RECOMMENDATIONS FOR THE USE OF SPREAD …docs.trb.org/prp/13-4352.pdf · Survey results indicate...

RECOMMENDATIONS FOR THE USE OF

SPREAD FOOTINGS ON SOILS TO SUPPORT HIGHWAY BRIDGES

By:

Naser M. Abu-Hejleh, Ph.D., P.E (Corresponding Author)

Geotechnical Engineering Specialist

FHWA Resource Center

4749 Lincoln Mall Drive, Suite 600, Matteson, IL 60443

Ph. 708-283-3550; Fax: 708 283 3550; E-mail: [email protected]

Khalid Mohamed, PE., PMP

Principal Bridge Engineer - Geotechnical

FHWA Headquarter

1200 New Jersey Avenue SE, Washington, DC 20590

Ph: 202-366-0886; Email: [email protected]

Daniel Alzamora, P.E.

Geotechnical Engineer

FHWA Resource Center

12300 W. Dakota Avenue, Lakewood, CO 80228

Phone: (720) 963-3214; E-Mail: [email protected]

Submitted to:

92th Transportation Research Board Annual Meeting

Washington, D.C.

No. of text words = 6495

No. of Figures (2) x 250 words/Figure = 500

No. of Tables (2) x 250 words/table = 500

Total number of words = 7495

TRB 2013 Annual Meeting Paper revised from original submittal.

Abu-Hejleh, N; Mohamed, K; and Alzamora, D. 2

ABSTRACT 1

Recent FHWA national surveys revealed that: (a) highway bridges supported on spread footings 2

bearing on competent and improved natural soils, and engineered granular and MSE fills have 3

been safely and economically constructed by many state departments of transportation (DOTs); 4

and (b) many DOTs may be missing an opportunity to save time and money by not actively 5

considering spread footings to support highway bridges. The goal of this report is to promote the 6

use of spread footings on soils when appropriate to support highway bridges. Perceived obstacles 7

in using spread footings are identified, and recommendations to address these obstacles are 8

developed and centered around: 1) deployment of AASHTO/FHWA technical resources; 2) 9

highlighting practices of DOTs that actively use spread footings, especially for selection of 10

spread footing; 3) performance review of bridges constructed with spread footings bearing on 11

soils; and 4) LRFD implementation for spread footings design. Excessive settlement of bridges 12

with spread footings bearing on soils is the main concern for the DOTs that do not consider 13

spread footings. To address this concern, detailed recommendations are presented for accurate 14

estimation of both the bridge tolerable settlements and the bridge settlements that impact bridge 15

performance. The paper demonstrates that bridges with spread footings bearing on soil perform 16

very well with respect to settlement, and concerns of bridge settlement should not limit DOTs 17

from using these spread footings. 18

19

20

21

22

23



BACKGROUND AND PURPOSE 1

2

The use of spread footings to support highway bridges has many advantages, mainly savings in 3

design, construction and maintenance costs as well as accelerating bridge construction (1-4). 4

Spread footing design is relatively simple and could be modified in the field if needed. Spread 5

footings are often considered in situations where pile driving or drilled shaft installations are not 6

recommended, such as to: a) accommodate the presence of aquifers, underground structures such 7

as utilities and obstructions beneath foundations; b) generate less noise, ground vibrations, and 8

movements of nearby structures, including residential and historical buildings; and c) reduce 9

excavation of contaminated soils such as in the case of drilled shafts. Construction of spread 10

footings utilizes common materials, and can be constructed with readily available labor, simple 11

and small equipment, and without the need for specialty construction contractors. The 12

construction process is often easier, faster, and its quality control is simple compared to deep 13

foundations. Because of these advantages, construction of spread footings is generally expected 14

to provide a safe work environment and fewer claims. Finally, the use of spread footing 15

alleviates the bridge bump problem, creating a safer and smoother transition between the bridge 16

and approach embankment (5,6). 17

18

A national FHWA survey of the geotechnical practices of the state DOTs was developed and 19

distributed in 2007. The states’ geotechnical engineers in forty four (44) states responded to this 20

survey. Survey results indicate that the average distribution of bridge foundation types 21

considered by state DOTs across the United States is approximately 24% spread footings (11.5% 22

founded on soils, 12.5% founded on rock) and 76% deep foundations (56.5% driven piles and 23

19.5% drilled shafts). Similar information was recently reported by Paikowsky et al. (7). The 24

FHWA national survey identified the states with significant and moderate use (>10%) of spread 25

footings on soils to support highway bridges (Table 1) and the states with limited or no use 26

(<5%). Based on this survey, the FHWA concluded that a number of state DOTs could save time 27

and cost if spread footings are incorporated in their selection process and used when appropriate 28

to support bridges. 29

30

Initially, the paper briefly discusses valid design conditions that limit the use of spread footings 31

on soils to support bridges (e.g., scour). Selection of spread footings may not be appropriate or 32

economical with these conditions. However, the FHWA believes that selection of spread footings 33

on soils to support bridges are underutilized by a number of state DOTs when the design 34

conditions are appropriate for their use. 35

36

The goal of this paper is to promote the use of spread footings on soils for the support of bridges 37

when appropriate by identifying the DOTs perceived obstacles in using spread footings and 38

presenting recommendations to address these obstacles. To achieve this goal, two additional 39

national surveys of DOTs were performed in 2009 to get more insight from DOTs regarding 40

their obstacles, practices and experiences with the use of spread footings on soils to support 41

bridges. One survey targeted states’ geotechnical engineers and the other targeted states’ bridge 42

engineers. The surveys revealed that excessive settlement of bridges is the main concern of 43

