Analysis and experimental studies of building glass facade

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1 Analysis and experimental studies of building glass facade Ján BUJŇÁK Professor, University of Žilina, Žilina, Slovakia Ján Bujňák, born 1950, received his civil engineering degree from the University of Žilina in 1973, his PhD in 1979, associated professor in 1986 and university professor in 1997.Actually he is rector of the University. Jaroslav ODROBIŇÁK Senior lecturer, University of Žilina, Žilina, Slovakia Jaroslav Odrobiňák, born 1975, received his civil engineering degree from the University of Žilina in Summary The exterior wall of the banking building 111 m high consists of 2274 glass plates. The three facade elements have failed and fallen down. For identification of this failure and avoiding a further imperfection, the complex investigation of the damage has been executed. The principle research procedures are given in the paper. Keywords: tall building, glass facade, dynamic instability, fatigue failure, glass behaviour. 1. Introduction The investigated building is built on a concrete foundation slab, some 15 m below surrounding ground level. The 3-story basement contains equipment facilities and the garages for 330 cars. The 33 above ground floors should house the banking offices. All in the building will be workplace of some people. The central concrete tower as the main bracing system carrying horizontal forces reinforces the building frame (Fig. 1). The character of the building is emphasised by the extensive use of glass on the external facades. This creates also a connection between the office levels, which have been designed as open space, and ensures natural light. The technical implementation of the exterior walls is based on double glass facades with a second facade consisting of single glass panel mounted at a distance of 500 mm. This solution makes it easier to control the indoor air conditions and creates a tight weather protection in front of the facade (Fig. 2). The external facade structures are made of an aluminium grid with 12 mm thick glass plates of Pilkington planar system of the type ESG HST Float Blank (Einschieben-Sichereitglass). The suspended glass plates are horizontally supported with six specially machined bolted connections and special brackets, which permit adjustment of tolerances. The outside glass of this double glass wall were the main topic of the assessment because the three glass units prefabricated in the factory have failed and fallen down.

2 Fig. 1 The look of the building Fig. 2 The scheme of the facade 2. The outer glass facade construction procedures The facade elements were made of floor-height 3.18 m and 1.25 m width. The glass parts prefabricated in the factory have been transported to the erection site by flatcars. There they were carefully stored or directly positioned in place. The facade pieces were hosted, fitted to support brackets and then permanently fastened in place. In all these operations, exactitude and safety had to be the important consideration. Though the optimal erection procedure and the rather advantageous erection plan, the three glass panels at the 9th, 14th and 19th floors successively failed (Fig. 3). Fig. 3 Failed glass panels at the 14th and 19th floor

3 3. Analysis of potential design deficiency As the initial explanation attempt, the detailed analyses of stresses and deflections due to exceptional load conditions have been executed. Slovak building code [1] specifies a basic value of wind force for different heights of building considering also the eventual protection from the wind offered by other structures. The slope and the shape of the surface should be taken into account by drag coefficients. For the wall units at the top of building, the wind pressure is 1.3 kn. m -2. The facade unit as glass plate was considered to be supported by six point fixed bearings capable of supplying the vertical and horizontal reactions plus the local restraining moments. The calculated maximum tensile stress at the bottom plate surface is only 7.9 MPa (Fig. 4), less than the glass tensile yield strength of 50 MPa. The calculation model and glass panel behaviour has been verified by the static loading tests (Fig. 5). The maximum measured deflection due to transferred artificial load in the quarter of the glass length 4.69 mm corresponds to the computed value of 4.23 mm. Fig. 4 Longitudinal stresses due to wind pressure Fig. 5 Static load test The intermediate brackets may follow the aluminium vertical columns deformation of structural grid. The corresponding central supports can be therefore more flexible. The glass panels can be alternatively considered to be supported rigidly only onto the edges at the floor level on each story. The stresses in the glass panel in this alternative static system as a plate at four supports are 15.0 MPa and the extreme deflection 9.15 mm corresponds very well to the tested value of 9.77 mm. The temperature variation has been examined as the future potential accidental effect. This thermal action was considered by uniform 45 o C change of temperature. The cooling down has been taken into account to respect also secondary hyper static effects due to restraint of deformations in the brackets. The distribution of calculated stresses for this temperature effects give the maximum stress MPa in the extreme anchorage support regions (Fig. 6). These resulting stresses do not exceed the half of glass yield strength.

