The AASHTO Standard Specifications for Highway Bridges require bridges to be designed to carry dead and live loads and impact, or the dynamic effect of the live load. Structures should also be capable of sustaining other loads to which they may be subjected, such as longitudinal, centrifugal, thermal, seismic, and erection forces. Various combinations of these loads must be considered as designated in groups I through X. (See Art. 11.5.1.)
The LRFD Specification separates loads into two categories: permanent and transient. The following are the loads to be considered and their designation (load combinations are discussed in Art. 11.5.4):
DC dead load of structural components and nonstructural attachments
DW dead load of wearing surfaces and utilities
EH horizontal earth pressure load
EL accumulated locked-in force effects resulting from construction
ES earth surcharge load
EV vertical pressure from dead load of earth fill
BR vehicular braking force
CE vehicular centrifugal force
CT vehicular collision force
CV vessel collision force
IC ice load
IM vehicular dynamic load allowance
LL vehicular live load
LS live load surcharge
PL pedestrian live load
TG temperature gradient
TU uniform temperature
WA water load and stream pressure
WL wind on live load
WS wind load on structure
Certain loads applicable to the design of superstructures of steel beam/ girder-slab bridges are discussed in detail below.
Dead Loads. Designers should use the actual dead weights of materials specified for the structure. For the more commonly used materials, the AASHTO Specifications provide the weights to be used. For other materials, designers must determine the proper design loads. It is important that the dead loads used in design be noted on the contract plans for analysis purposes during possible future rehabilitations.
Live Loads. There are four standard classes of highway vehicle loadings included in the Standard Specifications: H15, H20, HS15, and HS20. The AASHTO Geometric Guide states that the minimum design loading for new bridges should be HS20 (Fig. 11.l) for all functional classes (local roads through freeways) of highways. Therefore, most bridge owners require design for HS20 truck loadings or greater. AASHTO also specifies an alternative tandem loading of two 25-kip axles spaced 4 ft c to c.
The difference in truck gross weights is a direct ratio of the HS number; e.g., HS15 is 75% of HS20. (The difference between the H and HS trucks is the use of a third axle on an HS truck.) Many bridge owners, recognizing the trucking industries use of heavier vehicles, are specifying design loadings greater than HS20.
For longer-span bridges, lane loadings are used to simulate multiple vehicles in a given lane. For example, for HS20 loading on a simple span, the lane load is 0.64 kips per ft plus an 18-kip concentrated load for moment or a 26-kip load for shear. A simple-span girder bridge with a span longer than about 140 ft would be subjected to a greater live-load design moment for the lane loading than for the truck loading (Table 11.7). (For end shear and reaction, the breakpoint is about 120 ft). Truck and lane loadings are not applied concurrently
for ASD or LFD.
In ASD and LFD, if maximum stresses are induced in a member by loading of more than two lanes, the live load for three lanes should be reduced by 10%, and for four or more lanes, 25%. For LRFD, a reduction or increase depends on the method for live-load distribution.
For LRFD, the design vehicle design load is a combination of truck (or tandem) and lane loads and differs for positive and negative moment. Figure 11.2 shows the governing live loads for LRFD to produce maximum moment in a beam. The vehicular design live loading is one of the major differences in the LRFD Specification. Through statistical analysis of existing highway loadings, and their effect on highway bridges, a combination of the design truck, or design tandem (intended primarily for short spans), and the design lane load, constitutes the HL-93 design live load for LRFD. As in previous specifications, this loading occupies a 10 ft width of a design lane. Depending upon the number of design lanes on the bridge, the possibility of more than one truck being on the bridge must be considered. The effects of the HL-93 loading should be factored by the multiple presence factor (see Table 11.2). However, the multiple presence factor should not to be applied for fatigue calculations, or when the subsequently discussed approximate live load distribution factors are used. Impact. A factor is applied to vehicular live loads to represent increases in loading due to impact caused by a rough roadway surface or other disturbance. In the AASHTO Standard Specifications, the impact factor I is a function of span and is determined from
For LRFD, the impact factor is modified in recognition of the concept that the factor should be based on the type of bridge component, rather than the span. Termed dynamic load allowance, values are given in Table 11.3. It is applied only to the truck portion of the live load.
Live Loads on Bridge Railings. Beginning in the 1960s, AASHTO specifications increased minimum design loadings for railings to a 10-kip load applied horizontally, intended to simulate the force of a 4000-lb automobile traveling at 60 mph and impacting the rail at a 25 angle. In 1989, AASHTO published AASHTO Guide Specifications for Bridge Railings with requirements more representative of current vehicle impact loads and dependent on the class of highway. Since the effect of impact-type loadings are difficult to predict, the AASHTO Guide requires that railings be subjected to full-scale impact tests to a performance level PL that is a function of the highway type, design speed, percent of trucks in traffic, and bridge-rail offset. Generally, only low-volume, rural roads may utilize a rail tested to the PL-1 level, and high-volume interstate routes require a PL-3 rail. The full-scale tests apply the forces that must be resisted by the rail and its attachment details to the bridge deck.