DOTs in using spread footings on soils to support bridges. The paper focuses on addressing this 44

concern. 45

46



This paper continues and supplements FHWA efforts published in two recent publications: 1

FHWA Report (1): “Selection of Spread Footings on Soils to Support Highway Bridge 2

Structures.” The goal of this publication is to encourage consideration of spread footings on 3

soils to support bridges when appropriate. 4

Abu-Hejleh et. al. (8): “Implementation of LRFD Geotechnical Design for Bridge 5

Foundations.” The goal of this publication is to assist DOTs in the successful development of 6

LRFD Design Guidance for bridge foundations. 7

8

Although the paper uses the term “spread footings,’ to be consistent with recent AASHTO and 9

FHWA technical references, the recommendations of this paper are applicable to all types of 10

shallow foundation systems. 11

12

TABLE 1 States with Significant or Moderate Use of Spread Footings on Soils to Support 13

Bridges in Various US Regions (2007 National Survey) 14

States Spread Footings (%) Deep Foundations (%)

Soil Rock Driven Piles Drilled Shafts

Northeast States

Connecticut 50 25 20 5

Vermont 40 10 45 5

Massachusetts 35 15 20 27

New Hampshire 30 30 30 10

New York 30 15 47 3

New Jersey 30 20 40 5

Southwest States

New Mexico 30 10 30 30

Nevada 25 3 18 54

Northwest States

Idaho 20 10 60 10

Oregon 20 10 60 10

Midwest States

Michigan 10 5 80 5

15

Note: In the following sections of this paper, unless otherwise stated, “Spread footing” refers to 16

spread footings bearing on soils to support highway bridges. 17

18

DESIGN CONDITIONS THAT LIMIT THE USE OF SPREAD FOOTINGS ON SOILS 19

TO SUPPORT HIGHWAY BRIDGES (SCOUR) 20

21

The use of spread footings may not be suitable or economical under certain design conditions, 22

for example, presence of deep soft soil near the ground surface, very high lateral loads acting on 23

foundations (a major earthquake, flood, or vessel collision), and at sites with large scour or 24

liquefaction depths. These conditions need to be evaluated in the design of all foundation 25

alternatives using AASHTO and FHWA technical references to determine the: i) candidate 26

foundation alternatives in the preliminary design, including spread footings (with ground 27

improvements if needed); and ii) most appropriate (economical) foundation alternative. An 28

example of such evaluation for scour is briefly described next. 29



The 2007 and 2009 national surveys revealed that many state DOTs do not consider spread 1

footings at water crossings, mainly due to concerns with scour. According to AASHTO LRFD 2

(9) and HEC-18 (22) majority of bridge failures in the United States are due to scour. To avoid 3

these failures, spread footings need to be placed below the maximum scour depth for the design 4

events expected for the design life of the bridge. Appropriate hydraulic analysis and scour 5

evaluation need to be conducted in the design. AASHTO LRFD (9) defines the design flood 6

(needed to address the foundations LRFD service and strength limit states) and check flood or 7

superflood (needed to address the foundations LRFD extreme event limit states). AASHTO 8

LRFD Article 2.6.4.4 recommends placing the bottom of the spread footings below the total 9

scour depths determined for the check floods. The recently updated HEC-18 (22) describes the 10

scour evaluation and design procedures with the use of spread footings on soils to support both 11

piers and abutments. It recommends using more conservative floods than those used in 12

AASHTO, called “Scour Design Flood” and “Scour Design Check Flood,” for estimation of total 13

scour depths. It also recommends placing the spread footings below the total scour depth 14

determined for the Scour Design Check Flood. With the above AASHTO/FHWA 15

recommendations, it is expected that the use of spread footings may be found appropriate and 16

economical at stream crossings with low design total scour depth. 17

18

PERCEIVED OBSTACLES IN USING SPREAD FOOTINGS ON SOILS TO SUPPORT 19

HIGHWAY BRIDGES 20

21

The following obstacles are identified mainly based on our interpretation of the 2007 and 2009 22

FHWA national surveys information: 23

24

1. Limited knowledge and use of the AASHTO/FHWA technical references and training 25

courses for selection, LRFD design, construction, and performance of bridges on spread 26

footings (1-4; 8-20). For example, the following collected DOTs practices demonstrate that 27

some state DOTs do not have a design process for selection of the most appropriate 28

foundation type as recommended in the FHWA technical references: 29

Consideration of a standard or favorite foundation type to support bridges. 30

Having unrealistic concerns or perceived higher risk in the use of spread footings for 31

bridges support. 32

Costs are not considered in many cases during selection of appropriate foundation type. 33

Structural engineers often control this selection and tend to be conservative. Deep 34

foundations with higher costs are used in cases where the more economical spread 35

footings can be used. 36

Communication between the bridge (structural) and geotechnical engineers is one of the 37

main impediments for increased use of spread footings. Geotechnical groups may not be 38

consulted during the selection process of foundation type. Several respondents indicated 39

that education and training for both the structural and geotechnical engineers are required 40

to clarify their roles in the design process. 41

Selection of conservative or presumptive bearing geotechnical resistances in the final 42

design of spread footing. 43



Many of the obstacles described next can also be attributed to the limited knowledge and use 1

of AASHTO/FHWA technical resources for design and construction of spread footings. 2

2. DOTs with low use of spread footings may not be aware that a number of state DOTs 3

frequently use spread footings and that many highway bridges supported on spread 4

footings bearing on soils have been safely and economically constructed by DOTs with 5

very good performance. 6

3. Concerns regarding the use of spread footings bearing on engineered granular fill and 7

MSE fills. A number of state DOTs have allowed and constructed spread footings on natural 8

soils but not on engineered granular and MSE fills. These DOTs are concerned with the 9

quality and uniformity of placed fill and their effects on the performance (settlement) of 10

bridges. A state DOT reported that using spread footings on MSE walls to support bridge 11

abutments require longer spans, which would make them, cost more than using driven piles. 12

