Tributary area |
. |
The
proportion of the floor or roof load bearing down on an individual support
beam or |
column
is estimated by dividing the total loaded surface into tributary areas
corresponding |
to each
particular support member. The following model shows how the
tributary areas of |
a
reinforced concrete slab are allocated to each of its supporting columns.
The boundary |
of a support's tributary
area is demarcated as half the distance to any adjacent supports. |
. |
 |
 |
|
Fig. 208 - Tributary areas |
of columns supporting |
a concrete slab |
|
each square is 2' x 2' |
|
(training aid model) |
|
|
. |
The four
corner columns and their corresponding tributary areas are colored yellow.
The |
four
perimeter columns and their areas are red. And the central column
and area is blue. |
. |
Let's
say the slab is 14 feet by 14 feet for a total area of 196 square feet.
It has a dead load |
weight
of 286 lbs./sq. ft. and is designed to carry a live load of 90 lbs./sq.
ft. The following |
table
calculates the total maximum anticipated load for each column position. |
. |
Column |
Tributary area |
Live load |
Dead load |
Total load |
All columns |
corner |
4x4 = 16 |
16x90 = 1440 |
16x286 = 4576 |
6016 lbs. |
24064 lbs. |
perimeter |
4x6 = 24 |
24x90 = 2160 |
24x286 = 6864 |
9024 lbs. |
36096 lbs. |
center |
6x6 = 36 |
36x90 = 3240 |
36x286 = 10296 |
13536 lbs. |
13536 lbs. |
|
Total |
73696 lbs. |
|
. |
Table
8 - Tributary areas and loads for columns supporting a concrete slab |
. |
The
table indicates that the central column bears more than twice the load of
a corner |
column,
and one and one-half times the load of a perimeter column, at the maximum |
design
load. This is a very simplified presentation of the tributary area
concept, which can |
get much
more complex depending on the framing plan of the building. |
. |
If you
were designing this structure you would need to combine this load data
with other |
information on the maximum allowable deflection, critical buckling load,
maximum tensile |
and
compressive stress, maximum bending moment, safety factor, etc. in order
to select |
the
appropriate material and dimensions for its members.
|
. |
Now
let's see how structural members are combined to build complex
residential, public, |
commercial, and industrial buildings. |
. |
Back
to Knowhere |
 |
Page 124
- Building stability - Tributary area |
 |
|