Means and Methods for

Superstructure Evaluation

CG Gilbertson

March 22, 2011

Overview of load rating procedure

• Bridge Load ratings:

• Establish the safe load capacity

• Should be reviewed following inspections

• Are controlled by the weakest link

• Weakest link might not be within the superstructure – consider substructure

• Have a lower target reliability index to reflect reduced exposure period, consideration of site realities, and the economic considerations of rating vs. design

Superstructure

• In this workshop we will look only at

the bridge superstructure

• If other portions of the bridge system

are suspect, they should be fully

analyzed and considered in the load

rating calculations.

When are bridges load rated?

When a bridge is new and has not been rated

the bridge has had a significant alteration that may affect the capacity of the bridge

the bridge has incurred damage that affects the capacity

a key component of the structure has deteriorated such that the previous load rating is no longer valid

a request has been made to permit an overload vehicle to use the bridge

Qualifications

• The person charged with overall responsibility for load-rating bridges should have:

• PE

• 5-years bridge design/inspection experience

• Knowledge and skills for proper evaluation

Ref: The Manual for Bridge Evaluation

Governing References

• The Manual for Bridge Evaluation, Second Edition, 2011

• Bridge Analysis Guide, 2005

Edition w/ 2009 Interim Updates

MDOT Construction and

Technology Support Area

Additional References

• AASHTO LRFD Bridge Design Specifications, Customary U.S. Units: 5th Edition with 2010 Interim Revisions

• AASHTO Standard Specifications for Highway Bridges: 17th Edition

Overview of load rating procedure

• Three rating methods are available:

• Allowable Stress Rating (ASR)

• Load Factor Rating (LFR)

• Load and Resistance Factor Rating (LRFR)

• LFR was the previous standard for MDOT & the National Bridge Inventory System, HOWEVER, LRFR is now required for bridges designed with LRFD (designed after October 1, 2010)

General Procedure

• Determine applied loads

• Determine capacity of member*

• Conduct load rating calculations

• Submit results to MDOT

* Member capacity MUST include the affects of

deteriorated or damaged sections

Documentation

Hand calculations need to be well documented with appropriate references, inspection reports, and test data used to perform calculations

When software is used, input and output files should be printed and checked for “reasonableness” with at least rough hand calculations

Assumptions need to be clearly identified as well as the underlying reason for such assumptions.

Required Background Info

Design/construction info from original

design plans and as-built plans

Detailed information gathered during

bridge inspections

Information on modifications to original

structure (deck overlays for example)

Distress or deterioration to the structural

components

Four Components to Load Ratings

Federal Inventory

Federal Operating

Michigan Operating

Overload (Permit) Vehicles

Note: In LRFR these are referred to as:

Design (Federal)

Legal (Michigan)

Permit (Overload)

Load Rating Calculations

• Inventory

• Corresponds to customary design level of

stresses

• Incorporates existing conditions

• Results in a live load which can safely utilize

structure for an indefinite period of time

(HS-20 Truck) (NOTE: HL-93 for LRFR)

Load Rating Calculations

• Operating

• Describes maximum permissible live load for

bridge

• Limited load applications

• Loading at this level may shorten life of bridge

(HS-20 or HL-93 +Michigan’s 28 legal loads)

General Equation

Reserve Capacity for Live Load

Maximum Legal Live Load RF =

• The rating factor (RF) is useful in

understanding the factor of safety

on the bridge.

General Equation (LFR)

)(2

1

ILA

DACRF

RF = Rating factor for live load capacity

C = Capacity of the member

D = Dead load on member

L = Live load on member

I = Impact factor

A1 = Factor for dead load

A2 = Factor for live load

LFR Load Factors

• For both inventory

and operating:

• A1 = 1.3

• For inventory:

• A2 = 2.17

• For operating:

• A2 = 1.3

General Equation (LRFR)

RF = rating factor

C = capacity

γDC = LRFD load factor for structural components and attachments

DC = dead load effect due to structural components and attachments

γDW = LRFD load factor for wearing surfaces and utilities

DW = dead load effect due to wearing surfaces and utilities

γP = LRFD load factor for permanent loads other than dead loads = 1.0

P = permanent loads other than dead loads

γLL = evaluation live load factor

LL = live load effect

IM = dynamic load allowance

𝑅𝐹 =𝐶 − 𝛾𝐷𝐶 𝐷𝐶 − 𝛾𝐷𝑊 𝐷𝑊 ± 𝛾𝑃 (𝑃)