Many DOTs prefer to use abutments supported on piles placed in MSE walls. 13

4. Increased use of integral abutments. Dunker and Liu (21) and the FHWA survey indicate 14

that H-pile is the typical foundation type considered by DOTs for support of integral 15

abutments. This foundation type is considered “flexible” and therefore selected to 16

accommodate cyclic movements of the bridge superstructure with integral abutments. Spread 17

footings are considered “stiff” foundations (21) not appropriate with integral abutments. 18

Recently, DOTs use of integral abutments has significantly increased, and this reduced the 19

use of spread footings. 20

5. Limited use of bridge instrumentation program and load tests on spread footing. A 21

6. Concerns with construction and bridge performance problems due to inadequate 22

subsurface investigation program. Such concerns are perceived to be more critical for 23

spread footings than for deep foundations. Mainly, some DOTs are concerned with different 24

site conditions (DSC) during construction than those assumed in the design; for example 25

presence of soft soils and sudden rise in groundwater. These would require costly 26

modifications during construction or result in poor performance of foundations. 27

7. Concerns with bridge performance problems due to inadequate construction quality 28

program. Such concerns are perceived to be more critical for spread footings than for deep 29

foundations. An example of this problem is the unavailability of adequate experienced state 30

DOTs staff for bearing capacity verification of spread footings. This resulted in more 31

conservative designs and reduced the use of spread footings. 32

8. Use of conservative settlement analysis for bridges supported by spread footings. The 33

main concern of most DOTs with the use of spread footings is bridge settlement and potential 34

costly and difficult repairs to address settlement problems. This concern led to the use of 35

conservative tolerable settlements and conservative design methods to predict settlement. For 36

example: selection of unrealistic tolerable settlements (e.g., zero or 0.5 inch), not allowing 37

the use of spread footings on cohesive soils, and overestimating the loads considered to 38

compute settlements. 39

9. LRFD Implementation for spread footings design. Engineers from several DOTs have 40

expressed an interest in having some form of guidance from FHWA on the implementation 41

of LRFD design. Implementation of the new LRFD platform provides an excellent 42

opportunity for the DOTs to address the obstacles listed above, and improve their 43

geotechnical selection and design practices for spread footings. 44

45



RECOMMENDATIONS TO ADDRESS PERCEIVED OBSTACLES IN USING SPREAD 1

FOOTINGS ON SOILS TO SUPPORT BRIDGES 2

3

1. Deploy AASHTO LRFD design specification and FHWA technical references and 4

training courses (1-4; 8-20). The AASHTO/FHWA technical resources help to address 5

most of the obstacles described before and implement the recommendations presented later 6

since they: 7

Allow for the use of spread footings on all types of soils (competent and improved 8

natural soils, and engineered granular and MSE fills) to support bridges since they 9

provide the state-of-the- practice for their selection, LRFD design, and construction. 10

Require fair evaluation and consideration of all foundation types for bridges in the 11

preliminary design, including spread footings, and selection of the most appropriate 12

(economical) foundation type in the final design. 13

Provide training courses to address most of the DOTs perceived obstacles in using 14

spread footings on soils. These courses cover: a) application conditions and 15

advantages of spread footings; b) rational design process for selection of the 16

appropriate foundation type; c) roles of the project geotechnical and structural 17

engineers in the design process and importance of good communication and 18

cooperation between them, for example, in the settlement analysis; d) appropriate 19

settlement analysis of bridges supported on spread footings; and e) LRFD 20

implementation for spread footing design. 21

2. Review the FHWA national survey results (9) for: a) extent of use and performance of 22

highway bridges constructed on spread footing bearing on soil; b) practices of DOTs 23

for selection of spread footings; and c) good practices of DOTs for design and 24

construction of spread footings. This will help DOTs that have limited or no use of 25

spread footings to benefit from the experiences and practices of DOTs that frequently use 26

spread footings. Table 2 summarizes the approximate use and performance of highway 27

bridges constructed on spread footing bearing on soils in five regions: Northeast, Midwest, 28

Northwest, Southwest, and Southeast. DOTs reported good performance of their bridges 29

constructed on spread footings bearing on competent and improved natural soils, and 30

engineered granular and MSE fills (Table 2). Table 2 shows that the states’ DOTs use of 31

spread footings varies significantly across different regions in the USA and even among 32

states located in the same region. The use is up to 50% in the Northeast, up to 30% in the 33

Southwest, up to 20% in the Northwest, up to 10% in the Midwest, and almost none in the 34

Southeast region. There are DOTs with no or limited use of spread footings, not only in the 35

Southeast region but also in other regions (Table 2). In the Southeast region (Table 2d) and 36

Northeast region (Table 2e), some DOTs have extensive use of spread footings on rocks 37

but not on soils. It would be easier for these states to increase their use of spread footings 38

on soils. 39

3. Consider spread footing on granular and MSE fills and with semi-integral and 40

integral abutments. This recommendation can be implemented using AASHTO/FHWA 41

technical resources (1- 4, 6, 10, 12) and practices of DOTs that have successfully used 42

these applications, an example is Washington State DOT use of footings on granular fill 43

(20), New Mexico DOT use of footings on MSE fill (2), Colorado DOT use of semi-44

integral abutment on spread footings (6), and Tennessee DOT use of integral abutments on 45



spread footings (7). It is also suggested to consider these applications in selected pilot 1

projects that can be monitored, and in design-build and value engineering projects. 2

