Highway bridge loadings

Highwaybridgeloadings Charles F. G a l a m b o s Federal Highway Administration, Washington, D.C. 20590, USA

The paper presents a comparison of the A A S H T O design live Ioadings for bridges with various other loading situations. Comparisons are made with some European design live loads, with native and foreign legal loads, normal permit overloads, and abnormal permit loads. The results of a bridge load rating exercise are presented. Some actual bridge load histograms are given, as well as a comprehensive histogram based on the national Ioadometer survey for 1970. Fatigue Ioadings and damage are discussed in the light of actual and design Ioadings. It is concluded that it may be timely to increase the A A S H T 0 HS design Ioadings. To improve the bridge load rating process, it is suggested that some standard load rating vehicle and test method be employed. Further refinemenl of the fatigue design provisions for steel bridges do not seem warranted in light of the great variation of actual Ioadings on bridges. Introduction In the 1978 annual meetings of the American Association of State Highway and Transportation Officials (AASHTO) Bridge Subcommittee (there are four regional meetings), there was considerable discussion again about the adequacy of the present design loadings, especially in view of continued pressure by the trucking industry to ask for higher allowable loads, and also because of the recent increased awareness of the state of deterioration of many of the bridges. It is estimated that over I00 000 of the 600 000 highway bridges on all US road systems are structurally inadequate or obsolete for various reasons, and should be replaced. Some of these bridges are already being replaced through a special bridge replacement program and many more will be replaced as a result of the funds made available by the recently passed 1978 Surface Transportation Act. This act, for the next four years, makes available to the States 0.9, 1.1, 1.3 and 0.9 billion dollars expressly for bridge replacement. It is therefore a most appropriate time to examine bridge load design practice and philosophies to point out where changes for the better may be in order. The views expressed here are, of necessity, those of a researcher and not of a bridge design engineer, and they express private opinions and not official Federal Highway Administration policy. External loads from traffic only are considered. Design live loads The great majority of highway bridges in the United States have been, and still are designed according to the prevailing provisions of the 'Standard Specifications for Highway Bridges', l as adopted by the American Association of State Highway and Transportation Officials. The latest published full edition is the twelfth edition (1977). Selected interim sections are issued as needed. Generally, the specifications are republished every four years.

230

Eng. Struct., 1979, Vol. 1, October

The design live loads prescribed in the above specificationa are of two forms: (i), a uniform load per linear foot of load lane; and (ii), variations of axle loads in two standard trucks. The uniform load is supplemented by one concentrated load (or two for continuous spans), which varies for moment or shear. Some of the axle spacings of the design truck can be varied (within limits) to produce maximum stress effects. The heaviest loadings are illustrated in Figure 1, For interstate highway bridges, there is an alternate loading of two axles four feet apart, with each axle weighing 24 000 pounds. Whichever loading, lane load, truck load, or alternate loading produces the maximum effect is the condition which should be used in any specific case. These loadings, presently designated HS 20-44, have not been substantially changed since 1944. It is of interest to compare the AASHTO design live loads with those used in several other countries. This is shown graphically in Figure 2 for West Germany, the United Kingdom, Belgium, the Netherlands and the HS 20 AASHTO loadings. The method of comparison is by means of bending

HS20-44 E~

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32k

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Highway bridge Ioadings: C. F. Galambos 5000

combination is shown in Table 1. The values shown do not

include the slight increase in loads allowed in some jurisdictions due to statutory enforcement tolerances. The table shows that except for a few countries (Belgium, France, Italy and Spain), the allowable single axle and tandem axle loads are not too much different from the AASHTO allowables: 22 kips vs 20 kips, 35 kips vs 34 kips. The 5-axle vehicle allowable gross loads are somewhat greater in Europe, an average of 88 vs 76 kips, or about a 15% increase over the AASHTO allowable values. If Table I is now compared with Figure 2, it is clear that in the USA bridges are designed to carry a slightly smaller load than they are allowed to carry: 72 kips vs 76 kips, whereas in most European countries the design loads are much higher than the allowable loads. Although the above statement is true, it is of course quite a simple statement. Further discussion follows.

