ENCE 717 - Bridge Engineering



ENCE 717 - Bridge Engineering

Dr. C. C. Fu

Homework 1

(by Spreadsheet/MathCad; due)

For interior girder with girder spacing of 7', find maximum (Live Load plus Impact) moments of simply supported beams with span lengths 10' to 150' for every 10' increment based on AASHTO Standard Specifications (WSD/LFD) and AASHTO LRFD Specifications (LRFD).

Following procedures shall be followed:

1. Influence lines of all spans due to unit load shall be generated as intermediate step (see handout example; ordinates are used for concentrated load and areas are used for uniform load).

2. For WSD/LFD, maximum "unit" live load moments due to truck, lane, and interstate shall be calculated and the critical one shall be recorded.

3. Similarly, for LRFD, maximum "unit" live load moments due to [truck + lane] and [tandem (interstate) + lane] shall be calculated and the critical one shall be recorded.

4. Respective Impact Factor (IF) of WSD/LFD and LRFD shall be calculated and recorded.

5. Respectively Distribution Factor (DF) of WSD/LFD and LRFD shall be calculated and recorded.

(For LRFD calculation, assume preliminary design and thus Kg/(12Lts3) = 1)

(Hint: AASHTO Standard defines wheel load distribution factors and AASHTO LRFD Spec. defines axle load distribution factors)

6. Maximum (Live Load + Impact) moments = Max. "unit" live load moments x DF x (1 + IF)

7. Use Excel (or equivalent program) to record all items listed from procedures 1-6 (as columns) and span lengths as rows;

8. Draw lines of WSD/LFD and LRFD maximum moments on a graph;

9. To make a reasonable comparison, draw lines of [1.3 x 5/3 x LFD moment] and [1.75 x LRFD moment]

10. Your Conclusion!

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