Testing for façade and glass systems

Testing for façade and glass systems Mr. Joe Yu & Professor S.L. Chan

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Testing for façade and glass systems

Mr. Joe Yu B.Sc. (HKU)

Director Facadetech Laboratory Limited

and Professor S.L. Chan

Ph.D., FHKIE, MIStructE, FHKISC Department of Civil and Structural Engineering

Hong Kong Polytechnic University

Test vs. Computer Simulation by NAF-Shell for

a 3-side supported glass panel

Butt Joint, assumed free


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steel

glass

tensile stress

strain

Attack by Typhoon York

Young’s modulus of glass against steel.


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Testing is an important part for checking the adequacy of a sample in glass structures. New and innovative design systems are always requested for checking against several requirements. One major objective is to make sure the passed sample has the same detail as the one actually used on site otherwise the test is meaningless. Any remedial work carried out on a sample for making it passed must be recorded and the future installed system must be revised accordingly. In this aspect, a good test consultant must be qualified and sincere and, should be best employed or appointed by the project architect or engineer.


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Air Chamber

Test Specimen

Air system

Supply

Exhaust

control valve

to pressuremeasuring device

or

Flow meter

Arrangement for mock-up test

Transducersfor recordingdeflections


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Structural adequacy Is the system of adequate structural strength against design wind load plus a margin ? Are the deflection of the structural members too large ? Is there any plastic deformation ? PNAP 106 is a standard testing method in Hong Kong. The Australian New Zealand Standard AS/NZS 4284(1995) contains also the proof test which is 150% of the design wind pressure. This standard is similar to the SIRWET standard produced by CSIRO in Australia. An important point worthy of consideration by the Authority is the mandatory report of all failure cases and explanation to the cause should be made. This serves two purposes. First, remedial work will be undertaken instead of taking the failure test as “accident”. Secondly, future failure can be referred to the test failure and it will serve as a useful reference for engineers in checking the failure causes.

Full-scale test of a glass wall system Shear failure of glass fin near support due to concentrated stress


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Water leakage Is the system good in resisting water leakage during rain ? The sample test can be conducted by spraying water onto the sample with different values of air pressure. Many complaints from the occupants are related to this. Use static, cyclic and dynamic water penetration test method for this purpose plus site test for making certain the quality for the final product is good ? Refer to AAMA 501.1 for dynamic test, ASTM E331 for static and ASTM E547 for cyclic water penetration tests. For site test, AAMA 501.2 and 501.3 can be used. (AAMA refers to Architectural Aluminum Manufacturers Association in U.S.A.)


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Basic Glass properties Glass is strong in compression and weak in tension. Annealed glass – no heat treatment for tempering Tempered glass – protecting surface of glass by a compressive layer through rapid cooling 0.8% probability failure stress for annealed glass is about 20 N/mm2, 40 N/mm2 for heat strengthened and 80 N/mm2 for tempered glass. Young’s modulus is the same for all of them, 72 kN/mm2. However, tempered glass may have problem in Nichel Sulphide (NiS) – spontaneous breakage without warning

Dynamic Test to AAMA

Tempering in glass


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Water leakage during a laboratory test


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Air infiltration and exfiltration tests Will the system leak too much air to affect the air-condition performance ? Measurement of air leakage against some standard rates (see ASTM E-283) Lateral movement test Will the system be damaged for lateral movement of a building ? Stone cladding can be easily damaged for building movement when the gaps between stone panels are too small for the sake of aesthetic reason. In the Australian New Zealand Standard AS/NZS 4284(1995), the seismic test can be carried out by this form of lateral movement test.


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On-site Testing Obviously, the most direct and reliable test will be conducted on actual sample. To make sure that the actually installed system are adequate, sample test may become necessary. However, care must be taken to insure the test will not damage the sample after the test. For example, destructive test is not normally used. Also, test arrangement is needed to represent the actual response. One common test will be on embed or cast-in anchors to 100 or 150% of the design load. BS5080(1993) Parts 1 and 2 cover the tensile and shear load tests.

Test to BS5080 Part 1 (1993) Torque Test


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Design and analysis of glass panels and systems In Canadian and American code, nonlinear finite element method is suggested. It is economical in some cases but safer in others. For example, a panel under wind pressure will perform better than we expect because of the membrane action. A linear analysis cannot detect buckling of a glass fin under edge load which may occur the applied stress reaches the design cracking stress of glass.

Deformed shape of a glass panel

Stress Contour under wind pressure


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Design chart from linear analysis

Design chart from large deflection type of nonlinear analysis


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Stone Cladding

A Bank sued an architect firm for cracks in its headquarters building in Central !


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Testing used to design stone cladding 1 Introduction l Recent trends on Architecture include a growth in the

use of natural stone façade. l Use of thinner stones (30 mm) demand more

thorough investigation of the cladding and the attachment system to ensure their structural safety. l As a natural material, stone cladding have not been

manufactured to any designated quality/international standard. And also, because its breath of visual character often reflects a wide variability in engineering performance, particularly strength and durability. Strength can vary by several orders of magnitude between different type of stones.

