Tributary Area Load

P = q A_{T} w = q \times tributary width

A beam or column is assigned the floor load on its tributary area: halfway to adjacent members. For a girder spanning L with joists on spacing s, AT ≈ s L. Multiply uniform psf (or kPa) by AT to get the member load.

$q$ (psf or kPa): Uniform area load
$A_{T}$ (ft²): Tributary area
$P$ (lb or kN): Member load

Step-by-step solved example

Floor 80 psf. Beam spans 30 ft, tributary width 10 ft. Uniform line load on the beam?

Line load w = q × tributary width. Interior beam takes half the bay each side.

1. AT
AT = 10×30 = 300 ft².
2. Line load
w = 80 psf × 10 ft = 800 plf. Total P = 800×30 = 24,000 lb.
$w = 800 plf$

Answer: 800 plf (24 k total)

10 practice questions

0/10 correct

1.Beams at 12 ft spacing (equal bays). Interior tributary width =

2.Joists at 16 in o.c. Interior joist tributary width is

3.A corner column typically takes

4.q = 5 kPa, AT = 12 m². P =

5.Self-weight of the beam is

6.One-way slab tributary to a girder is a

7.Live-load reduction (ASCE) depends on

8.Double-counting a partition load in both q and a line load would

9.Cantilever slab tributary to the edge beam is

10.psf × ft² equals