Comparing ASD and LRFD for Load Rating of Timber

Comparing ASD and LRFD for Load Rating of Timber Bridges James Scott Groenier1, P.E, M.ASCE 1

Project Leader, Structures, USDA Forest Service, Missoula Technology and Development Center, 5785 Highway 10 West, Missoula, MT 59808, phone: (406) 329-4719, e-mail: [email protected]

Abstract Most timber bridges are currently designed in accordance with the American Association of State Highway and Transportation Officials (AASHTO) Standard Specifications for Highway Bridges, 17th Edition (SSHB), which follows an allowable stress design (ASD) approach. However, the Federal Highway Administration (FHWA) will be requiring all highway bridges designed after October 1, 2007 to follow the AASHTO LRFD (Load & Resistance Factor Design) Bridge Design Specifications, 3rd Edition. As the transition from ASD to LRFD nears, engineers need to be aware of the differences. Information about more recent changes to the design codes and differences between AASHTO’s Manual for Condition Evaluation and Load Resistance Factor Rating (LRFR) of Highway Bridges and AASHTO’s Manual for Condition Evaluation of Bridges, 2nd Edition (MCEB) will be presented. This paper will provide an overview of the pertinent issues related to timber bridge design and load rating. An example is included that illustrates the dramatic reduction in rated load capacity when calculated by the LRFD method. Introduction The objective of this paper is to review the differences between AASHTO’s Standard Specifications for Highway Bridges, 17th Edition and AASHTO LRFD (Load & Resistance Factor Design) Bridge Design Specifications, 3rd Edition. The Federal Highway Administration will require all new highway bridges to be designed using the LRFD methodology after October 1, 2007 (FHWA Memo, June 28, 2000). Currently the majority of concrete and many steel bridges are designed using the LRFD methodology. However, the FHWA requirement will now include timber structures. A table comparing ASD and LRFD design references is shown in the Appendix. The paper illustrates the effects of load rating existing timber structures using AASHTO’s new Manual for Condition Evaluation and Load Resistance Factor Rating (LRFR) of Highway Bridges. A table comparing ASD and LRFR load rating references is shown in the Appendix. There are many load rating differences between ASD and LRFR procedures. The significant changes are 1) increasing vehicle live load, 2) including a dynamic load or impact factor, 3) changing distribution of wheel loads to individual stringers and 4) modifying equations and adjustment factors.

Differences between ASD and LRFD Design Vehicles. The first major difference between ASD and LRFD methodologies is an increase in design vehicle live load from ASD to LRFD. The design live load (figure 1) for ASD (SSHB Section 3.7, Highway Loads) are the HS20-44 vehicle or the HS20-44 lane loading and the alternate military vehicle that is used for Interstate and other highways carrying heavy truck traffic. LRFD (Section 3.6.1.2, Design Vehicular Live Load) uses an HL-93 (figure 2), which consists of the design truck (similar to the HS20-44 vehicle) or the design tandem, whichever is greater, plus a superimposed design lane load of 640 pounds per linear foot.

Figure 1. ASD Design Vehicular Live Load

Figure 2. LRFD Design Vehicular Live Load (HL-93)

The ASD alternate military vehicle and the LRFD tandem vehicle have the same configuration with two axles spaced four feet apart. The magnitude of the load has changed from 24 kips (106.8 kN) in the ASD to 25 kips (111.2 kN) in the LRFD. The tandem vehicle typically controls for bridges from 10 feet (3.048 m) long to 40 feet (12.19 m) in LRFD and the alternate military vehicle controls for bridges longer than 11 feet (3.353 m) and less than 37 feet (11.28 m) in ASD. The LRFD superimposed loads are always greater than the ASD loads, but are supposed to more closely simulate current highway bridges loadings. For bridge rating, the ASD methodologies do not require use of the alternate military vehicle, but just the HS20-44 vehicle. In LRFR, both the HL-93 vehicle and design tandem must be checked. Then the larger value must be used concurrently with the lane load. For bridges over 10 feet (3.048 m) long, the design tandem load in conjunction with the lane load always controls. The rating vehicle loads under LRFD are always substantially greater and grow at geometric rate faster than those used in the ASD methodology. Dynamic Load Allowance. The second major difference between ASD, LRFD, and LRFR methodologies is that dynamic load allowance or impact factors are not required for wood under ASD for design (SSHB Section 3.8.1.2) and load rating (MCEB Section 6.7.4). The 1st and 2nd edition of LRFD included a dynamic load factor equal to 50 percent of that required for steel and concrete. The 3rd edition of LRFD (Section 3.6.2.3) has deleted this requirement. There is still a hold over in the LRFR (Section 6.7.5) and this impact factor increases the moment and shear values up to 16.5 percent. Wheel Load Distribution. The third major difference is the method used to distribute loads to interior longitudinal beams differs greatly between ASD (SSHB Section 3.23.2.2) and LRFD (Section 4.6.2.2.2). ASD calculates the distribution of wheel loads per wheel line load and then distributes the loads to each beam by dividing the beam spacing by a distribution factor. LRFD uses lane loads to distribute the loads to beams and divides the beam spacing by a different distribution factor. The distribution factors are not proportional and using plank decking for example, results in an approximately 20 percent greater load for interior stringers using LRFD procedures rather than ASD. Adjustment Factors. The adjustment factors for wood allowable stress values and base resistance values used in LRFD compared to ASD need to be considered. These values are found in Table 13.5.1A for ASD and Table 8.4.1.1.4-1 for LRFD. The conversion factor for converting ASD allowable stress values to LRFD base resistance values is 2.16 / (1000 * φ), where φ is the resistance factor for individual properties such as bending and shear. The LRFD values are based on dry-use condition and 10-minute live load duration. ASD used a load duration factor (CD) of 0.9 for dead load and 1.15 for live load. LRFD uses a time effect factor (CT) of 0.8, this is not consistent with the AFPA/ASCE Standard 16-95 Standard for Load and Resistance Factor Design (LRFD) for Engineered Wood Construction, which uses

