution factor assumes the permit vehicle is present in all loaded lanes and LRFD live load distribution equations are used.  Do not use LRFD formula 4.6.2.2.4-1 since mixed traffic calculations are not performed.

 

 

C6.4.5.1

Add the following:

Florida has chosen to apply a service limit state rating for permitting overload vehicles using load factors that include a reduced reliability factor. The live load factor is applied to a capacity calculated with the rating vehicle placed in all lanes. The load factor was developed to simulate a rating vehicle in the rating lane with adjoining lanes filled with legal vehicles (tractor trailers). The combined effect of these loads is multiplied by the multiple presence factor of 0.9 (Ontario Bridge Code).

6.4.5.2 Purpose

Add the following:

Bridges designed after January 1, 2005 are required to have rating factors for the FL120 permit truck. Rate the FL120 for both Strength and Service Limit State.

 

6.4.5.4.2 Load Factors

C6.4.5.4.2

Add the following:

       Since routine permits are evaluated using the FL120 permit truck and values of ADTT are not well known, a single load factor is specified for routine permit load rating.  Similarly, a single load factor is specified for single-trip permits.

6.4.5.4.2.1 Routine (Annual) Permits

Revise Table 6-6 as follows:

For all Permit Types, revise all the Load Factors by Permit Type to 1.35 except the escorted single trip load factor will remain 1.15.

Add the following:

The FL120 permit truck shall be considered as routine annual permit vehicle to be used to verify overload capacity of Florida bridges. The FL120 shall be checked  at  Strength  Limit State and  Service Limit State as noted in FDOT Table 6-1 and the minimum rating factor for new bridges is 1.0.

For spans over 200 feet assume the FL120 permit truck with coincident 0.20 kips per foot lane load. Assume the permit trucks are in each lane; do not mix trucks.  

The FL120 permit truck configuration is shown in the figure below:

 

   

 

C6.4.5.4.2.1

Add the following:

The FL120 permit truck is conceived to be a benchmark to past load factor design (LFD) practice in which the HS-20 truck was rated at the operating level with a load factor of 1.3.  A LRFR Permit Load rating for the FL120 permit truck equal to 1.0 is equivalent to an LFD operating rating for the HS-20 truck equal to 1.67. The axle spacing of the FL120 is not changed to emulate a truck crane.

It is reasonable to use the multiple-lane distribution factor for the permit load rating since  the force effects of the permit  trucks are similar to  the HL-93 notional load have been shown to be very similar.  Thus, this application is close to the intent of the AASHTO LRFR methodology where the HL-93 is placed in remote lanes.  The FL120 is intended to replicate the traditional HS20 operating rating where all lanes were occupied by the same truck.  Thus, the use of multiple-lane distribution factors is equally appropriate for the FL120 permit load rating.

 

6.4.5.5 Dynamic Load Allowance

End the first sentence after legal loads.

Add the following:

For exclusive-use vehicles with escort and speeds less than or equal to 5 mph, IM may be decreased to 0%.

 

6.4.5.8 Adjoining Lane Loading

When performing refined analysis for permit vehicles, combine the permit vehicle with the same permit vehicle in the adjoining lanes.  For spans over 200 feet, add a 0.20 kip per foot lane load to all vehicle loadings.

 

6.4.5.9 Multiple Presence Factors

For Permit load ratings, the LRFD multiple presence factors shall be equal to or less than 1.0.

 

6.5  CONCRETE STRUCTURES

6.5.2 Material

Add the following:

For concrete made with Florida aggregate calculate the modulus of elasticity by applying a 0.9 factor times the value found in the specifications.

 

6.5.4  Limit States

6.5.4.1 Design-Load Rating (01/07) 2007 Jan: Added "Prestressed deck/girder bridges with a continuous deck but without continuous girders shall be load rated as simple spans."

