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Although the testing of curtain wall systems for their effectiveness in resisting wind and rain has now become rather common practice, such testing is by no means a universal requirement of all curtain wall designs. Often tests are essential, but frequently they are not. When they are needed, they should be the proper kind of tests, governed by appropriate criteria, but unnecessary testing is obviously a waste of time and money. It is essential, therefore, that the architect, before specifying any tests for aluminum curtain walls, fully understands the reasons for testing, what tests are advisable, and what performance criteria should be established as reasonable requirements.

Present concepts of wall testing originated with the advent of metal curtain wall construction, though generally similar tests had been conducted on metal windows for some time prior to that. Two reasons in particular led to the introduction of wall testing at this time: first, it was not until factory-built wall systems came into use that representative pre-assembled units were available for evaluation by testing, and second, the problems of sealing against water leakage are far greater in metal-and-glass construction than in masonry construction. This is due both to the inherent nature of the materials themselves, and the fact that much larger wall units are used. Less water enters metal walls during a rainstorm than enters masonry walls, but whereas the relatively absorptive masonry construction absorb within themselves much of this water, later evaporating it, much of the water striking an impervious metal wall accumulates at the joints. These joints must be capable of accommodating movement, yet any water entering them must be drained back to the exterior or it may cause corrosion and may also appear conspicuously as a leak on the interior of the building.

The need for pre-testing of curtain walls may therefore be said to have been dictated by the very nature of the materials being used.

At some stage during its design development, any metal curtain wall should be tested for leakage of both air and water. But most standard types of wall, the walls which supply a large share of the market, have already been extensively tested, both by their manufacturers and by impartial testing agencies, during their design development, before they were placed on the market.

And, in addition, such walls have been extensively tested in actual use. When using an established wall system of this type, further testing for the specific job at hand is usually unnecessary, provided that no special features or design changes are incorporated and the installation is to be identical in all respects with the system tested.

With custom type walls, it's a different matter. Most walls in this category contain new and previously untried features, and therefore should be tested. It follows then, that by far the bulk of performance testing specified by the architect and performed by commercial testing agencies is concerned with custom designed walls.

In short, the need for testing depends upon both the type of wall being used and the circumstances of its use. If the manufacturer will certify that the wall, as it is to be installed on the building, has already been tested and qualified by an impartial authority and meets the specified criteria, further testing should be unnecessary. But when a previously unproven wall design is being used, thorough pre-testing is usually not only advisable but necessary.

The architect should be sufficiently informed regarding the nature and value of testing to determine what testing procedures, if any, are appropriate.

REASONS FOR, AND VALUE OF LABORATORY TESTING

In general, laboratory pre-testing of metal curtain walls is aimed at evaluating performance of the wall under exposure to simulated environmental conditions before full scale production of the wall system is begun. A secondary benefit of such testing is that, in constructing the test specimen, or mock-up, an opportunity is provided to check installation procedures, and in some cases this experience in itself leads to design improvements.

Laboratory tests may be conducted for either of two purposes: to provide the wall manufacturer himself with information about the performance of his design, or to provide official evidence and certification that the performance of the wall meets specified standards.

Design check, or "exploratory" tests are made during the development of the wall design, and are conducted by the wall manufacturer, usually with his own facilities and staff. Such test may be unrealistically severe, even to destruction, in order to disclose design weaknesses and suggest potential improvements. Acceptance tests are those which are conducted for the purpose of verifying that the wall conforms with the architect's performance specifications, or to prove its acceptability to the architect or owner. These tests are conducted, or witnessed and certified, by an impartial test agency designated or approved by the architect. Either the facilities of the agency or those of the manufacturer may be used, but in either case the results must be reported and certified by the agency.

It must be recognized, however, that even the most conscientious laboratory testing cannot reliably predict with accuracy the performance of the wall in actual use.

To a large degree, field performance depends upon the care used in installing the wall, on proper anchorage, the fit of mating parts and the effectiveness of field seals.

These, in turn, depend upon the alignment of the building frame, working conditions at the building site, quality of workmanship and proper supervision. Proper allowance for all of those unknowns cannot be made in laboratory testing, nor can the detrimental effects of time and aging be simulated.

Nevertheless, standard laboratory performance tests do have substantial value. Although they provide no positive proof that the wall when installed will function properly, they often do reveal design weaknesses or fabrication faults requiring correction, and the discovery of such deficiencies in advance of production may well save many times the cost of conducting the tests.

PERFORMANCE CHARACTERISTICS SUBJECT TO PRE-TESTING

Almost any type of performance can be pre-tested by using a proper full-size specimen of the wall and the proper testing facilities. There are three performance characteristics in particular that are commonly investigated ⎯ resistance to air infiltration, resistance to water penetration and structural adequacy ⎯ and standard methods have been developed for conducting such tests.

Other characteristics such as heat and sound transmission are also critical concerns in some cases and may require testing. All of these tests will be discussed, with the more common "standard" tests being examined in greater detail.