4 Fig. 6 Transverse σ x and longitudinal σ y stress distribution due to change of temperature 4. Research of imperfection effects on the glass wall behaviour The detail appraisal of the described overloading is sure to eliminate faulty design as a unique cause of glass facade failures. The material properties and the real strength in site were verified by standard quality assurance procedures. The overloading can be sometimes combined with an increased number of loading cycles resulting in fatigue failures. Moreover, stress concentrations can have a relatively important influence on the glass static strength, as this material is not sufficiently ductile. Fatigue failure may originate at the localized region of high internal stress, where an initial minute crack might be formed. Such spots can become potential sources of failure. Owing to this mechanism of fatigue failure, the glass panels were tested for the sensibility to spread the initial cracks to the surrounding region. Investigation of the fatigue strength of tested glass panels consisted in production of artificial stress raisers in the centre of the glass panels. The notches were cut at the tensioned side in the form of a line 100 mm long and 0.4 mm in depth. A repeated loading with the complete stress reversal was applied. The frequency 15 Hz of stress cycle has allowed reaching the number of two millions cycles in 37 hours. No fatigue failure as well as no remarkable crack propagation occurred even in the case of the more serious crossed notches. The impact strength of the facade panel is correlated with fatigue strength because both are related to brittles. To test impact resistance, the glass panel has been subjected to repeated impact loads. The glass plates were stressed by falling steel bar weighing 2.75 kg. The impact energy was absorbed by elastic and plastic deformations of glass with minimum of structural damage. Only when the fall height exceeded one meter, the sudden brittle failure has been developed (Fig. 7). Fig. 7 Impact test and failure

5 5. Dynamic behaviour Dynamic instability is distinct phenomena, which may occur under dynamic loading. When the frequency of the wind gusts would be nearly the same as the natural frequency of the facade plate, the resonance can be reached with gradual oscillatory motion. The micro seismic influences of street traffic, especially tramcars or haulage were suspected to produce similar dynamic instability effects. Therefore, the first five natural modes of glass panel vibrations and relevant frequencies were found by computing based on finite element method (Fig. 8). Semi rigid moment constraints with stiffness close to hinge behaviour were used in the calculation. The lower frequency 14.5 Hz corresponds to the first mode of the structural part. Dynamic loading tests have relieved that there is no danger of resonance. f 1 = 14.5 Hz f 2 = 17.3 Hz f 3 = 28.8 Hz f 4 = 32.5 Hz f 5 = 33.5 Hz Fig. 8 Calculated natural frequencies of the studied glass plates 6. Final comments and conclusion Only main measurements and inspection conclusions are given in the paper. Investigation of wind effects and temperature variation is described. Real support flexibility has also been taken into account. The influence of initial notches on the load carrying capacity, fatigue and impact strength, respectively, has been observed too. The theoretical and experimental research results have indicated that material properties of facade structural parts can assume safety behaviour even in the extreme load conditions. Thus, it was concluded that structural failures of tree glass panels could be probably caused by erection errors. The records in the construction diary indicated the minor failures and construction irregularities and confirmed lack of care during this process. To prevent all failures is not practically possible. But their analyses can provide issues for major disasters prevention in the future and the lessons from them to be learned. 7. References [1] Slovak Technical Code: STN Actions On Structures. SÚTN Bratislava, [2] Slovak Technical Code: STN Design of Steel Structures. SÚTN Bratislava, 1998.