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bridge approach span length to inner span length ratio

For a bridge with multi-span, it is not recommended to keep the length of inner span and approach span equal. the sagging moment for approach slab at mid of approach will be the largest in the case that bridge spans are equal. therefore, the recommended approach span to inner span length is 0.75 to 0.80. figure no:1 comparing the bending moment for the bridge with equal spans and with span ratio of 0.80. we can notice that for a bridge of approach (last) span=0.8*internal span. the sagging moment is equal for all spans.

Figure 1  

approach(last) span length to inner span ratio shall not be less than 0.40. if the selected ratio is less than 0.40. the bridge will be significantly vulnerable for uplifting force effects.figure 2 shows the uplifting force due to the short length of the approach span

Figure 2


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