Add the following:

For prestressed concrete bridges, perform Permit-Load ratings for:

1.  Service I transverse compressive and tensile stress checks in the deck of transversely prestressed bridges.

2.  Service III tensile stress checks in the longitudinal direction of all prestressed concrete bridges.

 

The stress limits given in FDOT Table 6-9B shall be satisfied by all prestressed concrete bridges.

 

Prestressed deck/girder bridges with a continuous deck but without continuous girders shall be load rated as simple spans.

 

C6.5.4.1

Delete the first sentence of the commentary.

6.5.4.2 Legal Load Rating and Permit Load Rating

6.5.4.2.2.1 Legal load Rating

Delete both sentences and replace with the following:

Legal load rating of prestressed concrete bridges is based on satisfying strength and service limit states (see FDOT Table 6-1)

 

C6.5.4.2.2.1

Delete the entire commentary.

 

 

 

 

FDOT Table 6-9B  Stress Limits for Prestressed Concrete Bridges (01/08) 2008 Jan: Added separate criteria for Longitudinal Tensile Stress-Nonsegmental Bridges. Changed Longitudinal Tensile Stress (Design Operating) for Segmental Bridges to 3 sq root f'c (agg. env) and 6 sq root f'c (mod. agg. env). Changed the Principal Tensile Stress for segmental bridges from 3.0 (Design Inv.) & 4.0 (Design Op.) to 3.5 sq root f'c.

Condition

Design Inventory

Design Operating, Legal, and Permit

Compressive Stress - All Bridges (Longitudinal or Transverse)

Compressive stress under effective prestress, permanent loads, and transient loads (Allowable compressive stress shall be reduced according to LRFD 5.9.4.2.1 when slenderness of flange or web is greater than 15)

0.60f'c

0.60f'c

Longitudinal Tensile Stress in Precompressed Tensile Zone -

Nonsegmental Bridges (including Post-Tensioned I-Girders)

For components with bonded prestressing tendons or reinforcement that are subject to not worse than:

 

 

(a) an extremely aggressive corrosion environment.  

3√f'c psi

7.5√f'c psi

(b) slightly or moderately aggressive corrosion environments.

6√f'c psi

7.5√f'c psi

Longitudinal Tensile Stress in Precompressed Tensile Zone -

Segmental Box Girder Bridges

For components with bonded prestressing tendons or reinforcement that are subject to not worse than:

 

 

(a) an extremely aggressive corrosion environment.

3√f'c psi   

3√f'c psi

(b) slightly or moderately aggressive corrosion environments.

6√f'c psi

6√f'c psi

For components with unbonded prestressing tendons

No Tension

No Tension

For components with Type B joints (dry joints, no epoxy)

100 psi comp

No Tension

Tensile Stress in Other Areas - Segmental Box Girder Bridges:

Areas without bonded reinforcement

No tension

No tension

Areas with bonded reinforcement sufficient to carry the tensile force in the concrete calculated on the assumption of an uncracked section is provided at a stress of 0.5fy (<30 ksi)

6√f'c psi tension

6√f'c psi tension

Transverse Tension, Bonded PT - Segmental Box Girder Bridges:

Tension in the transverse direction in the precompressed tensile zone calculated on the basis of an uncracked section (i.e. top prestressed slab) for:

 

 

(a) an extremely aggressive corrosion environment

3√f'c psi  

6√f'c psi

(b) slightly or moderately aggressive corrosion environments

6√f'c psi

6√f'c psi

Principal Tensile Stress at Neutral Axis in Webs  - Segmental Box Girder Bridges:

All types of segmental construction with internal and/or external tendons.

3.5√f'c psi tension

3.5√f'c psi tension

 

6.5.4.2.2.2 Permit load Rating

Delete the first sentence and replace with the following:

Permit load rating of prestressed concrete bridges is  based on satisfying Strength and Service limit states (see FDOT Table 6-1).

Delete the second paragraph.