4500 I,

4000

L

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3500 Z 3000

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25OO 2OOO 15OO

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I

70

80 q~O 100

A l l o w a b l e o v e r l o a d s or p e r m i t loads

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Figure 2

Comparison of design loads

moment calculations, taking account of impact factors and the respective allowable stress used in each country. A simply supported bridge was used, and the design vehicles were approximated by a conversion to a uniform distributed load spread out over a distance of 10 metres. The calculated values are only of theoretical interest; they cannot be considered as practical allowable values for the spans considered, but they are a good indication of the relative ultimate capacity of the structure. No lateral load reduction for more than one loaded lane was allowed for the comparison. What is immediately obvious from Figure 2 is that there is considerable variation between countries, and that by far the lowest design loading is the AASHTO loading. Another way of saying this is that these other countries build more conservatism into their bridges, since the design loads are very heavy compared with the AASHTO loads. In Europe, the design philosophies differ somewhat from those in the USA, in that the Europeans tend to design for longer life (120 years in the UK), and they worry more about being able to transfer heavy military loads. They also wish to be able to occasionally transfer extremely heavy peace-time loads (power plant generators, etc.) without unduly endangering the structural integrity of their bridges. Legal l o a d s Closely related to the above discussion on design loads are the legally allowed loads. These vary somewhat from State to State in the United States, and also differ a great deal in the European countries. A tabulation of axle loads (single and tandem), and gross loads for a 5-axle tractor-trailer

Occasionally, it happens that it is desirable, or necessary in the case of emergencies, to move a load that is heavier than the legal loads discussed above. The question then arises as to how heavy such a load can be and still not damage a specific bridge. This then brings us to the whole complicated question of the actual load rating of bridges. At this point, it is interesting to note that bridge engineers all over the world like to distinguish between 'normal overloads' and 'exceptional overloads'. In the USA there is the variation of the above in terms of stresses that translate into 'inventory' rating, and 'operating' rating. The reasoning behind the two ratings is the same - because of a certain conservatism in design, construction, material properties, and the fact that it is highly unlikely that two or more heavy vehicles should precisely meet at a critical section of a specific bridge, it is said that a certain amount of overloading will not harm the bridges. (We are for the moment leaving out the possibility of increased fatigue damage from the discussion.) In the US, this is based on a stress below 0.55 o yield. In several of the European countries, a specific weight has been assigned as the upper limit of such 'normal', unrestricted overloads. Some of these weights are tabulated in Table 2. Total loads, or gross vehicle loads are shown, but most of the countries also have axle load limitations, and one must have a permit to operate such vehicles; some of the permits are for single passages only, but many of them (such as for construction equipment) are for a season or a year at a time. Usually there is no restriction placed on speeds of travel and the vehicles mix in with the usual traffic stream, and no damage is thought to accrue from an unlimited number of such passages. Note that several of the values shown in Table 2 are also the legal lihaits for a 5-axle vehicle. The 'exceptional' overloads are only allowed occasionally, and are more carefully controlled. Usually, they have to have

Table 1 Maximum legal weight of freight vehicles (in kips) >=

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-

i,. ¢

:=

=

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t~tJ3

--

Single axle Tandem axle Gross 5-axle

29 44 84

22 44 110

22 35 97

22 35 79

29 46 84

22 35 84

26 42 97

22 35 97

22 35 86

29 46 84

22 35 -

22 31 -

~ ~"

~

E)
I

--J

22 45 72

20 34 76

24 40 80

18 29 71

Eng. Struct., 1979, Vol. 1, October

231

Highway, bridge Ioadings: C. F. Galambos Table 2 Allowable 'normal' overloads (in

kips)