Example : Typical variation in stone strength Stone type Flexural strength

(MPa) Compressive

strength (MPa) Granite 8 ~ 20 120 ~ 240 Sandstone 2.5 ~ 15 30 ~ 200 Limestone (High)

6 ~ 15 55 ~ 180

Limestone (Low)

2 ~ 10 10 ~ 90


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l Durability and chemical resistance Granite - Chemical resistance (stable)

- Change of temperature and dry/wet condition will cause gradual damage to stone strength overtime.

Limestone - usually produced by precipitation of calcium carbonate. Because of the relative solubility in acid of the constituent mineral (calite & dolomite). Attack by acid rain and other acid pollutants in the atmosphere.

Marble - usually produced by recrystallized of limestone. Susceptibility to acid attach is comparable to that of limestone.

Remark : Chemical resistance of natural stone is not

addressed in this topic. We only deal specially with strength testing.


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2 Need for stone testing l New stone without available information. l As suppliers only provide some mechanical

properties, strength of the stone in actual project need to be verified in order to confirm its suitability. l Stones on an existing building appear to have failure

by cracking at the fixing points. In particular, strength and anchor capacity has to be checked. l Full scale is recommended to test the fully stimulate

job condition and details. l Demand a consistent strength stone throughout the

project. Production test is needed to form part of the quality assurance during construction.

3 Stone test 3.1 ASTM test standard

ASTM standard

Subject

C 97 Absorption and Bulk Specific Gravity of Dimension Stone

C 99 Modulus of Rupture of Dimension Stone C 170 Compressive strength of Dimension

Stone C 295 Guide to Petrographic Examination of

Aggregates for Concrete C 880 Flexural Strength of Dimension Stone C 1201 Structural Performance of Exterior

Dimension Stone Cladding Systems by


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Uniform Static Air Pressure Difference C 1352 Flexural Modulus of Elasticity of

Dimension Stone C 1354 Strength of Individual Stone Anchorage

in Dimension Stone


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3.2 Test sequence 3.2.1 Preliminary / initial test

l Used as the basis for primary decisions regarding stone material structural performance. l The initial tests should be comprehensive enough

to statistically establish the variability of the material’s strength and properties. l Adequate to derive average values and variance

for design use. Proposed preliminary test a) Absorption and Bulk Specific Gravity (ASTM

C97) Assess the vulnerability of the stone to weathering and dry/wet damage. High density

Less microscope voids, faults and more intact crystalline structure. Hence, indicate a stone’s potential resistance to weatherability.

High absorption

More microcrack and more moisture to pass through and trap in the stone Hence, facilitate i) corrosion in anchor location, ii) chemical deterioration by acid/air pollutants suspended in atmosphere and iii) damage of strength under cycles of dry/wet weathering.


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b) Compressive strength (ASTM C170) No strictly recommended as compressive strength is rarely a limiting factor in the design of stone cladding. However, the initial compressive strength of a stone could be used as a parameter to indicate the consistence of that stone during production test.

c) Flexural strength (ASTM C880) ASTM C880 is a four point bending test to create a purely flexural tension-stress failure at the extreme of the tension face. Sample groups under different conditioning (wet/parallel, dry/parallel, wet/perpendicular, dry/perpendicular) would be tested to determine the flexural strength under critical condition. The stress mechanics measured in this test are similar to those created in the midspan of the cladding. The test result is useful to check the size of the panel under uniform distributed design load such as design wind load.


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Remark: l The test results vary in different stone finish

such as the flaming proceed in producing the Flamed Finish of a stone cladding may damage the finish structure up to 3 ~ 5 mm and lead to a lower flexural strength. Therefore, the test sample should include the stone finish and different stone finish should be tested.

l The test results also vary in different surface areas under maximum-stress. Larger region of the test specimen is at the maximum-stress, more opportunity or potential exists for a natural weakness of feature to occur in that region and lead to a lower flexural strength. As the failure area in C880 sample is quite small in comparison with the failure area that will exist for a full panel, modified sample size to actual job span and thickness is recommended.

d) Petrographic analysis (ASTM C295)

The test result is useful to evaluate the minerals present within the stone, check the consistent of the stone type at different quarry location and indicate the stone’s potential resistance to weathering. However, the results have to be assessed by Professionals.

e) Anchor test (ASTM C1354) The stone capacity of an anchor is difficult to accurately predict mathematically as relative stiffness of stone, anchor, infill material and back-


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up control how much of the anchorage device is actually resisting load is difficult to determine. Anchor tests are unique procedures specifically conceived and design to isolate and quantity the capacity of each individual stone anchorage in the configuration to be installed specified for the project.

f) Aged strength testing (CWCT) Natural stone suffer loss of strength through a combination of repeated stress fatigue and long term environment exposure. Thus in addition to standard tests, other specific tests should be established for different conditioning: l Dry/wet cycle l Thermal cycle l Residual static flexural strength after cycle

loading l Residual static anchor capacity after cycle

loading


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3.2.2 Full scale test before construction (ASTM C1201) The stone cladding and its anchorages require

higher ultimate resistance than glass and metal external wall because the stone has a greater range of uncertainties. Panel and fixing assemblies shall be accurately configured for testing to simulate job condition and details. The specimen should be tested to failure or max factor of safety (~ 4.5) that the structural engineer/consultant feel satisfactory.