The performance characteristics which are usually of greatest concern are structural performance under wind loading and the ability of the wall to prevent water penetration during heavy rain storms. These represent two levels of concern, however. Structural failure, of course, may endanger human life, so structural adequacy is a basic essential. The occurrence of water leakage will not likely be dangerous, but may cause discomfort and substantial property damage.

It does not follow, however, that structural testing is more essential than testing for water penetration. In fact, the order of importance, as far as the need for testing is concerned, is usually the reverse. The reason for this is that structural requirements are well recognized, can be calculated with reasonable accuracy, and are found to be amply satisfied in most cases. Resistance to water penetration, on the other hand, cannot be accurately calculated or predicted, but requires testing for verification, and is often found to be deficient.

A third common reason for testing is to determine resistance of the wall to air infiltration, and this is a matter of particular concern when the design includes a number of operating window units. Generally, the concern about air leakage is of secondary importance, though the amount of air passing through the wall must always be limited to a small amount, usually a specified maximum, in order to minimize heat loss and condensation.

Contrary to some beliefs, there is no direct and constant relationship between the amount of air infiltration and the amount of water penetration occurring in a wall. If the amount of air infiltration is high, the wall will likely be susceptible to water penetration also, but walls which are relatively airtight may also have serious water leakage problems. The resistance to both air infiltration and water leakage depend entirely upon the design details, and are essentially indeterminate, except by testing.

Varying degrees of importance are attached to the tests for these three characteristics which are tested by standard methods, and the architect may, of course, be selective in specifying them. Each type of performance is measured individually, by its own test, and only those which are considered to be in doubt need be tested. In the experience of most commercial testing laboratories, the water penetration test is always specified, the structural test not quite as frequently, and the air infiltration test far less often than either of the others. It should be recognized, however, that major expense of testing is the cost of preparing and instrumenting the test specimen.

After this is done, the difference in cost of running three or four different types of tests on the same specimen, rather than only one or two, is relatively small, provided of course that the laboratory is equipped to conduct all of the standard tests.

Thermal tests, as applied to aluminum curtain walls and windows, are of several types and are essential to the determination of energy-conserving capabilities. Efforts to legislate energy conserving measures into building codes may make testing of thermal performance as common a requirement in the future as testing for air leakage, water penetration and structural strength is at present. Sound transmission tests are also being required more often as designers strive for wall construction which effectively reduces transmission of air-borne noise.

THE TEST SPECIMEN

It is essential that the wall test specimen be, as nearly as possible, a faithful representation of the intended design.

It should be constructed just as the wall is to be installed on the building, using the same methods of support and attachment, similar conditions of continuity in all structural elements, the same type of glass, same sealants and so forth. As far as practicable, the building frame which supports the wall should also be simulated in the test set-up. In some cases, full-size steel or concrete framing has been constructed as part of the test structure, but in normal testing practice this expense is avoided by using heavy wood or steel members which provide equivalent stiffness and the same kind of support and anchorage as will be furnished by the actual building frame. In any case, all details of the intended anchorage system ⎯ the steel angles, clips, shims, brackets, bolts and welds ⎯ should be used on the test specimen just as detailed for the ultimate installation.

Whenever possible, the same parties who will later be installing the wall on the building should also construct the test specimen. This is desirable for several reasons: 1) it provides an installation more representative of field workmanship than would likely be provided by laboratory technicians, 2) it acquaints the parties involved with the details of construction and the critical aspects of the installation procedure, and 3) it often leads to valuable suggestions by the workmen themselves regarding minor revisions in details that will facilitate installation.

The size of the test specimen, as well as the selection of the wall area which it represents, are important considerations also. Usually the same test specimen is used for all three of the "standard" tests (air, water and structural), and the standard methods for these tests, which will be identified later, stipulate the general requirements as to size. Some types of thermal tests, if conducted by the same laboratory performing the standard tests, may also be performed on this same specimen.

Certain types of thermal testing, and tests for sound transmission, are done by different laboratories and require different kinds of test specimens which may not be quite as large or elaborate, though the tests themselves are more complex.

Some supplementary advice and recommendations regarding the nature of the test specimen, not provided in the standard test methods, should be noted. The area of wall represented by the specimen should include the most critical and vulnerable conditions, as illustrated in Figure 24. Horizontal joints between units, designed to accommodate movement, should be near the lower edge of the specimen, so as to collect rundown water from a sizeable area and, in some cases, when the extra cost is justified, especially with walls designed for use on tall buildings, it may be advisable to represent a corner condition, which is subject to the greatest pressures. If the wall design includes masonry piers or column facings

spaced not more than 7.5 m (25 ft) on centers, the width of the test specimen should be one full bay between such piers, plus the metal-to-masonry joints and representative width of masonry at both sides. The selection of the area of building facade to be represented by the specimen is, of course, the architect's decision. The choice deserves careful consideration, and should be made in consultation with the wall manufacturer and the testing laboratory.

A curtain wall specimen is erected in a chamber for conducting tests under static pressure.