Total load

84

88

110

97

92

84

110

92

113

72

an escort, travel very slowly along prescribed paths across a bridge, and be the only vehicle on the span. Again, in the US, the load allowed is derived from the allowable stress not beyond 0.75 a yield. This is called the operating stress. In other countries, careful stress calculations for 'exceptional' overloads are also made, although in Belgium and France, an actual upper gross load limit of 792 kips and 880 kips, respectively, exist, along with certain axle load limitations. It is recognized that monstrous loads of 800 kips take very special vehicles for safe transport across a structure. L o a d rating It is desirable for a number of reasons to have a good inventory and the best estimate ef the present live load capacity of our highway bridges, whether we are talking about a smaller local jurisdiction or even the entire combined highway system of a nation. Rules and regulations 2 for compiling an inventory and for the procedure to be used for the load rating of bridges have been issued in the United States by the Federal Highway Administration for the Federal Aid Highway Systems, and it is not the purpose here to dwell on details, except to point out that much of the load rating process still comes down to something called 'engineering judgment'. Substantial differences in loadcarrying capacity can result for the same structure depending on who looks at it, which calculation methods he uses, how he treats items like lateral load distribution, impact, effect of corrosion, settlement, scour, material strengths, and any number of engineering and non-engineering parameters. To illustrate what variations in load ratings are possible, an interesting experiment was performed recently by one of the committees of the Office of Economic Cooperation and Development (OECD) a headquarters in Paris, France. The assignment was as follows: having been given the design drawings of a specific bridge with the material properties (yield strength, ultimate strength, etc.) and a specific vehicle, a 5-axle tractor-trailer combination shown in Figure 3, each of the cooperating agencies was to assess the live load that this vehicle could carry. Canada, Norway, the province of Ontario, the United Kingdom, and the United States, all submitted answers as to what they thought prudent and reasonable. The bridge was a 70-foot simple span, non-composite steel beam and concrete slab structure, taken from 'Standard Plans for Highway Bridges - Steel Superstructures'. 4 The answers are presented in Table 3, and they vary from a low gross load for the vehicle of 95 kips to a high of 350 kips. Differences come about because of four general areas of variations: inventory ratings vs operating ratings, and working stress vs load factor calculations. Other differences arise from philosophical differences about impact factors, load distribution, live load-dead load ratios, and what fraction of the yield or fully plastic moment it seems reasonable to use. All these variations are tabulated in Table 4. There it

232

Eng. Struct., 1979, Vol. 1, October

is seen that the Canadian Standards Association is willing to use almost 100% of the fully plastic moment of the bridge, provided that the vehicle load path and speed are strictly controlled, and that the vehicle is the only one on the span, and that such a load happens very infrequently, and that supposedly the bridge was built according to plan, that the material properties are equal or stronger than was assumed in the design, and that the bridge has been maintained to stay brand new. Other jurisdictions proceed with somewhat more conservatism. Because as was just shown above, there can be such a variation in even as simple and common a structure as a 70 ft beam and slab bridge, many engineers feel that the only way to really rate a bridge is to actually test it with a moving test load. Properly conducted load tests with realistic vehicles can be of great help in assessing the load carrying capacity of a structure. However, such tests must be well planned and executed, with strain and deflection gauges placed at major points of interest. If this is done, the tests help to determine the actual lateral load distribution, the effect of bracing members, deck behaviour, joint behaviour (pinned, frozen, or intermittently free), the degree of composite action, the amount of help received from curbs, sidewalks, and railings, the impact factors, and the actual vibration characteristics and other actual bridge behaviour. A word of caution must be inserted here to say that as useful as such load tests are, they do not tell everything about a structure. The extent of fatigue cracking, for example, and the material properties such as crack growth rates and notch toughness, cannot be determined by load testing. Nor can the usual load tests tell what the ultimate carrying capacity of the structure might be.

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Figure 3 3S2 rating example vehicle. Axle weights: 1,8

kips;

2, 16 kips; 3, 16 kips; 4, 16 kips; 5, 16 kips

Table 3 Rating results for 70-ft

span

Canada

Unsupervised mixing

Ontario

UK

Norway

USA

166

.

.

.

.

350

.

.

.

.

-

198

-

-

-

-

304

-

--

-

--

--

-

103

and

Supervised and only vehicle Readily available permit Controlled special permit Inventory working stress Inventory factor Operating stress Operating

CSA S6

95

load .