0.8 for occupancy and 1.25 for impact loads. Also, the actual equations for LRFD differ from AFPA/ASCE Standard 16-95. The equations for bending in LRFD are: F = Fbo*CF*CM*CD*CT Mn = F*Sx*CS F = nominal resistance Fbo = base resistance in bending CF = size effect factor (section 8.4.4.2) CM = wet service factor (section 8.4.4.3)

(LRFD Equation 8.4.4.1-1) (LRFD Equation 8.6.2-1) CD = deck effect factor (section 8.4.4.4) CT = time effect factor (section 8.4.4.5) CS = stability factor (section 8.6.2) Sx = Section Modulus

From AFPA/ASCE Standard 16-95, the related equations are: F`b = Fb*CM*Ct*CL*CF*CV*Cfu*Cr*Cc*Cf*Cpt*Crt (AFPA/ASCE Table 2.6-1) Mu ≤ λ*φb*M`

(AFPA/ASCE Equation 5.1-1)

M` = F`b*Sx

(AFPA/ASCE Equation 5.2-2)

λ = time effect factor F`b = nominal resistance Fb = base resistance in bending CF, CM are the same as LRFD and the rest of the adjustment factors can be found in section 5.2.1 of AFPA/ASCE Standard 16-95. The largest discrepancy is with the time effect factor and deck effect factor. The deck effect factor is not found in AFPA/ASCE Standard 16-95, while the time effect factor gives engineers the leeway to use 1.25 for impact loadings. While the AFPA/ASCE Standard 16-95 has not been written specifically for bridges, the AASHTO standards generally have followed the 1991 AFPA National Design Standards for Wood. Load Rating Example The combination of the differences listed above greatly influence the design and load rating of bridges using LRFD and LRFR instead of ASD. The following example will illustrate the differences in load ratings for a timber bridge using the various methodologies and associated factors (figure 3). The rating factor serves as an indicator of the relative capacity of the bridge as calculated by each method. The bridge was designed using an HS20-44 vehicle in ASD. It will be load rated with ASD and LRFR incorporating various factors.

Figure 3. Typical Sawn Timber Bridge Section The timber bridge to be load rated has the following conditions: Type: Sawn Timber Stringer Bridge Deck: Wood Plank Beams: 6 x14 Rough Sawn, Southern Pine, Select Structural Beam Spacing: 16 inches on center between beams Length: 17 feet - 10 inch A series of MathCAD worksheets were developed for load rating timber bridges using ASD and LRFR for the examples of a timber bridge load rating from AASHTO’s Manual for Condition Evaluation of Bridges and Manual for Condition Evaluation and Load Resistance Factor Rating (LRFR) of Highway Bridges. The effect of increased design vehicle loads and vehicle wheel load distribution is shown for moment and shear values in Table 1. The numbers are unfactored values, but use an HS20-44 design vehicle for ASD and the tandem design vehicle plus lane load for LRFR.

LL Moment LL Shear

ASD ASD LRFD/LRFR With With Without Dynamic CD= 1.15 CD= 0.9 Load Allowance 23.8 ft-kips 23.8 ft-kips 40.0 ft-kips (32.3 kN-m) (32.3 kN-m) (54.2 kN-m) 6.1 kips 6.1 kips 7.6 kips (27.1 kN) (27.1 kN) (33.8 kN)

LRFR With Dynamic Load Allowance 45.8 ft-kips (62.1 kN-m) 8.7 kips (38.7kN)

Table 1. Comparison of Unfactored Moment and Shear Values Table 2 shows the differences between the two load rating methods with load duration factors for ASD of 1.15 (live load) and 0.9 (dead load), and LRFD/LRFR run with and without dynamic load allowance included. A load duration factor (CD) of 0.9 for dead load and a 1.15 for vehicle live load were reviewed, since AASHTO’s Manual for Condition Evaluation of Bridges uses the 0.9, but most engineers use the 1.15 load duration factor from Standard Specifications for Highway Bridges, 17th Edition to load rate bridges.