𝛾𝐿𝐿 (𝐿𝐿 + 𝐼𝑀)

LRFR Resistance Factors

Condition Factor

Good or Satisfactory – 1.00

Fair – 0.95

Poor – 0.85

System Factor

Reflects level of redundancy in structural

components

Table 6A.4.2.4-1

LRFR Load Factors

Load factors vary with:

Bridge type

Limit state

Type of vehicle

Rating level of evaluation (Design, Legal, Permit)

Table 6A.4.2.2-1 is provided in AASHTO

Manual for Bridge Evaluation to identify the

proper load factors

Live Load Factors in LRFR:

Table 6A.4.2.2-1

LRFR Load Factors (MI trucks)

The live load factors vary with the

average daily truck traffic and the weight

of the truck being analyzed

Tables 4a-1 through 4a-6 of the BAG

provide the live load factors for each of

the MI legal load vehicles for normal,

designated, and special designated

loadings, and ADTT of 5000, 1000, and

100

Table 4a-1 (BAG)

Finding Loads

• Dead Load

• Compute according to existing conditions

• Material unit weight must be at least the

value specified in the AASHTO Design

Specs.

• Composite beams:

• Separate loads acting on non-composite and

composite sections

Finding Loads (Cont.)

• Dead Load (Cont.)

• Be careful with decks built prior to 1965 due

to significant deck thickness variations

• Nominal dead load values should be based

on plan dimensions with allowance for

construction tolerances

Finding Loads (Cont.)

• Live Load

• HS-20/HL-93 Truck or AASHTO Design

Specifications lane load

• 28 Michigan Legal Trucks

• Wheel Load – If the deck is the weakest link,

this is normally not the case, wheel loads

are used to find the bending moment stress

of the deck

Finding Loads (Cont.)

• Truck Loads

• Number of loaded lanes and transverse placement of wheels in accordance with AASHTO Design Specs. and:

• Roadway widths 18-20 ft, 2 design lanes, each half the width, centered live load

• Widths less than 18 ft, 1 design lane

• Fewer traffic lanes may be warranted depending on traffic moments and volume

Finding Loads (Cont.)

• Normal

• Lowest set of max loadings that apply to all

roads in Michigan

• Designated (typical for counties and cities)

• Local or state owned roads that allow loads

heavier than normal

• Special Designated

• Typically applies to interstate roadways

Finding Loads (Cont.)

• Lane Loads

• AASHTO lane loads may be used for all span

lengths where it will result in greater effects than

the standard truck.

Load effects

• BAG Tables • Simple spans

• HS-20 truck

• 28 Michigan legal trucks

• Moment and shear effects

• AASHTO Manual for Bridge Evaluation • Simple spans, trusses, through girders

• HS-20 truck

• HL-93 truck

Overload Trucks

BAG Tables

Simple spans, 5 to 300 feet

Class A, B, and C

20 vehicles for each class

Moment and shear effects

How to use the Tables (BAG: Chapter 10)

• Determine the controlling vehicle

• Search all vehicles listed in tables for the largest

moment or shear effect of each truck type

(be sure you are in the proper table)

• This vehicle will produce the lowest rating factor

(but not necessarily the lightest vehicle weight)

• We will talk about determination of lightest vehicle

weight (posted weight) later

How to use the Tables (Cont.)

Controlling 2-Unit Truck

Load Distribution

• The amount of live load transferred to a

single member of the bridge structure

should be in accordance with current

AASHTO Design Specs.

Impact Affect

• The affects of impact should be accounted

for in the load rating assessment

• For LFR

• Determine in accordance with current AASHTO

Design Specs

• For LRFR analysis:

• IM = 33% of live load effect

• May be eliminated for permit vehicles if they are

restricted to less than 10 mph.