ROOF COPING

EXTENDSONEBAY AROUNDCORNER

C

A B

WALL

X

UNIT

BUILDINGCORNER

HORIZONTALJOINTS BETWEENUNITS

Areas of a curtain wall which can be used for test specimens are shown on an elevation of a typical curtain wall.

Partial Building Façade Illustrating Possible Selection of Areas to be Represented in Test Specimen.

Area A – Normal Choice

Area B – Better Choice, but more elaborate and more expensive Area C – Most complex and expensive, but may be advisable in

some cases

Area X – Inadequate and unacceptable FIGURE 24

ORDER OF TESTING

The three "standard," and by far most common, tests are generally conducted, quite logically, in order of severity of loading. First is the air infiltration test, which usually employs loads of from 75 to 300 Pa (1.57 to 6.24 psf). If the wall contains operable window units, for example, it is essential that this test be conducted on a dry wall, because the wetting of some types of weatherstripping may improve their sealing ability by as much as 100%, and only a test of the dry material can give a true measure of its capability of sealing against air infiltration. The second test is normally the test for water penetration. This may require uniform loading of 390 to 580 Pa (8 to 12 psf), depending on the criteria specified. The last test in the series, then, is the structural test, in which no less than the full design (wind) loading is applied. The amount of this loading varies widely, depending on design requirements, but may be as high as five to ten times the loading used for testing for water penetration.

Tests for thermal and acoustical properties, if such are required, may generally be run at any time. Some of these tests, as previously noted, require special laboratory facilities and a different type of test specimen. If only critical interior surface temperatures and the effectiveness of thermal breaks are to be determined, the same specimen as is used for the standard tests may be employed, and it is customary to conduct such tests before subjecting the specimen to the water penetration test.

TEST FOR AIR LEAKAGE

This test is always conducted by the so-called "static"

method, using an air chamber. Briefly, the procedure consists of constructing a relatively airtight assembly in the form of a large box, with the wall test specimen constituting or being contained in one of the two large sides of this box. Air is then supplied into, or exhausted from this assembly by means of a blower system, producing a pressure differential across the specimen, and the amount of air passing through the specimen itself is carefully measured. A schematic drawing of such a test assembly is shown in Figure 25.

The procedure used for this test is described in ASTM E 283, "Test Method for Determining the Rate of Air Leakage Through Exterior Windows, Curtain Walls and Doors Under Specified Pressure Differences Across The Specimen." This standard, which originally was applicable only to the testing of windows, was later revised to include the testing of walls and doors. It supersedes the old NAAMM Standard TM-1-68T which was specifically intended for testing air leakage as well as water penetration and structural performance of curtain walls.

The ASTM standard method of test for air leakage prescribes the testing procedure but, unless otherwise specified, calls for air leakage tests to be conducted at 75 Pa (1.57 psf), representing the velocity pressure of a 11 m/s (25 mph) wind. The 75 Pa (1.57 psf) is normally used for testing residential and commercial windows. Air leakage should not exceed 0.2 L/s•m (0.37 cfm/ft) of crack under this pressure. A pressure of 300 Pa (6.24 psf) representing a 22 m/s (50 mph) wind is used for testing monumental windows. Here too, air leakage should not exceed 0.2 L/s•m (0.37 cfm/ft) of crack. For windows having high thermal performance requirements air leakage should not exceed 0.2 L/s•m (0.37 cfm/ft), and for certain architectural applications it may be desirable to limit the leakage to considerably less than this. Refer to AAMA 101, "Voluntary Specifications for Aluminum and Poly (Vinyl Chloride) (PVC) Prime Windows and Glass Doors," for more specific values. Pressures higher than 75 Pa (1.57 psf) may also be used to test curtain walls, but for walls tested at 75 Pa (1.57 psf) good performance through the fixed glass and panel area would require that the air leakage not exceed 0.08 L/s•m (0.06 cfm/ft) of gross wall area. Since the permissible air infiltration of a curtain wall is usually specified in Liters per second per square meter (cfm per square foot) of projected wall area, a reasonable criterion for good performance ⎯ though subject, of course, to modification as dictated by circumstances ⎯ would be 0.08 L/s•m (0.06 cfm/ft) of wall area plus the air leakage in L/s (cfm) per linear meter (foot) of crack for the operable window units (if any) contained in the wall.

TESTS FOR WATER PENETRATION

Two different methods are used for testing the resistance of walls to water penetration. One of these is the static method, using an air chamber, as described for the air infiltration test. The procedure is similar to that of the air test, except that higher pressures are used, and the outdoor (high pressure) side of the wall is subjected, while under pressure, to a uniform application of water at a specified rate. The other method, referred to as the "dynamic"

method, employs a wind generator ⎯ usually an aircraft motor and large propeller ⎯ to simulate wind (and provide the test pressure), while water is fed into the air

method, employs a wind generator ⎯ usually an aircraft motor and large propeller ⎯ to simulate wind (and provide the test pressure), while water is fed into the air