Design Loads for Awnings and Canopies
Load for which awning and canopies may need to
be designed can be categorized as follows:
Dead Load
This is the self-weight of the awning or
canopy frame, fabric and hardware. This load must always be
included with other design loads since it is always acting
on the structure. For instance, if one were designing an awning
for 20 psf snow load, and the structure itself weighed 2 psf,
then the design for snow should actually account for 22 psf
total load.
Wind Load
This load, as well as snow load, are usually
the most critical loads on awnings and canopies.
Important aspects of wind load are:
A. Speed or Velocity
Basic wind pressure is a function of its speed. Basic wind
pressure (psf) can be computed as the product of 0.00256 times
the square of the wind speed (mph). It can be readily observed
then, for example, that the wind forces on an awning are four
times greater if the wind speed is doubled, and the forces
are nine times greater if the wind speed is tripled. Design
wind speeds are generally shown on maps published in the building
code. Local codes may require higher design wind speeds.
B. Exposure
This is a general category for the amount of protection from
the wind that is afforded by the surrounding environment.
Structures located in wooded areas, for instance, do not have
to be designed for as much wind force as a structure located
in an open area.
C. Gusts
These are short-term excursions of velocity above the steady
design velocity, which must be accounted for in the design.
D. Drag, Lift
Drag is the wind-induced pressure toward the fabric surface,
and lift is the pressure away from the fabric surface. Wind
forces on an awning system act in different directions (toward
or away from the fabric surface depending on a variety of
factors). When designing an awning frame, all these factors
must be taken into account.
E. Return Period
This term is used to describe the time interval which is the
basis for establishing the required design wind speed. For
most applications the return period is 50 years. This simply
means that the required design wind speed is that which has
a 0.02 statistical probability of occurring once in 50 years.
Loss and safety experts have determined that it is an acceptable
level of risk and have based code design requirements on it.
Snow Load
Required design snow loads are established
by maps published in the building code. As in the case for
wind, sometimes local requirements are more stringent. On
the other hand, in many localities there is no requirement
for snow load design. Check with the local department of building
and safety.
Some important considerations about designing
for snow are:
A. Ground Snow
The beginning point for snow design, this is the pressure
of the designed snow load occurring at ground level.
B. Exposure
A categorization of the amount of protection afforded by the
surroundings. interestingly, the exposure factor works opposite
of the way it works for wind. Whereas a wooded environment
would result in a lowering of the wind forces on a structure,
a wooded environment would result in higher snow loads than
an open environment would.
C. Flat Roof Snow Load
This is the design load occurring at the actual roof level,
and results from factoring the ground snow load by a coefficient
accounting for exposure and height. Many times the flat roof
snow load can be as little as 0.6 or 0.7 times the ground
snow load. For example, the snow map or the code may indicate
a 20 psf ground snow load; the actual design pressure required
for an awning may be as little as 12 psf.
D. Drifting
Building codes require that the phenomenon of drifting snow
be accounted for in the design of roofs; this includes awnings
and canopies. While it is beyond the scope of this publication
to discuss this in detail, the effects of drifting snow can
be significant. The codes describe the procedure for designing
with snow drifting in mind.
E. Return Period
See discussion under Wind Load.
Live Load
These are loads that are associated with
the forces related to human occupants, furniture, equipment,
etc. Since these loads are movable, the live load stipulation
is an allowance for the most severe anticipated condition
or case. Common code requirements for roof live loads are
from 12 to 20 psf. Provided that the case of ponding water
is properly addressed, live loading is not a practical requirement
in the design of awnings. Some codes do not require a live
load design, and others greatly reduce the requirement.
Ponding
Addressed elsewhere in this publication,
this is a potential load on an awning or canopy and must be
addressed in one of several ways:
A. Design for ponding must be taken structurally
B. Keeping the fabric properly supported
and taut will avoid the problem.
C. Remove snow before it melts and ponds
water.
D. All the above.
Seismic Load
These are loads due to earthquakes or earth
tremors. The design process for earthquake loads is also too
elaborate to be included in this publication.
However, awnings and canopies tend to fare
well in earthquakes for the following reasons:
A. They are lightweight; lightweight structures
do not have a lot of mass, therefore, relatively small seismic
forces are likely to be developed. F= ma.
B. They are generally small, secondary structures.
Compared to the structures to which they are attached, which
are subject to significant destructive forces due to their
larger mass, these structures are relatively unaffected. F=
ma. Although seismic design requirements are not rigorously
pressed in geographical areas not significantly affected by
earthquakes, most model codes contain the provision in current
editions.
Return to Awnings
|