C6.5.4.2.2.2

Delete the first and second  paragraphs.

Florida has elected to use a service limit state for permit analysis and has removed the check for stress in the reinforcing at the strength limit state.

6.5.7 Minimum Reinforcement (07/07) 2007 July: Revised perTDBC07-03.

Delete equation 6-4 and use LRFD Equation 5.7.3.3.2-1.

Add the following:

See SDG 4.1.5 for clarification of the appropriate application of minimum reinforcing at the ends of simply supported bridge girders.

 

6.5.9 Evaluation for Shear (01/09) 2009 Jan: Revised in response to NRCHP Report 579 and to bring requirements in sync with SmartBridge and reduce the number of Variance requests. 2007 Jan: Added clarification for shear analysis requirements for load rating

Delete the second sentence and replace with the following:

Design and legal loads shall be checked for shear.

Add the following:

For shear load rating using the Strength Load Combinations I & II, use the area of stirrup reinforcement intersecting the plane created by the theta (q ) angle. The plane will extend a distance of 0.5 dv cot ( q ) either side of the design section under review but not past the center-line of the bearing. This concept is shown in LRFD [Figure C5.8.3.2-2].

 

6.5.12 Temperature, Creep and Shrinkage Effects

Delete the sentence and replace with the following:

At the service limit state, all prestressed concrete bridges shall include the effect of uniform temperature (TU), when appropriate), creep (CR), and shrinkage (SH).  In addition, temperature gradient (TG) shall be included for post-tensioned beam and box girder structures. See FDOT Table 6-1 for clarification.

 

 

 

6.6  STEEL STRUCTURES

6.6.1  Limit States (07/07)  2007 July: Per TDB C07-06, Removed section.

 

6.6.4  Limit States

6.6.4.1 Design-Load Rating

Delete both paragraphs and replace with the following:

Bridges shall not be rated for fatigue.  If the fatigue crack growth is anticipated, Section 7 of the Guide Manual for Condition Evaluation and Load and Resistance Factor Rating of Highway Bridges can be used to estimate the remaining fatigue life.

 

C6.6.4.1

       Add the Following:

       The estimate of the remaining fatigue life of Section 7 of the Guide Manual requires a historical record of past truck traffic in terms of average daily truck traffic (ADTT) and projected future traffic.  Many times, conservative recreation and projection of traffic volumes produces a worst case scenario which results in low remaining fatigue lives or totally exhausted fatigue lives.  As fatigue life estimates are based upon statistical evaluation of laboratory tests, different levels of confidence are presented in Section 7.  The minimum expected fatigue life, the evaluation fatigue life and the mean fatigue life are based upon approximately 98%, 85% and 50% probabilities of cracking, respectively.  Judgment must be used in evaluating the results of the fatigue-life estimates.

6.6.13  Fracture-Critical Members (FCMs)

As with all other steel members, the appropriate system factors of FDOT Tables 6-3A or 6-3C shall be applied in the ratings of FCMs.

Steel members which are traditionally classified as FCMs may be declassified through analysis if the material satisfies the FCM fracture-toughness of LRFD Table 6.6.2-2.  After the approval of an exception based upon an approved refined analysis demonstrating that the bridge with the fractured member can continue to carry a significant portion of the design load, the member may be declassified and treated as a redundant member.  See LRFD Article C6.6.2.  After declassification, the member may be rated using a system factor of 1.0.

C6.6.13 (new)    

Only FCMs which are fabricated from material meeting the FCM fracture-toughness requirements are candidates for declassification.  Newer bridges designed, fabricated and constructed since the concept of FCMs was introduced should meet this material requirement.  The demonstration of non-fracture criticality must include an analysis of the damaged bridge with the member in question fractured and a corresponding dynamic load representing the energy release of the fracture.  Acceptable remaining load carrying capacity may be considered equal to the full factored load of the strength I load combination associated with the number of striped lanes.

