Structural Adequacy ASTM E

The test set-up in accordance with ASTM E330 is very similar to ASTM E283 and ASTM E331. Basically an air pressure difference is applied across the wall sample and measurements of deflection or distortion are made. Like previous tests the test method presupposes a face sealed wall where the air pressure difference is applied across the outer cladding which forms the air barrier. This results in an important limitation with respect to rainscreen wall systems. While ASTM E330 effectively tests the curtain wall framing and anchors to the structure it does not test the structural adequacy of any panels or outer cladding and its attachment (in an interior air sealed rainscreen wall) nor does ASTM E330 test the capacity of any glass in the assembly.

In order to effectively specify a structural test programme at least three performance parameters are critical. These include:

• design wind/air pressure • deflection criteria • overload criteria

These are discussed individually in the following sections. Design Pressures

Procedures to determine the design wind pressure for testing are well established. Building codes provide information to develop minimum wind pressures and this information can be supplemented by wind tunnel tests. Additive to the wind pressure should be stack and mechanical pressurization pressures previously determined for air leakage testing, although the stack and mechanical pressures are typically very low compared to the design wind pressure. Test pressures are usually applied and held for 10-15 seconds to partially reflect wind gust effects.

Deflection Criteria

Industry recommendations for wall framing deflection tested at the design pressures are L/175 (where L=span). This specification is based primarily on glass edge support conditions. It may result in excessive deflections unless an upper limit on deflection is provided. This limit is determined by the

specifier but is often 9 to 12 mm (3/8 to 1/2 inch)

The L/175 deflection criterion is an industry recommendation only and can be altered by the specifier if the project demands. Deflection limits of L/200 are quite common and deflection limits of L/240 are also not unusual. The less the allowable deflection the deeper or heavier the framing and there is an increased cost associated with this.

ASTM E330 Test Limitation

Testing a rainscreen wall in accordance with ASTM E330 does not test any elements outside of the air barrier such as spandrel panels, spandrel glass, feature elements or anchors that attach these elements to the mullions or rails. Spandrel panels, etc. require separate tests. It is also important to note that this procedure does not in any way certify any glass.

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Most references and specifiers only address the deflection of the aluminum framing. Glass deflection and spandrel backpan deflection can also be an issue. Glass, properly designed for strength, is quite flexible and will deflect substantially during a structural load test. Deflection ratios of L/50 are not uncommon. While glass deflection limits are normally not specified, in cases where there are large lites or public perception is important, the glass deflection specification may be appropriate. Extra attention is typically provided for main floors, observation areas or airport control towers. Deflection limits for sheet metal backpans are critical. An overly flexible backpan may snap back and forth creating noise, may move to release firestops or load smoke seal sealants or impair the pressure equalization performance of the spandrel cavity. In the case of backpans deflection, ratios are not appropriate and fixed limits should be imposed. Most critical is the deflection at the floor slab level.

Overload Criteria

The application of the specified test air pressure difference and subsequent measurement of deflections allow the confirmation of a serviceability limit state. The specified load must be applied without exceeding a specified limit state (deflection limit). Testing then normally proceeds to an overload test. This test is conducted at 1.5 times the design pressure.

Deflection Limits – Sheet Metal Backpans

The deflection of sheet metal backpans at the level of the floor slab should be limited to 6 mm (1/4 inch).

The 1.5 factor originates in the working stress approach to structural design (WSD) and recognizes test guidelines published by the Aluminum

Association. It also reflects a load equivalent to 90 per cent of the minimum factor of safety (1.65) implicit in the WSD method. This coincidentally corresponds to the limit states approach to structural design whereby 90 per cent of the calculated member, or system, resistance must equal or exceed 150 per cent of the calculated load on that member, or system. At the 1.5 times overload condition there is to be no plastic deformation of the structure. In other words the materials stresses stay below the yield point of the aluminum.

However, it is important to recognize that the 1.5 overload condition does not test all components equally as different components have different safety factors. For example the shear failure of an anchor bolt at the 1.5 condition does not indicate a marginally undersized bolt, but a bolt almost 40 per cent undersized. Therefore, the overload test provides useful information on the primary framing members and their interaction but does not truly qualify all components especially anchors, fasteners, welds and glass.

Some specifiers request that testing proceeds to destruction of the sample. Testing to destruction can be useful if information on the post-glass breakage behaviour is required or if concern is expressed regarding the adequacy of anchors. It is often more useful to test anchors or small components in separate, more controlled tests rather than in the large scale mock-up. Given the inconsistencies in the results of the structural test methods, structural calculations for the curtain wall should also be submitted and reviewed. Historically, calculations were only done in-house by the curtain wall contractor as a rough check on portions of the wall. Today, the

calculations form a vital part of project documentation. As such, aside from technical accuracy and completeness, the presentation of the calculations is critical. Unfortunately calculations are often presented in an unorganized haphazard manner and cover only the major building elements. Incomplete or unorganized calculations make the verification of a design more difficult and therefore less likely.

When requesting calculations, the following should be outlined as requirements:

Curtain Wall Calculations

• Calculations must be numbered, preferably in logical groupings that allow the logical insertion of corrected pages. Each page and section should be titled.

• Typed computer output does not guarantee clarity. Often a logically ordered, well referenced hand-written set is far superior (provided handwriting is legible).

Overload Test Limitation

Overload testing does not ensure a consistent level of safety with respect to all curtain wall components. Typically anchors, fasteners, welds and glass are not fully evaluated in an overload test.

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• Sketches of the element being analysed help immensely in the understanding of the calculations.

• A clear definition of the imposed loads and their origin is mandatory. • Analysis should be conducted using accepted techniques and

nomenclature common to the applicable design method.

• The basic material properties and allowable stresses/loads should be presented along with their origin.

• Appendices should be included and could contain section properties, bolt data, adhesion data or repetitive form calculations.