Civil Engineering Reference
In-Depth Information
The hope is that this uniform distribution will simplify the design of
residential buildings with two common structural systems: roof rafter sys-
tems with either a ridge board or a supporting ridge beam. Buildings with
roof trusses are not eligible for the simplifi ed ridge-to-eave unbalanced load.
For these systems, the axial and fl exural stresses in the truss components are
functions of truss geometry, which is not standardized.
6.2
Curved Roofs
Because of the complicated geometry of a curved roof, ASCE 7-10 presents
the balanced and unbalanced load cases in Figure 7-3.
6.2.1
Balanced Loads
For the case in which the slope at the eave of the curved roof is less than 30°
(i.e., Case 1), the balanced load is uniform near the crown and trapezoidal
adjacent to the eaves. The uniform load extends over the low-sloped portion
of the roof where
C
s
is 1.0 as determined from Figure 7-2. For example, if
C
t
=
1.1 and the roof had an unobstructed, slippery surface, the uniform
load region would extend to the point where the roof slope was greater than
10° (see Figure 7-2b). Beyond the uniform load region, the balanced load
is assumed to decrease linearly to a value corresponding to the sloped roof
snow load at the eave. The sloped roof snow load at the eave is determined
by multiplying the fl at roof snow load by the roof slope factor associated
with the slope at the eave. To simplify the snow load diagram, the sloped
roof snow load,
p
s
, is established at selected points and a linear interpolation
between the selected points is used. This method approximates a curved roof
snow load, yet avoids the need to evaluate the roof slope and
C
s
for every
point along the curved-roof surface.
When the eave slope is between 30° and 70° (i.e., Case 2), the balanced
load is uniform near the crown (where
C
s
=
1.0) and there are two trapezoi-
dal loads—a middle trapezoid and an edge trapezoid—located near the eave.
The middle trapezoid starts where
C
s
1.0 and terminates where the roof
slope is 30°. The intensity of the balanced load at the termination point is
p
f
multiplied by
C
s
corresponding to a 30° roof slope. The edge trapezoid
extends from the 30° roof slope to the eave. The value of the balanced load
at the eave equals
p
f
multiplied by
C
s
for the roof slope at the eave. If the
roof slope at the eave is greater than 70° (i.e., Case 3) then the trapezoidal
edge load is replaced with a triangular edge load. The triangular edge load
terminates where the roof slope is equal to 70°.
<
6.2.2
Unbalanced Loads
The unbalanced load case for a curved roof has zero load on the windward
side and, for the simplest case, a trapezoidal load on the leeward side. In all
cases, the unbalanced load at the crown is 0.5
p
f
. For Case 1, in which the