6.6.14 Double-Leaf Bascule with Span Locks

Evaluate all appropriate load combinations at Strength Limit State II. Apply the full load to the cantilever leaf of the bascule bridge assuming the span locks are not engaged to transmit live load to the opposite leaf.

 

6.8 POSTING OF BRIDGES

Add the following:

Posting avoidance is the application of engineering judgment to a load rating by modifying the specification defined procedures through use of variances and exceptions.

 

6.8.3 Posting Analysis  (01/07) 2007 Jan: Added Section.

Add the following before the existing text:

Before weight limit posting is recommended, posting avoidance strategies should be discussed and approved by the FDOT and may require additional analysis.

 

A.6.1 LOAD AND RESISTANCE FACTOR RATING FLOW CHART

Replace the flowchart with FDOT Figure 6-1.

 

A.6.2 LIMIT STATES AND LOAD FACTORS FOR LOAD RATING

Delete all three tables and use FDOT Table 6-1.

 

B.6.2 AASHTO LEGAL LOADS

       Delete section a) and use the Florida legal trucks defined in article 6.4.4.2.1.

 

D.6 - ALTERNATE LOAD RATING

D.6.1 GENERAL

Add the following paragraph:

Use the 17th Edition of the AASHTO Standard Specification with the allowable stresses shown in FDOT Table 6-9B.

 

D.6.1.4 Application of Standard Design Specifications (01/07) 2007 Jan: Added Section D.6.1.4

Add the following before the existing text:

When using the AASHTO Standard Specifications for Highway Bridges, follow explicitly the guidance in the Specifications. All deviations from the Specifications require approval by the FDOT.

 

D.6.6 NOMINAL CAPACITY

D.6.6.3 Load Factor Method

D.6.6.3.3 Prestressed Concrete (07/07)  2007 July: Revised perTDB C07-03.

After the 5th RF equation, add the following heading:

 Operating Rating

After the last paragraph, add the following:

See SDG 4.1.5 for clarification of the appropriate application of minimum reinforcing at the ends for simply supported bridge girders.

 

 

D.6.7 LOADINGS

 

D.6.7.2 Evaluation for Shear

Delete the last sentence.

 

E.6 RATING OF SEGMENTAL CONCRETE BRIDGES

E.6.2 GENERAL RATING REQUIREMENTS

Add the following:

Six features of concrete segmental bridges are to be load rated at the Design Load (Inventory and Operating) Levels. Three of these criteria are at the Service Limit State and three at the Strength Limit State, as follows:

At the Service Limit State:

   " Longitudinal Box Girder Flexure

   " Transverse Top Slab Flexure

   " Principle Web Tension

At the Strength Limit State:

   " Longitudinal Box Girder Flexure

   " Transverse Top Slab Flexure

   " Web Shear

In accordance with AASHTO LRFR Equation 6-1, the general Load Rating Factor, RF, shall be determined according to the formula:

 

Where:

For Strength Limit States:

C = Capacity = (φc x φs x φ ) Rn.

φc = Condition Factor per Article 6.4.2.3.

φs = System Factor per Article E.6.4.2.4.

φ = Strength Reduction Factor per LRFD.

Rn = Nominal member resistance as inspected, measured and calculated according to formulae in LRFD with the exception of shear, for which, capacity is calculated according to the AASHTO Guide Specification for Segmental Bridges.

For Service Limit States:

C = fR = Allowable stress at the Service Limit State (FDOT Table 6-9B).

 

E.6.8 APPENDIX E6 STEP-BY-STEP SUPPLEMENT

See Section E.6.8.

 

F.6 POSTING AVOIDANCE

See Section F.6.

 G.6 LOAD RATING SUMMARY and DETAIL SHEETS (01/09)  2009 Jan: Revised Prestressed Concrete Bridges Sheet.  2007 Jan: Added new section.

See Section G.6   for the Load Rating Summary detail sheets.