Structural Concrete - Hassoun

896 Chapter 22 Prestressed Concrete Bridge Design
minimum factor for the permanent loads are specified in Table 22.9. The minimum factors are
to be used in the strength limit states to produce maximum effects of live load. In other words,
when the effect of live load is opposite to that of permanent load, the minimum combinations are
to be used.
22.5 GRAVITY LOADS
Gravity loads include permanent and live loads that are caused by the self-weight of the bridge and
the weight of objects on the bridge. Such loads are applied in a downward direction (toward the
center of the earth).
22.5.1 Permanent Loads (AASHTO 3.5)
Permanent loads include dead loads and earth loads that remain on the bridge for the entire service
life. Such loads include DC, DW, EV, EH, ES, and DD specified in Section 22.4.2.
22.5.2 Live Loads (AASHTO 3.6)
The scope is limited to gravity live loads, which include vehicular live load (LL) and pedestrian
live load (PL).
22.5.2.1 Vehicular Live Load (AASHTO 3.6.1.1). A typical bridge is designed for several longitudinal
traffic lanes of equal width that are defined as traffic lane and design lane. The traffic
lane is the amount of traffic that the traffic engineer plans to route across the bridge whose
width is associated with a traffic lane of, typically, 10 to 14 ft. The design lane is the number
of lane designation that the bridge engineer uses for live-load placement to produce maximum
load effects.
Generally, the number of design lanes should be determined by taking the integer pat of the
ratio ω/12.0, where ω is the clear roadway width in feet between curbs and/or barriers. Engineering
judgment shall be used to ensure that the number of design lanes is not less than the number of traffic
lanes. In case where the traffic lanes are less than 12.0 ft wide, the number of design lanes shall be
equal to the number of traffic lanes, and the width of the design lane shall be taken as the width
of the traffic lane. Possible future changes in the physical or functional clear roadway width of the
bridge should be considered. Roadway widths from 20.0 to 24.0 ft shall have two design lanes, each
equal to one-half the roadway width.
22.5.2.2 Multiple Presence of Live Load (AASHTO 3.6.1.2). The extreme live-load effect shall
be determined by considering each possible combination of number of loaded lanes multiplied
by a corresponding multiple presence factor to account for the probability of simultaneous lane
occupation by full HL-93 design live load. Multiple presence factors have been included in the
equations for distribution factors and, therefore, shall not be applied in conjunction with load
distribution factors except where a lever rule is used. The multiple presence factors are given
in Table 22.10.
22.5.2.3 Design Vehicular Live Load (AASHTO 3.6.1.2). Although the automobile is the most
common vehicular live load on most bridges, trucks cause the critical load effects. Therefore,
AASHTO design live loads attempt to model the truck traffic that may occur independent of or
combined with other truck loads.