Page 234 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 5 LOAD AND RESISTANCE FACTOR RATING AND REDESIGN 209

5.5 SELECTION OF TRUCK LIVE LOADS
5.5.1 The Role of the Highway Administration
In the U.S., truck loads such as HL-93 are based on the combined intensity of truck and lane

loads. Three types of traffic volumes can be summarized as:
1. Recommended truck loads are HL-93. Legal and permit loads for the state in which the
bridge is located are also applicable.

2. High through traffic on interstate and major highways between states or cities where the
average number of trucks per lane per day exceeds 1000 and the average of all vehicles
exceeds 4000.
3. Moderate traffi c on arterial roads serving property and business access where the average
number of trucks per lane per day is between 250 and 1000, and the average of all vehicles
is between 1000 and 4000. Recommended truck loads are either HL-93 or other as specifi ed
by the state in this category.
4. Limited traffic on local roads serving small communities where the average number of trucks

per lane per day is less than 250 and the average of all vehicles is less than 1000. Recom-
mended truck loads are for the state or county in which roads are located.
Older bridges with occasional truck load are posted for the maximum weight of truck per-
missible to prevent overstress.
5.5.2 Historical Perspective of Live Load Vehicles Specified for Design

1. According to increasing traffic demands, truck loads have been upgraded in the U.S. Major

changes occurred in 1944 and 1993. These have been supplemented by special truck loads
for which permits are required. There are restrictions on use by permit trucks such as use
during night time windows with very light traffi c and police escort. Weigh bridges are in-
stalled to monitor the maximum weights of trucks.
2. HS-20 truck: An idealized truck load of 72 kips is applied using the HS-20 vehicle load

configuration, which has not changed since 1944 when it was first introduced as a trac-
tor-semi-trailer combination. In 1944 the idealization was based on a three-axle group of
vehicles known as 3-S2’s. Load length varied between 28 feet and 44 feet.
3. Interstate or tandem load: The drawback with the above idealization was the long length
between axles that did not govern for the design of smaller span bridges. It was felt that a
smaller distance was needed between axles, of minimum 4 feet spacing. During construction
of the interstate system, the two-axle tandem load of 25 kips each was introduced. Those
states that adopted HS-25 also increased the tandem load by 25 percent.

4. Some states have specified rating vehicles and legal vehicles.
5.5.3 Details of 2007 LRFD Specifi cations
1. Separate analyses are required to obtain an envelope of bending moments and shear forces

for each set of vehicles. Basically, stress, fatigue, and deflection limits are applicable based
on selected vehicle types causing maximum effects.
• HS-20 truck 4 lane load (HL-93) as shown in Figure 5.10
• 90 percent HS-20 truck pair 4 lane load (alternate to HL-93 tandem pair spaced at 50 ft
min.; 14 ft typical spacing between 32 kips axle)
2. Tandem truck 4 lane load (alternate HL-93) as shown in Figure 5.11.
• 90 percent tandem pair 4 lane load (alternate to HL-93 tandem pair spaced at 50 ft min.;
14 ft typical spacing between 32 kips axle)