Load Paths and
Tributary Area Examples
© T. Bartlett Quimby, 2007
A Beginner’s Guide to Structural
Mechanics/Analysis
Alaska State Fairgrounds
Farm Exhibits Building
Palmer, Alaska
Long Span Roof Truss Girders
Mezzanine Area
Awning Roof with
Hip Beam
A large open exhibit building with long
span truss girders.
Awning Roof
Each joist supports
an area equal to its
span times half the
distance to the joist
on either side.
The joists transfer
their loads to the
supporting truss
girders.
Roof deck transfers load
to supporting joists.
Load rests on roof deck
The pier supports half the
area supported by the truss
Each truss
supports
girdergirder
plus area
from an
other
area equal
to itselements
span times
structural
that it
half the
distance to the girder
supports.
The truss girders
on either side.
transfer their loads
to the supporting
piers and columns.
Long Span Roof Load Path
The girders are not
single span so the
tributary area for the
columns cannot be
graphically determined
The area tributary to a
joist equals the length of
the joist times the sum of
half the distance to each
adjacent joist.
The area tributary to a girder
equals the length of the
girder times the sum of half
the distance to each adjacent
girder.
Columns Support Girders
Girders Support Joists
Metal Deck/Slab System
Supports Floor Loads Above
Joists Support Floor Deck
Mezzanine Floor System
The point load consists of
the reaction from the two
supported joists which
equals the tributary area (1/2
the cantilever span times the
Exterior
spacing
joist
of the
carried
cantilevers)
load to
the
times
supporting
the pressure
cantilever
load on
the floor
beam
plus ends
the self weight
of the joist.
The load diagram for the
cantilever (excluding self wt)
consists of a single point
load at the end of the
Deck
carries load to edge
cantilever.
joist and wall.
Cantilever Loads
End Wall
Framing
The beam-columns do
not support
any roof
For lateral
pressures,
the
load,
they
are
here
to
siding spans between the
resist
lateral forces
that
horizontal
girts (yet
they fancy
receive
from
another
word
forthe
a
girts. They support
an
beam!)
The
girts
support
the
area that extendshalf
from
siding
to the
adjacent
girts.
locations
half
way to the
This is the
tributary area
adjacent
beam-columns
one
girt.
on each sidefor
and
from
floor to roof as shown.
The girts transfer their
lateral load to the
The
beam-columns
supporting
beamtransfer their
lateral
columns.
loads equally to the roof
and foundation.
Hip Beam
This beam picks up load from
joists of varying lengths. In
this case the resulting load
distribution would have a
linearly varying component.
The illustrated area is part of
the tributary area at the roof
deck level.
The hip beam also picks up a
point load reaction from a pair
of the roof girders.