Other Loads

• Deflection

• Do not consider except in special

circumstances

• Longitudinal

• Operating level

• Reduce speed if inadequate

• May be needed to evaluate substructure

Other Loads (Cont.)

• Wind

• Only for moveable bridges, suspension

bridges, high-level structures

• Thermal

• Only for long-span bridges, concrete arches

Other Loads (Cont.)

• Environmental

• Operating level

• Stream Flow

• Only if critical for structure stability

• Ice Pressure

• Only for substructure

Capacity of Member

• Nominal capacity based AASHTO Specifications

• Structural steel

• Reinforced concrete

• Prestressed concrete

• Load factor methods for timber and masonry are

not available – use ASR or LRFR

Condition of Bridge Members

• Bridge ratings should be based on

recent and thorough bridge inspections

• Adequate data should be available to

evaluate the affect of damage.

• Description

• Location

• Extents

It is EXTREMELY important

to consider distress!

Map Cracks Hairline Cracks

Spalling or

Delamination

Narrow Cracks w/

Water or Corrosion Water Stains at

Joints

Longitudinal Cracks

on Deck

Common Forms of Deterioration

Common Forms of Deterioration (Cont.)

Medium Cracks w/o

Water

Evidence of

Displacement

between Beams

Medium Cracks with

Water or Corrosion

Wide cracks w/ Water

or Corrosion

Spalling w/ Exposed or

Corroded Reinforcement Shear or Flexure

Cracking

Incorporating Distress into Load

Rating

Material and Structural Distress

• Material Related Distress

• Chemical attack

• Weathering

• Structural Distress

• Spalls

• Cracks

• Corrosion

• System malfunction (shear keys, diaphragms)

Incorporating Distress

• Material Distress

• Change f’c to represent actual concrete

strength

• May use core tests to establish

Incorporating Distress (Cont.)

• Structural distress

• Change cross-sectional properties to reflect loss of section

• Moment of inertia

• Area of beam

• Area of reinforcement

• Don’t forget the location on the beam (moment is critical near mid-span, shear at supports)

Structural Distress Example

rating design distressA A A

2( )rating design distress distressI I I A d

Impact of Distress on Load Rating

Description of Distress

Inventory

Load

Rating

Percent of

Original

Rating

Controlling

Equation

No Distress 1.26 100% Prestress

steel tension

Spalling of concrete (5.9%) 0.97 77.0% Prestress

steel tension

Spaling of concrete (5.9%)

Corrosion of strands (30%)

0.72

57.1%

Flexural

Strength

Impact of Distress on Load Rating

Legal-Load Truck

Max Weight (U.S. Tons)

1-unit 42

2-unit 77

3-unit 82

Note: Above table represents

Designated Loading

Posting Load Restrictions

• Determine which vehicles have a load effect greater than allowable, they cannot safely use the bridge at max load • Set RF=1 and solve for the moment/shear

• Check tables for trucks exceeding this moment/shear

• The “controlling” truck in this group is the one that has the largest M/W or V/W ratio

• Divide the allowable moment/shear by this ratio to determine the max weight of that truck type under the operating/legal load level

Posting Load Restrictions (Cont.)

• The posted limit is the total weight on the

controlling truck which will produce the

allowable load effect

Posting Load Restrictions (Cont.)

)(2

1

ILA

DACRF

RF = Rating factor for live load capacity

C = Capacity of the member

D = Dead load on member

L = Live load on member

I = Impact Factor

A1 = Factor for dead load

A2 = Factor for live load

Use the same equation and values used to conduct the load

rating. Substitute the controlling truck for the live load. Multiply

the result by the weight of the truck to get the maximum

allowable weight for that particular truck classification.

Basic Submittal Process

• Owner or appointee enters data into the

Online Inventory Inspection Program

• MDOT manages database

• MDOT submits reports to the Federal

Highway Administration for the National

Bridge Inventory Records

Records

• All assumptions used in the calculations

must be clearly identified at the

beginning of the load rating

• A summary table should be provided

with the load rating calculations to

clearly summarize the results

References:

Bridge Analysis Guide, 2005 Edition with 2009 Interim Update. MDOT Construction and Technology Support Area

The Manual for Bridge Evaluation, Second Edition, AASHTO, Washington DC. (2011)