PL-1 represents the forces delivered by an 1800-lb automobile traveling at 50 mph, or a 5400-lb pickup truck at 45 mph, and impacting the rail system at an angle of 20. PL-2 represents the forces delivered from an automobile or pickup as in PL-1, but traveling at a speed of 60 mph, in addition to an 18,000-lb truck at 50 mph at an angle of 15. PL-3
represents forces from an automobile or pickup as in PL-2, in addition to a 50,000-lb vantype tractor-trailer traveling at 50 mph and impacting at an angle of 15.
The performance criteria require not only resistance to the vehicle loads but also acceptable performance of the vehicle after the impact. The vehicle may not penetrate or hurdle the railing, must remain upright during and after the collision. and be smoothly redirected by the railing. Thus, a rail system that can withstand the impact of a tractor-trailer truck, may not be acceptable if redirection of a small automobile is not satisfactory.
The LRFD Specifications have included the above criteria, updated to include strong preference for use of rail systems that have been subjected to full scale impact testing, because the force effects of impact type loadings are difficult to predict. Test parameters for rail system impact testing are included in NCHRP Report 350 Recommended Procedures for the Safety Performance Evaluation of Highway Features. These full-scale tests provide the forces that the rail-to-bridge deck attachment details must resist.
Because of the time and expense involved in full-scale testing, it is advantageous to specify previously tested and approved rails. State highway departments may provide these designs on request.
Earthquake Loads. Seismic design is governed by the AASHTO Standard Specifications for Seismic Design of Highway Bridges. Engineers should be familiar with the total content of these complex specifications to design adequate earthquake-resistant structures. These specifications are also the basis for the earthquake extreme-event limit state of the LRFD specifications, where the intent is to allow the structure to suffer damage but have a low probability of collapse during seismically induced ground shaking. Small to moderate earthquakes should be resisted within the elastic range of the structural components without significant damage. (See Art. 11.11.)
The purpose of the seismic design specifications is to . . . establish design and construction provisions for bridges to minimize their susceptibility to damage from earthquakes.
Each structure is assigned to a seismic performance category (SPC), which is a function of location relative to anticipated design ground accelerations and to the importance classification of the highway routing. The SPC assigned, in conjunction with factors based on the site soil profile and response modification factor for the type of structure, establishes the minimum design parameters that must be satisfied.
Steel superstructures for beam/ girder bridges are rarely governed by earthquake criteria.
Also, because a steel superstructure is generally lighter in weight than a concrete superstructure, lower seismic forces are transmitted to the substructure elements.
Vessel Impact Loads. A loading that should be considered by designers for bridges that cross navigable waters is that induced by impact of large ships. Guidance for consideration of vessel impacts on a bridge is included in the AASHTO Guide Specification and Commentary for Vessel Collision Design of Highway Bridges. This Guide Specification is based on probabilistic theories, accounting for differences in size and frequency of ships that will be using a waterway. The Guide is also the basis for the LRFD extreme-event limit state for vessel collision.
Thermal Loads. Provisions must be included in bridge design for stresses and movements resulting from temperature variations to which the structure will be subjected. For steel structures, anticipated temperature extremes are as follows:
Moderate climate: 0 to 120F
Cold climate: -30F to 120F
With a coefficient of expansion of 65 107 in/in/F, the resulting change in length of a 100-ft-long bridge member is
If a bridge is erected at the average of high and low temperatures, the resulting change in length will be one-half of the above.
For complex structures such as trusses and arches, length changes of individual members may induce secondary stresses that must be taken into account.
Longitudinal Forces. Roadway decks are subjected to braking forces, which they transmit to supporting members. AASHTO Standard Specifications specify a longitudinal design force of 5% of the live load in all lanes carrying traffic in the same direction, without impact. The force should be assumed to act 6 ft above the deck.
For LRFD, braking forces should be taken as 25% of the axle weights of the design truck or tandem per lane, placed in all design lanes that are considered to be loaded and which are carrying traffic headed in the same direction. These forces are applied 6.0 ft above the deck in either longitudinal direction to cause extreme force effects.
Centrifugal Force on Highway Bridges. Curved structures will be subjected to centrifugal forces by the live load. The force CF, as a percentage of the live load, without impact, should be applied 6 ft above the roadway surface, measured at centerline of the roadway.
Sidewalk Loadings. In the interest of safety, many highway structures in non-urban areas are designed so that the full shoulder width of the approach roadway is carried across the structure. Thus, the practical necessity for a sidewalk or a refuge walk is eliminated. There is no practical necessity that refuge walks on highway structures exceed 2 ft in width.