4. Consider load tests on spread footings and instrumentation of bridges with spread 3

footings, and review documented performance data from load tests and instrumented 4

bridges. Performing load tests and instrumentation programs would help DOTs verify the 5

design, construction, and performance of spread footings, which would help increase use of 6

spread footing to support bridges. Also, these programs provide performance data for the 7

reliability calibration and improvement of the LRFD geotechnical design methods for 8

spread footings (Abu-Hejleh et. al., 2010). Consider the FHWA technical references to 9

implement this recommendation (1,9,16,17), and experiences of DOTs that implemented 10

these programs, such as Minnesota (23), Ohio (24), and Colorado (6). 11

5. Deploy adequate subsurface investigation and construction procedures, and 12

construction quality control program. This would help DOTs to minimize risk during 13

construction, improve performance of spread footings, and justify the use of spread 14

footings. Consider the AASHTO/FHWA technical references to implement this 15

recommendation (2-4, 10, 15, 18). Minnesota (23) and Ohio (25) DOTs reported successful 16

use of cone penetration test (CPT) for more accurate prediction of settlement of spread 17

footings. 18

6. Deploy accurate settlement analysis of bridges supported on spread footings. This will 19

help DOTs develop less conservative and more economical LRFD design specifications to 20

address the service limit state for settlement of bridges supported on spread footings 21

bearings on soils. 22

7. Deploy accurate LRFD design bearing resistances for spread footings bearings on 23

competent and improved natural soils, and engineered granular and MSE fills. This 24

will help DOTs to develop less conservative and more economical LRFD design 25

methods/procedures. 26

8. Based on the above recommendations, develop LRFD guidance for spread footings 27

that consists of: 28

LRFD specifications and process for selection of the most appropriate foundation type 29

that would allow for selection of spread footings in the design when appropriate (main 30

goal of this paper). 31

LRFD specifications and process for design of spread footings that include more 32

accurate and economical design methods of spread footings than presently used by 33

many DOTs. 34

State DOTs could implement the recommendations from the above list that meet their needs. The 35

remainder of this paper will help DOTs with implementation of the recommendation on the 36

settlement of bridges supported on spread footings on soils (main concern of state DOTs). A new 37

FHWA report (9) is being prepared to help DOTs with implementation of all other 38

recommendations described above. 39

40

41

42

43

44



TABLE 2. Use and Performance of Bridges with Spread Footings on Soils in Five US 1

Regions: (a) Midwest States 2

State

2007 Survey

Estimated (%) 2009 Survey:

Use and Performance Soils Rocks

Michigan 10 5 Hundreds of bridges with spread footings on soils were

constructed in Michigan (70% before 1980, reduced to 50% by

1990, and currently 10%). “Their overall performance is

adequate, as bridges with deep foundations.“

Illinois 5 10 With piers on hard tills and dense sand. One bridge with MSE

abutments. “No performance or movement problems with

these bridges.”

Wisconsin 7.5* 10 Roughly 75 bridges supported on stiff natural soils are

constructed in the last 10 years. Very limited use with MSE

walls. With multi-span bridges, piers, and abutments. “These

bridges are performing well or as good as bridges with

piles.”

Indiana 1 5 Recently allowed spread footers for interior pier support on

some grade separation bridges only in glacial tills, IGMs and

engineered fills and in process to allow them over MSE walls.

Successfully used in the Accelerate I-465 project. “There has

been no visible evidence of excessive settlement.”

Minnesota 7 2 Recently used spread footing in simple span bridges (at

abutments only) on dense sand and gravel, ~ 4 bridges per year,

and with MSE walls. Use will increase in corridor and design

build projects. Employed with ground improvement in a value

engineering project. “Bridges appeared to be in fine shape

and perform well.”

Ohio 5 1 Since 1998, built 244 bridges with spread footing on MSE walls

and rocks. The use of spread footings on MSE walls is not

permitted. The current use is with dense sand and in few cases

with very stiff clays. “No major problems are observed”

Missouri Little* 5

Iowa 0 5 No use

*Not reported in the 2007 survey or taken from the 2009 survey 3

4

5

6

7

8

9

10

11

12

13

14



(b) Northwest Region (Mostly Spread Footings on Granular Soils) 1

State

2007 Survey



Idaho 20 10 Not aware of any performance issues with spread footings

Oregon 20 10 For piers and abutments. Not aware of any performance

issues with spread footings

Washington 10 * A very long history with successful use of spread footings

on compacted granular embankments and with piers to

support bridges. FHWA (20) reported very good conditions

of 148 bridges. Presently, not aware of any performance


Nebraska 10* * Not aware of any performance issues with spread footings

Montana 5 5 For piers and abutments. Not aware of any performance


Wyoming 5 17 Not aware of any performance issues with spread footings

Alaska 30% for

abutments and

<10% for piers*

Mostly to support abutments on MSE wall embankments.

With piers on very dense glacial till. Not aware of any

performance issues with spread footings

Hawaii 7 2 For piers. Not aware of any performance issues with spread

footings

South Dakota 0 5 No use

North Dakota 0* * No use


3

(c) Southwest Region 4

State

2007 Survey

Estimated (%)* 2009 Survey:


New Mexico 30 10 Extensive use of spread footings on MSE walls (30 out

of the 55 bridges in the I-25/I-40 interchange).

“Performed well, better than deep foundations as

there is no bridge bump.”

Nevada 25 3 Considered with all types of bridges. Saved money. “No

known Issues, they are performing well.”

Arizona 20 5 Performance is not reported, but expected to be OK

California 5% (30% -50% in

South California) *

Significant savings. Considered with all bridge types.

