Loads & anchoring: Awnings & canopies
Fabric Architecture | Sourcebook 2005
A major consideration in designing a large awning or canopy for a structure is how it will be secured. Anchorages—elements used to fasten an awning system to a building and the ground—stabilize the awning.
Most anchoring is done using one of four tried-and-true methods: friction, keying, combined friction and keying, or adhesion.
The various awning shapes and sizes will pose unique anchoring challenges. Projects having especially heavy weights and/or unusual spans typically will require an engineer’s help. In the process of determining the best anchoring system for a job, you should consider these four items:
- the amount and direction of loads imposed on the frame
- the location of the anchors
- the reaction loads at the anchors
- details and material of the parent structure.
The first three items are primarily of concern to the fabricator or engineer, although the architect should be somewhat familiar with them. The fourth item, however, is something an architect is generally best suited to evaluate.
Because they determine the strength and number of anchorages needed, the loads imposed on the frame bear further discussion, as does the parent structure, since its compositional materials can determine the anchor type.
The four types of loads awnings and canopies need to withstand are wind, snow, ponding and drift. In certain regions, seismic loads also may deserve consideration. Ponding occurs when rain or melted snow collects on the fabric, causing it to sag and add to the awning structure’s weight load. Ponding and snow loads are dead loads on a canopy; wind creates a live load.
Winds can act upon awnings and canopies in numerous ways, subjecting them to uplift, wind shear, gusts and steady winds, not to mention the infrequent tornado, tropical storm and hurricane.
The designer or engineer working on an awning must understand which, if any, of these forces will be at work on the awning once it is in place. If wind pressure is calculated incorrectly—or not calculated at all—uplift or down force may rip awning anchorings loose or cause the rafters or tubes of the frame to buckle.
Before hitting the drawing board, become familiar with local building codes that have jurisdiction over the area in which the awning will be placed and use them to determine design loading for the structure. Also, consult elementary engineering statics to resolve wind-and snow-load pressures at the awning’s supports.
Within the code books, you’ll find information about basic wind speeds by location. From this you can calculate the height of the proposed awning. By checking these numbers against the code book’s conversion chart, you can determine a basic wind pressure.
The amount of wind load an awning or canopy will be subjected to is not only determined by the regional location; it’s also a function of the local site environment. If the awning’s parent structure is in a built-up environment, surrounding trees and buildings can provide a sheltering effect, reducing the wind load. In contrast, if the building site is in a wide-open space or along the shore of an ocean or large lake, where the wind’s approach is unimpeded, the wind-load factor may have to be increased beyond the nominal design value. Snow loads, however, will be lower in such open areas because snow can blow off the structure.
Wall surface and fasteners
Wall surface is an important factor in the anchoring equation. The wide selection of fasteners narrows to those that work best with the parent structure’s material—be it brick, wood, concrete, steel or other material.
It is essential that the anchors be attached to a building’s structural element. Consequently, the best awning anchor for almost any building is a through-bolt, which is drilled through the entire structural support surface and fastened with a nut on the back. Awning anchors attaching to an accessible steel or wood substrate are easily through-bolted.
However, when the frame must be anchored to an inaccessible wood substrate and cannot be through-bolted, there may be no choice but to use lag screws. The strength of lag-screw anchorages is significantly affected by the species, condition and thickness of the wood. Apart from the strength of the screw itself, the limiting factor of the wood must be examined when making this kind of connection (see "Lag screw formulas").
For anchoring in structural brick, hard-steel self-tapping anchor screws or expansion bolts are recommended. Several types of fasteners are available for use in concrete. Three common types are expansion anchors, where the bolt contains a wedge and is surrounded by a sleeve (hitting or screwing the bolt forces the wedge against the sleeve and the edges of the hole); hooked-end anchor bolts, which are set in wet concrete; and chemical anchors, which use epoxy to keep anchoring screws in place.
Some awning manufacturers disagree about the advantages and disadvantages of chemical anchors.
A chemical anchor is normally used in masonry. The epoxy works by bonding to everything it touches, creating a keying that is unable to be pulled through. The two basic types of chemical anchors are capsules and gun-mixed (also called injection method). Capsules are intended for solid, not hollow, masonry installation. The capsule, a glass tube containing hardener and resin, is placed in a predrilled hole. Drilling through the tube with an anchoring screw breaks the capsule, mixes the epoxy, and releases it.
The gun-mixed or injection technique, used for solid or hollow masonry, delivers the pre-mixed adhesive through a specially designed gun that is inserted into the pre-drilled hole. Next, the anchor screw is inserted. Within the solid part of the substrate, the adhesive bonds itself to the strata. Inside the hollow portions, the adhesive mushrooms out and keys itself into the unit.
Chemical anchors do not exert expansion force, have high loading capacity, are easy to use, and offer good cure time. They can eliminate the blow-out, splitting and spalling problems that can occur with sleeve and drive-pin anchors. When cured, the adhesive and imbedded anchor become a part of the base material—even in poor-quality materials such as sandstone, decaying brick and loose mortar. Anchors of this type can be used closer together than fasteners that depend on wedging.
Chemical anchoring products have a shelf life of one to three years. Using them overhead is difficult because dripping may occur.
Temperature greatly affects the cure rate of the adhesives, a fact installation crews must understand. In hot weather, the anchor screws must be set immediately before the epoxy hardens. In cold temperatures, the cure time greatly increases; if the temperature drops too low, the chemicals cannot be used. In any case, the chemical must cure before any load is placed on the anchor, which can increase labor costs if crews must wait or leave and return later.
Most modern brick is veneer, not a support surface. Unlike structural brick, a modern brick veneer cannot accept through- bolts or expansion bolts. Expansion bolts bear pressure against the sides of the wall; such sideways force could break apart the mortar joints.
Another modern material, Dryvit (essentially a 76mm-thick [3 in.] plastic foam covered with concrete), also will not support anything mounted to it. These and similar veneers require special anchoring treatment.
New construction may allow the building designer, awning designer, and awning fabricator to collaborate and install anchors before veneer goes up. In an existing building, an anchor support—a 51 by 102mm or 51 by 152mm [2- by 4-in. or 2- by 6-in.] wooden board—can be added behind the veneer at the height where the awning’s top rail will be. This solution also works for metal buildings, particularly those constructed of thin, corrugated metal.
For most applications, brackets are used in conjunction with the various fastening methods. One of the most popular is the Z bracket, which essentially serves as a cradle for the awning framework. Z brackets come in aluminum, galvanized steel, and zinc-plated steel. The type used depends on the region’s weather conditions and the awning fabricator’s preference.
For canopies and non-retractable awnings, there are six chief methods of attaching fabric cover to frame: lacing, track systems, screws, staples, staple-in extrusions, and snaps or hook-and-loop fasteners.
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