Note:  Updated March 2018 in response to 3 negative user feedback comments for more information.  Please thumb this up!

A canopy is a structure which provides overhead protection from the elements such as rain, snow, or sunlight. Canopies can be attached to a structure or they can be free standing with their own supports. They can be constructed of a variety of materials including steel, concrete, aluminum, wood, or even fabric. According to ASCE 7-10, canopies are to be designed to resist wind loads, roof live loads, dead loads, snow loads, rain loads, seismic loads, wind-on-ice loads, and weight of ice loads.

ASCE 7-10 permits canopies to be designed for wind loading using equations for Open Buildings or Other Structures and Building Appurtenances depending on a variety of factors to be discussed further below. In order to determine the applied wind pressures, ASCE 7-10 has a variety of factors that affect wind loading such as wind speed, risk category, exposure category, flow type, etc. The code requires us to apply all of the aforementioned loads simultaneously, however we are allowed to reduce these loads based upon the load combinations specified in ASCE 7 and design our system based on the least favorable loading scenario applied in all directions.

Along with ASCE 7-10, canopies are designed in conjunction with the governing building code in the municipality in which the project is located [i.e. Florida Building Code Sixth Edition (2017) or the 2012/15/18 International Building Codes] which can contain other design limitations such as allowable member deflections and the newer use of ASCE 7-16 which is forthcoming and has many additions to the below.

Free Standing

Free standing canopies are self-supporting roof systems without walls such as pavilions or walkway covers. These structures are designed using the Open Buildings portion of ASCE 7-10. The wind flow for these structures can either be clear or obstructed depending on obstructions below the roof. Obstructed wind flow is considered when 50% or more of the area below the surface of the roof is composed of objects inhibiting wind flow such as a canopy atop a concrete basin at a wastewater treatment plant. Canopies with less than 50% obstructions below the roof surface would be considered clear. Components & cladding and MWFRS (main wind force resisting system) wind loading are used to design the different elements of these structures. See ASCE 7 Main Wind Force vs. Components & Cladding Explained (MWFRS vs. C&C) for a description on when to apply C&C vs. MWFRS wind loading.

Host Attached

Host attached canopies rely on the superstructure for stability such as suspended canopies, louvered roofs, or fabric awnings. Wind loading on host attached canopies depends on the size of the canopy in relation to the superstructure and its location on the building. There is a new section in ASCE 7-16 called Attached Canopies on Buildings which address wind loading on these structures. However due to the lack of provisions in ASCE 7-10, host attached canopies are designed using the Roof Overhangs sections of the code. When the canopy is small in respect to the building which is common in commercial applications (see image below), the canopy is to be designed as a Roof Overhang Component. Wind tunnel tests (see images below) have confirmed that the entire canopy should be designed using C+C wind loading.

Roof Overhang Components

MWFRS (Main Wind Force Resisting System) Roof Overhangs

When the canopy is similar in size to the building which is common in residential applications (see image below), the canopy is to be designed for MWFRS Roof Overhang wind loading.

Trellises and Sunshades

Trellises and sunshades are porous which allow wind/rain/snow to pass through them and are utilized to reduce sunlight. The porous nature of these roofs doesn’t allow a pressure differential to occur between the opposing faces of the roof surface and therefore are designed using the Other Structures and Building Appurtenances portions of ASCE 7-10. These wind pressures are applied vertically and laterally to the structure to develop the least favorable loading conditions. In addition, trellises and sunshades are required to resist a 300-pound concentrated load placed anywhere on the structure to simulate a maintenance worker standing on the roof surface.

Fabric Awnings

Fabric awnings have provisions which permit the design of the structure with the fabric at a reduced force and the structure without the fabric at the code minimum full design load.  From FBC/IBC chapter 31:

Florida Building Code 2017:

3105.5.1 Loads. Rigid awnings and canopy shutters shall be designed to resist the loads set forth in Chapter 16 of this code except that structures or parts thereof which are intended to be removed or repositioned during periods of high wind velocity shall be designed in their open or extended position to design pressures based on a basic wind speed of minimum 115 mph, 3-second wind gust with applicable shape factors and to resist not less than 10 psf (478 Pa) roof live load.

ASCE 7-10 requires that “roofs-fabric construction” utilize 5 psf uniform load, and 300 lb concentrated load. Since the IBC and FBC sections seems to override these requirements, it’s acceptable to design with or without the ASCE loads, provided the IBC and FBC (17) sections are followed strictly.

 

From FBC (2014)

3105.4.2 Design of the structural framing members shall be based on rational analysis, using the applicable wind loads of Chapter 16 as shown below.

3105.4.2.1 The wind design loads for any fabric or membrane-covered structure designed with a quick removal or breakaway membrane or fabric at wind velocities of 75 mph, shall be based on the following criteria:

1. Minimum wind speed 105 mph.

2. Exposure Category B, C or D as defined in Chapter 16.

3105.4.2.2 The wind design loads for any fabric or membrane covered structure designed with a permanent or non-removable fabric or membrane, shall be based on the following criteria:

1. Minimum wind speed velocity as required in Chapter 16, using Figure 1609C.

2. Exposure B, C or D as defined in Chapter 16.

3105.4.3 The fabric portions of awnings and fabric-covered frames shall be securely laced, tied or otherwise fastened to the frame; no rafter or front bar will be permitted in pockets; and in no case shall a rolling curtain be caused to operate over a canopy frame.

 

From INTERNATIONAL BUILDING CODE (2012 & 2015):

RETRACTABLE AWNING.
3105.3 Design and construction. Awnings and canopies shall be designed and constructed to withstand wind or other lateral loads and live loads as required by Chapter 16 with due allowance for shape, open construction  and similar features that relieve the pressures or loads.

 

Article provided and updated by Zachary A. Rubin, E.I.  Additional fabric commentary by F. Bennardo July, 2017

Mar 28, 2016   5561    Codes & Standards, Engineering Principles    
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