“Performed very well, no indications of poor

performance.”

Utah 5 5 Mostly single span bridges. “Performed well”

Colorado 3 bridges on soils* Two bridges on MSE walls, 3rd

bridge on 2:1 approach

embankment. “All performing well.”

Oklahoma Rarely used*

Texas/ Kansas 0 No use




(d) Southeast Region (% Use from 2007 Survey) 1

State Soils

(%)

Rocks

(%)

State Soils

(%)

Rocks

(%)

Tennessee 1 40 Kentucky <1* 40*

Florida 1 0 Georgia 2-3* 7-8*

Alabama 5 10 Mississippi 5 0

North Carolina 0 10 South Carolina Rarely used*

Arkansas 1 22 Louisiana 0


3

(e) Northeast Region 4

See FHWA (19) for Documented Good Performance of Bridges in this Region 5

State

2007 Survey



Connecticut 50 25 Used at every opportunity. Good performance

Vermont 40 10 Good performance

Massachusetts 35 15 Good performance

New Hampshire 30 30 To support abutments and piers. On MSE wall

abutments. Used at every opportunity. Good

performance

New York 30 15 To support abutments and piers. On MSE wall

abutments. Good performance

New Jersey 30 20 Used at every opportunity. On MSE wall abutments.

Good performance

Delaware 13* 4* Where feasible, more recent use. Good performance

Pennsylvania 10 60 Good performance

Rhode Island 10* * In glacial till. Good performance

Maine 2 45 Used at every opportunity. Good performance

Virginia 10 30 To support abutments and piers. On MSE wall

abutments. Good performance

Maryland 15* Good performance

West Virginia 0 20 No use

District of

Columbia

Rarely used*


7

SETTLEMENT ANALYSIS FOR BRIDGES SUPPORTED ON SPREAD FOOTINGS 8

9

Types of Settlements and Bridge Performance Problems 10

11

There are three types of settlements: 1) settlement of bridge foundation, SF; 2) settlement of the 12

bridge, SB; and 3) settlement of the bridge that impacts bridge performance, SBP. 13

14

Total settlement of bridge foundation (SF) is generated from loads transferred to the foundation 15

soil during: 16



Construction of the bridge substructure, which may include placement of spread footings, 1

columns for piers, abutment and wing walls, and earth fill behind the abutment. 2

Construction of the bridge superstructure (girders, and deck) 3

After construction due to traffic loads. 4

5

Bridge settlement (SB) is equal to the settlements of bridge foundation (SF) that is generated 6

during and after placement of bridge superstructure. The bridge settlements at various foundation 7

locations during these stages lead to (Figure 1): 8

Bridge uniform settlements due to uniform foundation settlements. 9

Bridge differential settlement when the foundations do not settle uniformly. This 10

differential settlement can develop between footings of adjacent bents (piers and abutments), 11

footings of adjacent columns within the same bent, or due to the rotation of a single footing. 12

Bridge angular distortion (or rotation) defined as the differential settlement between 13

any two points (often at the foundation locations) divided by the distance between these 14

two points (span length), as demonstrated in Figure 1. 15

Differential settlement between the bridge and its associated structures (e.g., 16

underground utilities, drainage grades, wing walls, and approach roadway). 17

18

Excessive bridge settlement can lead to the following problems that may impact the bridge 19

performance: 20

Structural distress and cracking of the bridge superstructure. The angular distortion may lead 21

to increased internal structural shear and moment stresses and deformations that are generally 22

manifested by cracks in the bridge deck and girders at the support locations. 23

Reduction of the bridge clearance, rideability problems within the bridge and between the 24

bridge and approach roadway (bridge bump problem), and safety and aesthetic problems. 25

Damage to the structures associated with the bridge (e.g., underground utilities) due to 26

differential settlement between the bridge and these structures. 27

28

Finally, settlement of the bridge at foundation locations that impact bridge performance, SBP, can 29

be less than bridge settlement, SB, because some of the bridge settlement that occur during 30

placement of the bridge superstructure could be accommodated by the structure or corrected for 31

during construction with little or no consequence to the integrity of the bridge or the structures 32

associated with the bridge as will be discussed later. 33

34



1 2

FIGURE 1 Types of bridge settlements (2) 3

4

Addressing the Service Limit State for Settlement of Bridges Supported on Spread 5

Footings 6

7

The project’s structural, geotechnical, and construction engineers need to work closely during 8

bridge design and construction to avoid or minimize bridge performance problems that could 9

develop due to foundation settlement. For example, the bridge and foundation elevations can be 10

slightly raised in the final design to compensate for the anticipated foundation post-construction 11

settlement. This could address concerns due to bridge clearance problems and differential 12

settlement between the bridge and its associated structures due to settlement of bridge 13

foundations. 14

15



The potential distress and cracking of the bridge superstructure due to bridge differential 1

settlement and angular distortion often control the design for settlement. Bridge differential 2

settlements often develop due to site variability, non-uniform applied stresses on foundations, 3

and construction sequence. Bridge differential settlement should be minimized during: 4

Design by sizing the foundations for uniform bridge settlements at all foundation locations, 5

SB. 6

Construction by changing sequence of construction and adopting other construction measures 7

that could reduce bridge differential settlements and increase bridge tolerance to differential 8

settlements during construction. 9

10

Presently, load and resistance factors for the settlement analysis at the service limit state are 11

assumed one (1) in the AASHTO LRFD (10) because scientifically-based load and resistance 12

factors are not yet developed. Future research is still needed to develop reliability-based 13

settlement analysis for bridges. Until that time, it should be ensured that the current settlement 14

analysis is on the conservative side. The following conservative assumptions are recommended 15

in the current settlement analysis (see also 1,2): 16

Assume the computed bridge differential settlement between two points equals to the larger 17

of the computed bridge settlement at both points as shown in Figure 2. In another way, zero 18

settlement is assumed for the point with the smaller settlement value (Figure 2). 19