.

.

.

111

working --

--

136

--

168

-

270 306

184 --

load

factor W o r k i n g stress Load factor G r o s s l o a d in kips

. --

.

. -

.

Highway bridge Ioadings: C. F. Galambos Tab/e 4

Variations in load rating parameters Working stress

Load factor Load multiple Impact

Capacity of structure

Live

Dead

Capacity of structure

0.712 0.712

0.256 0.256

0.55 fyS 0.75 fyS

5/3 1

1 1

0.8 fyS 0.8 fyS

0.560

0.400

0.636 fyS

1.3

1.2

0.870 fyS

Design situation 0.586 Unsupervised and mixing 0.586 Supervised and only vehicle 0.686 Readily available permit 0.630 Controlled special permit 0.607

0.256

-

2.35

1.400

0.703 fyZ

0.256

-

1.933

1.286

0.831 fyZ

0.256/3

--

1.192

1.166

0.972 fyZ

1.35

1.169

0.92 fyZ

1.15

1.169

0.92 fyZ

Inventory Operating

USA Norway Canada

CSA S6

Ontario

Fraction of lane per stringer

0.45 0.15

- -

-

fy, yield strength of steel; Z, plastic section modulus of stringer; S, elastic

section

On older bridges, where no design or construction plans exist, load tests can serve the most useful purpose in determining whether the normal truck traffic can continue to use the bridge, or whether some load restriction has to be imposed. The bridge rating crew of the Province of Ontario, Canada, does a great deal of load testing of the kind just mentioned and they generally find that most bridges are stronger than one thinks they are. s

the figures. The double peak of the gross weight histograms seems to be typical of many American tests, showing in general empty tractor-trailer freight vehicles (25-30 000 pounds) and loaded ones (67-75 000 pounds). A composite loading histogram made up of a number of studies nationwide for the year 1970 is shown in Figure 10. Note again the double peak. A very legitimate observation is often made in connection with bridge loading history studies. And that is that the

Actual loads

modulus of stringer; CSA, Canadian Standards Association

9'0

In recent years, a great deal of actual weighing of vehicles has taken place in the USA, much of it for route planning purposes or for legal load enforcement, yet for the most part, such truck weighing is not directly applicable to bridge loading problems. We have therefore encouraged and participated in specific bridge loading history studies in which care is taken to weigh every vehicle (above a certain relatively minimum weight) crossing the structure in some representative time period. Usually, such studies also include the collection of strains (stress ranges) in selected members of the structure at the same time. Several typical samples of bridge loading histograms are shown in Figures 4-9. Both axle load distributions and gross vehicle weights are shown. The mean load, standard deviation and number of cases weighed is also shown on some of

72

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Gross vehicle weight, (kips)

Figure 5 Histogram for gross vehicle weight, all trucks, Shaffer Creek Bridge, 1968. N = 249; # = 40.5; a = 20.6; c.o.v. = 0.51

15

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Axle weight s (kips)

Histogram for axle weight, all axles, all trucks, Shaffer Creek Bridge, 1969. N = 3540; # = 8.4; o = 4.1 ; c.o.v. = 0.49 Figure 4

56

0

2:5

5 )

7:5

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12'5

15-O 17'5

20-0

Axte weight, (kips) Figure 6 Histogram for axle weight, all axles, all trucks, C.B. & Q. Bridge, 1969. N = 6751 ; # = 10.1 ; a = 3.9; c.o.v. = 0.39

Eng. Struct., 1979, V o l . 1, O c t o b e r

233

Highway bridge Ioadings: C. F. Galambos

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u_

1

40 50 60 7o 80 30 Gross vehicte weight, (kips) Figure 7 Histogram for gross vehicle weight, all trucks, C.B. & Q. Bridge, 1969. N = 1482; ;~ = 46.0; a = 18.7; c.o.v. = 0.41 10

