Lessons · Engineering · Which area a member actually carries
A member carries the area that drains to it
Tributary area is the piece of floor whose load a given beam or column actually takes: halfway to the neighbour on each side. Multiply that area by the load per square metre and you have what the member carries, before any arithmetic about the member itself.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.
What it is for
Getting this wrong is the quiet way to be wrong by a factor of two. An internal beam with a neighbour on both sides takes half a span from each -- a full bay -- while the edge beam takes half of that, and the two are easy to swap when reading a grid.
How to think about it
Draw the grid, mark the midline between your member and each neighbour, and shade what is enclosed. Width times span is the area. Internal members take from both sides, edge members from one.
Worked example
Beams at 3 m centres, spanning 6 m.Grid first, always.
An internal beam takes 1.5 m each side: 3.0 m wide.Halfway to each neighbour.
Area = 3.0 x 6.0 = 18 square metres.Width times span.
An edge beam takes 1.5 m one side only: half the area.Nine square metres.
Your turn
Beams at 4 m centres span 5 m. Write the tributary area of an internal beam, in square metres.
Internal beam, 4 m centres, 5 m span: area square metres
Solve one, graded on the server
The trap
Giving an edge beam a full bay. It has a neighbour on one side only, so it takes half, and the error doubles everything after it.