For sample fails after passing the min factor of safety (2.0 ~ 2.5), the failure mode (panel or anchorage) acts as an important indicator of the critical portion of the stone cladding in the actual job condition.

3.2.3 Production test Repeat flexural strength test in the critical

conditioning at certain interval in job to ensure the consistent in strength of the stone used.


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4 Analysis of test data 4.1 Bases on CWCT 4.1.1 For project with aged-strength test

Calculate of test results Mean, Xm = (X1 + X2 + ….. +Xn) / n Standard deviation, s = {[S(Xi – Xm)2] / (n – 1)}0.5

Coefficient of variation, V = 100 * s / Xm Then, Variation factor (VF)

V Granite Limestone Marble 0 – 5% 2.0 3.0 2.5 5 – 10% 2.5 3.5 3.0 10 – 20% 3.0 4.0 3.5

Above 20% 3.5 4.5 4.0 Durability Factor (DF) Fraction of initial flexural strength (%) = Flexural strength after 300 thermal cycle * 100% Initial flexural strength Fraction of initial flexural

strength (%) DF

100% 1.0 95 – 75% 1.2 75 – 60% 1.5 Less 60% 1.8

Stone flexural safety factor (FSF)


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FSF = VF * DF Stone anchorage safety factor (ASF) ASF = FSF * 1.4


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Design load = Xm – k * s FSF or ASF

Where k = factor of distribution depend on tolerance and confidence level Adopted from BS2846:Part s and specified to a confidence level of 95%.

Number of test (n) k-factor 5 3.41 10 2.36 15 2.07 20 1.93 30 1.78 40 1.70 50 1.65

>50 1.645

4.1.2 For project without aged-strength test

Stone type Stone flexural safety factor (FSF)

Granite 4.0 Limestone 6.0

Marble 5.0 Stone anchorage safety factor (ASF) ASF = FSF * 1.4


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4.2 Bases on General Specification of ASD, clause 16.52 Design load = Xm / FOS Value deviation =

Xmax – Xm *100% or Xmin – Xm *100% Xm Xm whichever is greater.

Value deviation

Granite Marble Limestone

Flexural strength

0 – 10% 3.0 4.0 5.0 10 – 20% 4.0 5.0 6.0

Above 20% 6.0 7.0 8.0 Anchor capacity

0 – 10% 4.5 6.0 7.5 10 – 20% 6.0 7.5 9.0

Above 20% 8.0 10.0 12.0


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Further comments related to structural safety of stone cladding Owing to the variability of stone properties, a larger factor of safety is normally used and also the factor varies with the type of stone. We need to determine an appropriated one for the sample type. Understanding stress in a stone panel is extremely important since no stress re-distribution is permitted for brittle material like stone. Flexibility for movement due to building sway, thermal expansion etc. is important and stress concentration should be considered. Test does not cover shear strength since failure is due to tension crack resulted from shear force (i.e. Mohr stress circle for finding the maximum tension stress due to shear stress). Thus, it is more suitable to carry out anchorage test directly in order to determine the support capacity. In fact, even we know the shear strength, we can hardly assume a correct failure shear surface and very often the tested and broken specimen has a failure surface far from the assumed surface obtained from 450 projection. Jointing system should provide some sort of flexibility for movement and vertical and horizontal supports should best be separated for taking load in order to insure a well-defined load-path. A structural determinate system is better than an indeterminate system here, which is opposite to our usual perception


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for ductile structures, because of the clearer load path and lack of stress re-distribution in brittle material and the non-reliance of flexibility to share load.

Conclusions Understanding structural behaviour and properties is essential for safe design. Design concept for brittle material like glass and stone is very much different from ductile material like steel. Testing is a means of gathering confirmation of the adequacy of the system against design assumptions and collecting of properties for the particular type of materials. Quality testing such as by HOKLAS laboratories with experience in façade and stone testing is essential. We, professionals, should always be open-minded to accept and scrutinize new technology in order to remain competitive under challenges from globalization.


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References 1. Structural Uses of Glass in Buildings, the Institution

of Structural Engineers, December 1999. 2. So, A.K.W. and Chan, S.L., Stability and strength

analysis of glass wall systems stiffened by glass fins, Finite Element in Analysis and Design, 23, 1996, pp.57-75.

3. Guide to the selection and testing of stone panel for external use (CWCT)

4. Modern stone cladding (ASTM) 5. New stone technology, design and construction for

external wall systems (ASTM) 6. General specification of ASD, 1993. 7. ASTM standard, Volume 04.07 (2000).

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Testing for façade and glass systems

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