Set settlements of structures associated with the bridge (e.g., approach slab) to zero, so 20

differential settlement between the bridge and structures associated with the bridge becomes 21

equal to settlement of the bridge. 22

Assume the bridge tolerable settlement equals to the bridge tolerable differential settlement. 23

24

By accepting the above assumptions, the governing equation for the service limit state for 25

settlement of a bridge can be simplified as: 26

27

SBP ≤ SBT …………………………………………..... (1) 28

29

Where SBP is the computed bridge settlements at various foundation locations that impact bridge 30

performance, and SBT is the bridge tolerable settlement. Note that the service limit state is to 31

ensure adequate performance of the bridge and not the bridge foundation. Adequate 32

performance of the foundation will be ensured by meeting the strength limit state for the bridge 33

footing. 34

35



1 2

3 FIGURE 2 Conservative design assumptions for computations of bridge differential 4

settlement and angular distortion (1) 5

6

The remainder of this paper will provide recommendations to estimate SBT and SBP. 7

8

Bridge Tolerable Total Settlement 9

10

The bridge performance problems described previously vary from project to project, depending 11

on the types and details of the bridge and structures associated with the bridge. Therefore, the 12

best approach to address these problems is to assign project specific bridge tolerable settlement 13

that would address all possible performance problems for the bridge and its associated structures 14

during their design service lives. After consultation with the project’s geotechnical and 15

construction engineers, the project’s structural engineer needs to finalize the project specific 16

bridge tolerable total settlement, SBT, based on Option 1, and with consideration of Options 2 and 17

3 described next. 18

19

Option 1. Develop project specific criteria based on structural tolerance of the bridge, 20

tolerance of the structures associated with the bridge, bridge design life, importance of the 21

bridge, and past experience. The concerns with bridge structural distress due to excessive 22

angular distortion often govern selection of bridge tolerable settlement, SBT. Bridge structural 23

tolerance is a function of the bridge stiffness, bridge type (simple or multispan, steel or concrete), 24

type of connections between the superstructure and support abutment or piers (pinned or fixed, 25

joints, integral, semi integral or simply supported abutment), and construction procedure, 26

tolerance, and stages. The effect of all these factors at various construction stages can be 27

examined in the design through structural analysis of the bridge when subjected to various 28

potential tolerable settlements. Paikowsky (26) suggested preliminary tolerable settlement 29



criteria based on the structure type (concrete or steel), construction stage (before or after bridge 1

completion), and type of bent (abutment or pier). 2

3

Option 2. Based on reported practices of DOTs that successfully constructed bridges with 4

spread footings. Most DOTs used tolerable settlement of 1 inch. A number of DOTs that 5

frequently use spread footings (Maine, Massachusetts, and California) reported tolerable 6

settlements as high as 2 inches. According to FHWA (3), bridges are often designed for tolerable 7

differential settlement of less than 1 inch for continuous span bridges and 1.5 to 2 inches for 8

simple span bridges. 9

10

Option 3. Based on documented settlement measurements of bridges that performed well 11

during their service design lives. These settlements represent lower limit for tolerable 12

settlements, as higher settlements might work as well. Based on data for 56 simple span bridges 13

and 119 continuous span bridges, FHWA (2) recommended angular distortion of 0.005 for 14

simple span bridges (0.008 in AASHTO, 10) and 0.004 for continuous span bridges. Paikowsky 15

(26) recommended to consider these angular distortions for span length longer than 50 ft, and not 16

to consider them for rigid frame structures and integral abutments. Based on measured 17

movements of 28 bridges constructed with spread footings on compacted fill, FHWA (20) 18

concluded that bridges can tolerate differential settlements of 1 to 3 inches without significant 19

distress. Based on measured settlements of 21 bridge footings, FHWA (19) reported good 20

performance of these bridges with settlement up to 1 inch. FHWA report (1) presented the 21

measured settlements for 78 bridges performing well during service, where 69 bridges had less 22

than 1 inch settlement. 23

24

Estimation of the Settlement of the Bridge Spread Footings 25

26

Total foundation settlement is the summation of elastic or immediate settlement that occurs 27

quickly while the loads are being applied during construction (mostly for granular soils) and 28

time-dependent consolidation settlement that occurs during and after construction (mostly for 29

clay soils). Spread footings for bridges are normally not considered with organic and highly 30

plastic soils that generate creep settlement. AASHTO (10) indicates that transient live loads may 31

be omitted in the time-dependent consolidation settlement of foundations bearing on cohesive 32

soils but need to be considered for granular soils. FHWA (2) discusses the estimation of 33

consolidation settlements of footings on cohesive soils and structural fills. 34

35

According to AASHTO LRFD (10), settlement of footings on cohesionless soils can be 36

estimated using the elastic theory or the empirical Hough method. These methods generally 37

generate conservative settlement estimates. FHWA (19) concluded that the Hough method is the 38

least accurate and most conservative method. Sargand et al. (25) employed the measured 39

settlement of spread footings at various bridges in Ohio to evaluate various settlement prediction 40

methods using SPT and CPT data and found that: 41

The modified Schmertmann method (27) is more reliable than other methods for normally 42

consolidated (loose) sands. The preloading effect (due to compaction) was incorporated by 43

using higher soil stiffness. 44

Using CPT data (continuous subsurface data) in the settlement prediction methods provides 45