Moment Inventory Rating Factor Moment Operating Rating Factor Shear Inventory Rating Factor Shear Operating Rating Factor

ASD With CD= 1.15

ASD With CD= 0.9

LRFD/LRFR Without Dynamic Load Allowance

LRFR With Dynamic Load Allowance

1.08

0.83

0.56

0.49

1.46

1.13

0.72

0.63

1.09

0.84

0.64

0.55

1.47

1.14

0.83

0.72

Table 2. Comparison of Rating Factors A rating factor of 1.00 means the structure is rated to just carry the design vehicle. A rating factor below 1.00 means that the bridge has inadequate capacity, while a rating factor above 1.00 means that the bridge has excess capacity. The results show substantial reduction in the rating factor when the LRFR rating methodology is used rather than ASD. The combination of heavier live loads, distribution of wheel loads, duration of load factors, and dynamic load allowance factors have resulted in lower rating factors. The bridge has a rating factor greater than 1 for an HS20-44 vehicle load using the ASD methodology and is good for an HS20-44 load. When we look at the rating factor for LRFR, we find that the rating factor is only 49 percent due to the larger vehicle load and other factors. Summary There are many differences between AASHTO’s Standard Specifications for Highway Bridges, 17th Edition, AASHTO Load & Resistance Factor Design (LRFD) Bridge Design Specifications, 3rd Edition, and the AASHTO’s new Manual for Condition Evaluation and Load Resistance Factor Rating (LRFR) of Highway Bridges. The changes need to be taken into account when designing and load rating timber bridges for federal, state, and local highways. These new LRFD specifications include numerous minor adjustments and several significant changes from the current ASD, such as increasing vehicle live load, including a dynamic load or impact factor, changing distribution of wheel loads to individual stringers, and modifying adjustment factors from the existing AF&PA/ASCE Standard 16-95 Standard for Load and Resistance Factor Design (LRFD) for Engineered Wood Construction. Lastly, the new load rating vehicle coupled with the other changes to the LRFR code may greatly decrease the rated load capacity for existing timber bridges.

References AASHTO (2002). Standard Specifications for Highway Bridges, 17th Edition. American Association of State Highway and Transportation Officials, Washington, DC. 1028 pp. AASHTO (2003). Manual for Condition Evaluation of Bridges, 2nd Edition, 2001 and 2003 interims. American Association of State Highway and Transportation Officials, Washington, DC. 148 pp. AASHTO (2003). Manual for Condition Evaluation and Load Resistance Factor Rating (LRFR) of Highway Bridges. American Association of State Highway and Transportation Officials, Washington, DC. 256 pp. AASHTO (2004). AASHTO LRFD (Load & Resistance Factor Design) Bridge Design Specifications, 3rd Edition. American Association of State Highway and Transportation Officials, Washington, DC. AFPA/ASCE Standard 16-95 (1996). Standard for Load and Resistance Factor Design (LRFD) for Engineered Wood Construction. American Society of Civil Engineers, New York, NY. FHWA (2000). Use of LRFD for Bridge Design Concurrence Memo, dated June 28, 2000. U.S. Department of Transportation, Federal Highway Administration, Washington, D.C.

Appendix

Section

Loads and Load Factors Structure Analysis Decks and Deck Systems Design Vehicle Live Load Impact Factor/Dynamic Load Allowance Load Combinations Distribution of Wheel Loads

Standard Specifications for Highway Bridges, 17th Edition

AASHTO Load & Resistance Factor Design (LRFD) Bridge Design Specifications, 3rd Edition

3.22 3.2.2 3.25 3.7.1/3.7.6

3.4 4 9.9 3.6.1.2

3.8.1.2

3.6.2.3

3.22.1A 3.23.2.2 Table 3.23.1 13

3.4.1-1 4.6.2.2.2 Table 4.6.2.2.2a-1 8

Wood Structures Design Values / Base Resistance Adjustment Values

13.5

8.4.1

13.6.4.1 / 13.6.5.3.1

8.4.4 / 8.6 / 8.7

Deflection Criterion

13.4.3

2.5.2.6.2

Table A1. Comparison of Bridge Design References

Item

Manual for Condition Evaluation of Bridges, 2nd Edition

Manual for Condition Evaluation and Load Resistance Factor Rating (LRFR) of Highway Bridges

3 6.5 6.62 6.5.1 6.7 6.7.2

4 6.4.2 6.1.5 / 6.3 Table 6-1 6.4.3.2.2 / 6.7.4.1 6.4.3.2.1

Inspection Rating Factor Structure Analysis Load Combinations Loads and Load Factors Design Live Load Impact Factor/Dynamic Load Allowance Wood Structures Decks and Deck Systems Distribution of Wheel Loads Tabulated Stresses Adjustment Values

6.7.4

6.7.5

6.6.2.7 Use AASHTO ASD Use AASHTO ASD Use AASHTO ASD Use AASHTO ASD

6.7 6.7 Use AASHTO LRFD Use AASHTO LRFD Use AASHTO LRFD

Example Load Rating

Example B3 - p. 114

Example A4 - p. A-75

Table A2. Comparison of Bridge Load Rating References