24.02.2017 Views

Structural Concrete - Hassoun

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

22.5 Gravity Loads 899<br />

Design<br />

tandem<br />

(a) Design tandem + design lane loading<br />

25 K 25 K<br />

4 ft<br />

0.64 klf<br />

Live loading<br />

combinations<br />

Uniform lane<br />

loading: 0.64 klf<br />

Design truck<br />

(HL 93)<br />

(b) Design truck + design lane loading<br />

32 K 32 K 8 K<br />

14 to 30 ft 14 ft<br />

0.64 klf<br />

(c) Negative moment<br />

8 K 32 K 32 K<br />

32 K 32 K 8 K<br />

14 ft 14 ft >= 50 ft 14 ft 14 ft<br />

0.64 klf<br />

Figure 22.8<br />

The AASHTO HL-93 design loadings and load combinations.<br />

The loads are placed in such a way as to generate maximum live-load effects in the combinations<br />

described above, among which the critical condition governs the design. These combinations<br />

are summarized in Fig. 22.8.<br />

22.5.2.3.5 Fatigue Load (AASHTO 3.6.1.4). One truck design with the variable axle spacing set<br />

at 30 ft shall be considered in the fatigue and fracture limit state to take into account the cyclic live<br />

loading. The dynamic load allowance must be included.<br />

Since the fatigue and fracture limit state is defined in terms of accumulated stress-range cycles,<br />

load should be specified along with the frequency of load occurrence, namely, average daily truck<br />

traffic (ADTT).<br />

22.5.2.4 Pedestrian Loads (AASHTO 3.6.1.6). A pedestrian load of 75 lb per liner foot (or<br />

0.075 ksf) shall be applied to all sidewalks wider than 2.0 ft concurrently with the vehicular<br />

load.<br />

If bridges are designed for only pedestrian and/or bicycle traffic, then 85 lb per liner foot<br />

(or 0.085 ksf) live load shall be applied. Pedestrian and/or bicycle bridges shall be designed for<br />

maintenance truck and/or other incidental vehicles that are intended to used.<br />

22.5.2.5 Dynamic Load Allowance: IM (AASHTO 3.6.2). Roadway roughness and irregularity<br />

induces the vehicle suspension systems to oscillate compression and extension. Axle force

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!