0

20

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20 15

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90

frame into stiffer members. Fatigue cracks therefore grow that are deflection related. The latter case seems to be a design related problem. The fatigue damage caused on bridges, in a global sense, is a function of the number of live load stress ranges applied to the bridge. The stress ranges vary more or less linearly with vehicle gross load for the main bending members of bridges. For deck elements and floor beams, the wheel and axle loads produce the stresses. Enough crack growth information is available from laboratory fatigue tests and from actual bridge damage observations on various cracked details, for a relationship between loading and damage usable for design purposes to be developed. One such relationship is shown in Figure 11. The 'damage factor' incorporates several relationships between average daily truck traffic, desired design life, ratio of actual vehicle to design vehicle weights, and Miner's hypothesis of damage. A fuller description of the reasoning used is given elsewhere, l° but the above was the basis for the latest fatigue provisions for the steel design portion of the AASHTO Specifications. In Figure 11, it is seen that by far the greatest amount of damage is done by the heavily loaded, but still legal vehicles of 60 to 80 kips gross loading. Figure 11 is based on the composite truck weight histogram shown in Figure 10. It is not the purpose of this paper to dwell on fatigue problems, except by way of coming around to the question of what is the proper fatigue loading spectrum to be used

100 16

Figure 8 Truck-weight histogram constructed from east-bound

Y

weighing data obtained from Westport (solid line) and adjusted values for Bridgeport test site (broken line)

weights obtained are not really representative of day-in, day-out traffic, because the illegally overloaded truck drivers soon learn about the tests and then take an alternate route to go around the test site. The only way to avoid this is to monitor weights all the time, and some States are doing this with built-in weighing stations. There is also considerable research and testing going on worldwide on equipment and schemes for weighing trucks in motion, most of them with pavement platforms of one kind or another, but some of them are also using the bridge as the weighing mechanism. 6-9 Many of the schemes are workable and in the next few years, much more attention will be paid to the monitoring and enforcement of loads.

F

--m

cr u_

i

10

20

3O

40

50

60

70

80

90 1OO

Gross toed, (kips) Figure 9 Tractor-trailer gross weight distribution from locations in Ohio. All MR locations 2328 semi-trailers

Fatigue The interest in the actual loads on the highways is by no means just academic. Loads produce stresses and stresses, even small ones when repeated enough times, will cause a flaw to grow and possibly even cause the rupture of a member, but at the least, will make it necessary to spend money for crack repairs. There is increasing evidence of fatigue crack growth in American steel highway bridges. Often, the problem originates in secondary members in which a relatively large initial flaw exists, possibly due to poor fabrication practice and careless shop inspection. Several instances of the complete fracture of a major load carrying welded plate girder have been experienced in recent years, caused by initial flaws in gusset plates and horizontal and vertical stiffeners. An even greater number of fatigue problems come about when flexible members

234

Eng. Struct., 1979, V o l . 1, O c t o b e r

50 4C =z 3.0 2.0 10

%

G

S'0 60 70 80 Gross vehicte weight (kips) Figure 10 Composite gross vehicle weights, 1970 3'0

4 '0

90

100

Highway bridge Ioadings: C. F. Galambos t.8

Based on these thoughts on highway loadings, the following conclusions and recommendations are offered:

1.6

(I) It would seem that it is time to increase the AASHTO HS design loadings. There is something disquieting about having the lowest comparative design load in the industrialized western world. However, such an upgrading of the loading should not be done without a thorough economic study of the long-range consequences. (2) Although not treated in great detail in this paper, it is believed that considerable benefits in terms of longer pavement and bridge life would result from a universal and strict enforcement of legal allowable loads. In this respect, reference should again be made to the fatigue damage curve o f Figure 11. This curve does not show the influence of illegal loads. (3) Considerable improvements are yet to be made in the process of the load rating of bridges. It is believed that some standard load testing vehicle and scheme can be a very good tool in the rating process, especially when used in conjunction with bridge replacement priority settings. (4) In light of the considerable variation in actual live loads and the lack of enforcement of legal loads in some jurisdictions, it would seem counter-productive to further refine the fatigue live load design provisions for steel bridges. It may, in fact, be more prudent in the long run to avoid fatigue problems as much as possible and imitate the concrete provisions of the specifications.