more accurate results compared to SPT data (series of isolated data). 46



Based on the above, it is recommended to use the Schmertmann’s modified method for 1

estimation of immediate settlement of spread footing on cohesionless soils. This method is 2

described and demonstrated with examples in the FHWA (2, 4) using SPT data. It can more 3

accurately be applied using CPT data (25). According to FHWA (4), the use of SPT data with 4

this method often overestimates settlement (conservative). 5

6

DOT Good Practice: Minnesota DOT attributed its increased use of spread footings on soils to 7

the deployment of more accurate field testing procedures (e.g., CPT) and advanced computer 8

programs (e.g. SIGMA, FLAC) to predict foundation settlement. 9

10

Estimation of Bridge Settlement that Impact Bridge Performance 11

12

Most of the immediate settlement of footings on cohesionless soils occurs rapidly during 13

construction. According to FHWA (19), the average settlement of 21 bridges supported by 14

spread footings on cohesionless soils was 1 inch, with 0.75 inch occurring before placement of 15

the bridge decks and only 0.25 inch occurring after construction of the bridge deck. FHWA (1,2) 16

suggests that settlement analysis should consider the stages at which loads are applied to the 17

footings and cause bridge settlements. Accordingly, certain loads (not all the loads applied on the 18

footings) should be considered in the estimation of the bridge settlement that impact 19

performance, SBP. 20

21

Bridge foundation settlement, SF, that occurs before placement of the bridge superstructure 22

should not be considered in the computation of bridge settlements SB and SBP. Several references 23

(2,4) reported that some of the bridge settlements that occur during placement of the bridge 24

superstructure (SB) can be accommodated by the structure or corrected during construction with 25

little or no consequence to the integrity of the bridge, depending on bridge type, and construction 26

method and sequence. For example, it may be possible to correct for settlement of the spread 27

footings due to the placement of girders in the final constructed grades. Then, this settlement 28

would not impact performance of the bridge and should not be included in the computation of 29

SBP. FHWA (1) reported that settlements that are relevant to the performance of the bridge 30

structure are often 25 to 50 % of the footing total settlement. According to FHWA (19), 31

foundation settlements which occur prior to bridge deck construction may not adversely affect 32

the bridge, and for continuous span bridges, only post-deck settlement can cause bending 33

moments and stresses in the structural frame. 34

35

Based on the above, portions of the foundation settlements (SF) that occur during and after 36

placement of the bridge superstructure may not impact the performance of the bridge (structural 37

distress, or clearance) or performance of the structures associated with bridge. This portion of 38

settlement should not be considered in the estimation of bridge settlement that impact bridge 39

performance, SBP. LRFD design example for a 15 ft wide bridge spread footing with a tolerable 40

settlement of 0.9 inch is discussed in the FHWA report (1). As discussed in Section E.4.1of this 41

report, the computed foundation settlement, SF, is 0.87 inch and computed bridge settlement that 42

impact performance, SBP, is 0.36 inch (41% of SF). Since 0.36 inch is much less than 0.87 inch, 43

foundation width smaller than 15 ft can be considered to meet the service limit state for 44

settlement. In this case, most likely, the strength limit state would control the design (footing 45

width). 46



In the design process, it is recommended that the project geotechnical engineer computes the 1

bridge foundation settlement, SF, at various stages during and after construction. The project 2

structural engineer need to discuss and agree with the project geotechnical and construction 3

engineers on the stages that would impact bridge performance, and to use the settlements at these 4

stages to compute the bridge settlement that impact bridge performance, SBP. 5

6

It is widely accepted that the service limit state would generally control the design for spread 7

footing of highway bridges because of the tight bridge tolerable settlement often required in the 8

design (e.g., ≤ 1 inch), and the large footings (>10 ft wide) that generate deeper DOSI (depth of 9

significant influence) and larger settlements. Many state DOTs use the large foundation 10

settlement, SF, and not SBP to address the service limit state, leading to the settlement often 11

controls the design and becomes of concern to state DOTs. However, the settlement that impact 12

bridge performance, SBP needs to be considered to address the service limit state not the 13

foundation settlements, SF. Since SBP is smaller (could be much smaller in some cases) than SF, 14

the strength limit or extreme event limit states may control the design (footing width), and not 15

the settlement at the service limit state. 16

17

Based on the above, bridges with spread footings bearing on soil can perform very well with 18

respect to settlement as confirmed with the national survey results discussed before. 19

20

SUMMARY AND RECOMMENDATIONS 21

22

The use of spread of footings is significant in the Northeast (up to 50%) followed by the 23

Southwest (up to 30%) and Northwest (up to 20%). The use of spread footings is low in the 24

Midwest region (0-10%), and almost none in the Southeast region. The goal of this paper is to 25

promote the use of spread footings on soils when appropriate to support bridges. 26

27

Recent FHWA national surveys revealed that: (a) highway bridges supported on spread footings 28

bearing on competent and improved natural soils, and engineered granular and MSE fills have 29

been safely and economically constructed by many DOTs; and (b) many DOTs may be missing 30

an opportunity to save time and money by not actively considering spread footings to support 31

bridges. DOTs reported good performance of their bridges constructed on spread footings 32

bearing on soil, with performance similar to bridges supported on deep foundations. The 33

presented settlement analysis for bridges with spread footings in this paper demonstrated that 34

these bridges can perform very well with respect to settlement. Therefore, concerns of bridge 35

settlement should not limit state DOTs from considering spread footings on soils to support 36

highway bridges. 37

38

This paper will be of immediate interest to DOTs with limited or no use of spread footings on 39

soils to support bridges since it provides recommendations to address their concerns in using 40

these spread footings. Implementation of the new LRFD platform provides an excellent 41

opportunity for DOTs to change and improve their selection and design practices for spread 42

footings by implementing the recommendations presented in this paper. A new FHWA report (9) 43

is being developed to help DOTs with implementation of all recommendations presented in this 44

paper. 45

46



ACKNOWLEDGEMENT 1

2

The authors wish to thank Jennifer Nicks, PhD, from the FHWA for her in-depth and 3

comprehensive review of this paper and providing excellent suggestions to improve it. 4

