Influence Lines

A bending moment influence line for any given point X on a structure is a line whose ordinate at any point B gives the bending moment at X when a load is placed at B (keep reading it and look at the example below - it will make sense eventually!). Influence lines can be obtained for bending moments, shear forces and reactions.
A bending moment influence line is drawn for one point only which may not necessarily be the point of maximum bending. A bending moment diagram, by comparison with a influence line, will give the bending moment at all points for one position of load.

Example - Simply Supported Single Span


There is an Excel spreadsheet called InfLine which you can download from the Design Spreadsheets web page. The proforma calculates bending moment influence lines for single span or multispan continuous line beams at critical sections.

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For more information contact David Childs