1.4 i

"t.

12

o u

o

1.O

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~ 0"8 0

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02O

30

40 50 60 70 80 90 Gross vchicNt weight, (kips) Figure 11 Probable damage caused by various truck weights

10(D

for the design of highway bridges. The AASHTO Bridge Specifications have a definite difference in fatigue design philosophy between concrete and steel bridges. In concrete structures, the specification writers seem not to want to tolerate any hint of fatigue problems in the reinforcement in that the allowable live load design stress range has been set high above (near 21 ksi, depending somewhat on minimum stress and reinforcement deformations) and damaging fatigue loading likely to be experienced in the expected life of the structure. (This lbhilosophy seems to work, in that to the best of the author's knowledge, there have been no instances of fatigue ruptured reinforcing bars on highway bridges.) In the steel design portion of the specifications, the elaborate table presented relates geometric details to allowable stresses and to the repetitive number of these allowable stresses, which were derived as shown above. It is clearly evident that the steel fatigue design specifications concerning allowable stresses are much closer to what actual bridge members experience daily, when compared to the reinforcement in concrete structures. For instance, the allowable fatigue stresses in concrete reinforcement, when compared to those for steel members, would say that for over 2 000 000 cycles of loading, one should not allow any detail lower than type B. It appears that some re-thinking of the AASHTO fatigue specifications may be in order. Summary and conclusions For the most part, this paper presents a comparison of the AASHTO design live loadings with various other loading situations. Comparisons are made with some European design live loads with native and foreign legal loads, normal permit overloads, and abnormal permit loads. The results of a bridge load rating exercise are presented. Some actual bridge load histograms are given, as well as a more comprehensive composite histogram from 1970. Fatigue loadings and damage are discussed in the light of actual and design loadings.

A final note is added to the policy makers charged with the establishment of highway bridge design and allowable loads. The time is past when each mode of transportation such as highways, railways and canals can independently set its own limits on sizes and weights. Transportation of goods and people has become highly interchangeable between modes and any changes in weight limits must be made with the coordination and cooperation of all the affected sectors of transportation. References 1 'Standard Specification for Highway Bridges' (Twelfth edn), 1977, American Association of State Highway and Transportation Officials, Washington, D.C.20004 2 Manualfor Maintenance Inspection of Bridges (Second edn), June 1974, American Association of State Highway and Transportation Officials, Washington, D.C. 3 'Evaluation of Load Carrying Capacity of Existing Road Bridges, Report of Committee CM-2', OECD, Paris, France. (To be published in 1979) 4 'Standard Plans for Highway Bridges Volume II, Structural Steel Superstructures', Federal Highway Administration, Washington, D.C.20590, April 1968 5 Bakht, B. and Csagoly, P. F. 'Testing of Perley Bridge, Ontario', Ministry of Transportation and Communication, Research and Development Division, Rep. RR 207, January 1977 6 'Weighin Motion Instrumentation', Rep. No. FHWA-RD-78-81, Moses, F. and Kriss, M., Federal Highway Administration, Washington, D.C. 20590, June 1978 7 FothergiU, J. W. et al. 'Feasibility of Utilizing Highway Bridges to Weigh Vehicles in Motion', Volume I, Rep. No. FHWA-RD75-33, Federal Highway Administration, Washington, D.C., November 1974 8 Moses,F. and Goble, G. 'Feasibility of Utilizing Highway Bridges to Weigh Vehicles in Motion', Volume lI, Report No. FHWA RD 75-34, Federal Highway Administration, Washington, D.C., October 1974 9 Siegel,H. J. 'Feasibility of Utilizing Highway Bridges to Weigh Vehicles in Motion', Volume III, Rep. No. FHWA RD 75-35, Federal Highway Administration, Washington, D.C., November 1974 10 Fisher,J. W. 'Bridge Fatigue Guide - Design Details', American Institute of Steel Construction, New York, N.Y. 10020

Eng. Struct., 1979, Vol. 1, October

235