5

The authors wish to thank Ben River, PE; Justice Maswoswe, PhD, PE; Barry Siel, PE; and Silas 6

Nichols, PE (all from the FHWA) for contacting their contact states and compiling their survey 7

information. 8

9

Finally, the authors also like to acknowledge Scott Anderson, Ph.D, PE from the FHWA for his 10

review and continuous support to this work. 11

12

REFERENCES 13

14

1. FHWA (2010). Selection of Spread Footings on Soils to Support Highway Bridge 15

Structures. FHWA RC/TD-10-001, Authors: Samtani, N. C., and Mertz, D.R. FHWA, 16

U.S. Department of Transportation. 17

2. FHWA (2006). Soils and Foundations: Reference Manual. FHWA-NHI-06-088, Authors: 18

Samtani, N.C., and Nowatzki, E.A. FHWA, U.S. Department of Transportation. 19

3. FHWA (2002). Geotechnical Engineering Circular 6, Shallow Foundations. FHWA SA-20

02-054, Author: Kimmerling, R.E. FHWA, U.S. Department of Transportation. 21

4. FHWA (2001)*. Shallow Foundations: Reference Manual. FHWA-NHI-01-023, Authors: 22

Munfakh, G., Arman, A., Collin, J.G., Hung, J.C.-J., and Brouillette, R.P. FHWA, U.S. 23

Department of Transportation. *In 2012, the training course was updated in accordance 24

with the AASHTO Bridge LRFD Specification, 5th Ed., 2010. 25

5. Briaud, J-L., James, R.W., and Hoffman, S.B. (1997). Settlement of bridge approaches, 26

NCHRP Synthesis Report 234, Transportation Research Record, Washington, DC. 27

6. Abu-Hejleh, N., Hanneman, D., White, D. J., Wang, T. and Ksouri, I., (2006). Flowfill and 28

MSE Bridge Approaches: Performance, Cost, and Recommendations for Improvements. 29

Report No. CDOT-DTD-R-2006-2, Colorado Department of Transportation. 30

7. Paikowsky, S.G., Canniff, M.C., Lensy, K., Kisse, A., Amatya, S., and Mugangam R. 31

(2010). LRFD Design and Construction of Shallow Foundations for Highway Bridge 32

Structures. NCHRP Report 651, Transportation Research Board, Washington, D.C. 33

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Recommendations for the Use of Spread Footings on Soils to Support Highway Bridges. 38

FHWA, U.S. Department of Transportation. 39

10. AASHTO (2010). AASHTO LRFD Bridge Design Specification”, 5th

Edition with 2010 40

Interim Revisions. AASHTO, Washington, D.C. 41

11. FHWA (2011). LRFD Seismic Analysis and Design of Transportation Structures, Features, 42

and Foundations. FHWA-NHI-11-032, Authors: Kavazanjian, E. Wang, J-N.J, Martin, 43

G.R, Shamsabadi, A., Lam I., Dickenson, S., and Hung, C.J. FHWA, U.S. Department of 44

Transportation. 45



12. FHWA (2009). Design and Construction of Mechanically Stabilized Earth Walls and 1

Reinforced Soil Slopes. FHWA NHI-09-083, Authors: Berg, R.R., Christopher, B. R. and 2

Samtani, N. C. FHWA, U.S. Department of Transportation. 3

13. FHWA (2006). Ground Improvement Methods. FHWA NHI-06-019 (Vol. I) and FHWA 4

NHI- 06-020 (Vol. II), Authors: Elias, V., Welsh, J., Warren, J., Lukas, R., Collin, J.G. and 5

Berg, R.R. FHWA, U.S. Department of Transportation. 6

14. FHWA (2005). LRFD for Highway Bridge Substructures and Earth Retaining Structures: 7

Reference Manual. FHWA-NHI-05-094. FHWA, U.S. Department of Transportation. 8

15. FHWA (2002). Geotechnical Engineering Circular 5: Evaluation of Soil and Rock 9

Properties. FHWA-IF-02-034. Authors: Sabatini, P.J., Bachus, R.C.,Mayne, P.W., 10

Schneider, J.A., and Zettler, T.E. FHWA, U.S. Department of Transportation. 11

16. FHWA (1998). Geotechnical Instrumentation. FHWA-HI-98-034, Author: Dunnicliff, J. 12

FHWA, U.S. Department of Transportation. 13

17. FHWA (1997). Large Scale Load Tests and Data Base of Spread Footing on Sand. FHWA 14

RD-97-0680, Authors: Briaud, J-L., and Gibbens, R. FHWA, U.S. Department of 15

Transportation, Washington, D.C. 16

18. FHWA (1996). Geotechnical Differing Site Conditions. FHWA Geotechnical Engineering 17

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Transportation. 19

19. FHWA (1987). Spread Footings for Highway Bridges. FHWA RD-86-185, Authors: 20

Gifford, D. G., Kraemer, S. R., Wheeler, J. R., McKown, A. F. FHWA, U.S. Department 21

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20. FHWA (1982). Performance of Highway Bridge Abutments on Spread Footings on 23

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24. Sargand, S. M., and Masada, T. (April 2006). Further Use of Spread Footing Foundations 34

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45

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