Consequently, no live load need be applied. Current safety standards eliminate refuge walks on full-shoulder-width structures.
In urban areas, however, structures should conform to the configuration of the approach roadways. Consequently, bridges normally require curbs or sidewalks, or both. In these instances, AASHTO Standard Specifications indicate that sidewalks and supporting members should be designed for a live load of 85 psf. Girders and trusses should be designed for the following sidewalk live loads, lb per sq ft of sidewalk area:
For LRFD a load of 75 psf is applied to all sidewalks wider than 2 ft.
Structures designed for exclusive use of pedestrians should be designed for 85 psf under either AASHTO specification.
Curb Loading. For ASD or LFD, curbs should be designed to resist a lateral force of at least 0.50 kip per lin ft of curb. This force should be applied at the top of the curb or 10 in above the bridge deck if the curb is higher than 10 in. For LRFD, curbs are limited to no more than 8 in high.
Where sidewalk, curb, and traffic rail form an integral system, the traffic railing loading applies. Stresses in curbs should be computed accordingly.
Wind Loading on Highway Bridges. The wind forces prescribed below, based on the AASHTO Standard Specifications, Group II and Group V loadings, are considered a uniformly distributed, moving live load. They act on the exposed vertical surfaces of all members, including the floor system and railing as seen in elevation, at an angle of 90 with the longitudinal axis of the structure. These forces are presumed for a wind velocity of 100 mph.
They may be modified in proportion to the square of the wind velocity if conditions warrant change.
Superstructure. For trusses and arches: 75 psf but not less than 0.30 kip per lin ft in the plane of loaded chord, nor 0.15 kip per lin ft in the plane of unloaded chord.
For girders and beams: 50 psf but not less than 0.30 kip per lin ft on girder spans.
Wind on Live Load. A force of 0.10 kip per lin ft should be applied to the live load, acting 6 ft above the roadway deck.
Substructure. To allow for the effect of varying angles of wind in design of the substructure, the following longitudinal and lateral wind loads for the skew angles indicated should be assumed acting on the superstructure at the center of gravity of the exposed area.
When acting in combination with live load, the wind forces given in Table 11.4 may be reduced 70%. But they should be combined with the wind load on the live load, as given in Table 11.5.
For usual girder and slab bridges with spans not exceeding about 125 ft, the following wind loads on the superstructure may be used for substructure design in lieu of the more elaborate loading specified in Tables 11.4 and 11.5:
Wind on structure
50 psf transverse
12 psf longitudinal
Wind on live load
100 psf transverse
40 psf longitudinal
Transverse and longitudinal loads should be applied simultaneously.
Wind forces applied directly to the substructure should be assumed at 40 psf for 100- mph wind velocity. For wind directions skewed to the substructure, this force may be resolved into components perpendicular to end and side elevations, acting at the center of gravity of the exposed areas. This wind force may be reduced 70% when acting in combination with live load.
Overturning Forces. In conjunction with forces tending to overturn the structure, there should be added an upward wind force, applied at the windward quarter point of the transverse superstructure width, of 20 psf, assumed acting on the deck and sidewalk plan area.
For this load also, a 70% reduction may be applied when it acts in conjunction with live load.
For LRFD wind load calculations, see Art. 13.8.2.
Uplift on Highway Bridges. Provision should be made to resist uplift by adequately attaching
the superstructure to the substructure. AASHTO Standard Specifications recommend
engaging a mass of masonry equal to:
1. 100% of the calculated uplift caused by any loading or combination of loading in which
the live-plus-impact loading is increased 100%.
2. 150% of the calculated uplift at working-load level.
Anchor bolts under the above conditions should be designed at 150% of the basic allowable stress.
AASHTO LRFD Specifications require designing for calculated uplift forces due to buoyancy, etc., and specifically requires hold down devices in seismic zones 2, 3, and 4.
Forces of Stream Current, Ice, and Drift on Highway Bridges. All piers and other portions of structures should be designed to resist the maximum stresses induced by the forces of flowing water, floating ice, or drift.
For ASD or LFD, the longitudinal pressure P, psf, of flowing water on piers should be calculated from
P = KV^2
where V velocity of water, fps, and K constant. In the AASHTO Standard Specifications, K = 1.4 for all piers subject to drift build-up and for square-ended piers, 0.7 for circular piers, and 0.5 for angle-ended piers where the angle is 30 or less.
In the ASSHTO LRFD Specifications, the pressure P, ksf, is calculated from
where V velocity of water, fps, for design flood and appropriate limit state, and CD is a drag coefficient (0.7 for semi-circular nosed pier, 1.4 for square ended pier, 1.4 for debris launched against pier, and 0.8 for wedge nosed pier with nose angle 90 or less).
For ice and drift loads, see AASHTO specifications.
Buoyancy should be taken into account in the design of substructures, including piling, and of superstructures, where necessary.