Seismic Behavior of Point Supported Glass Panels

Challenging Glass 3 – Conference on Architectural and Structural Applications of Glass,

Bos, Louter, Nijsse, Veer (Eds.), TU Delft, June 2012.

Copyright © with the authors. All rights reserved.

Seismic Behavior of Point Supported Glass

Panels

Luís Martins, Raimundo Delgado

Faculty of Engineering of the University of Porto, Portugal, www.fe.up.pt

Rui Camposinhos

Scholl of Engineering Polytechnic of Porto, Portugal, www.isep.ipp.pt

Tiago Silva

FACAL - Façade Engineering, Portugal, www.facal.pt

Due to its transparency today’s architectures often rely on glazed façade solutions

to execute the building envelope. During a seismic event, glass brakeage and fall out can occur and threaten occupants and passers. So in earthquake prone regions

this must be accounted in the design project; however, there is little research on the

behaviour of glazed façades under seismic loads. This papers focus on the results of a project under development to ascertain right well dimensioned and adequate

solutions to glass facades using point fixing systems under seismic or wind actions.

Numerical analyses were performed, using finite element commercial software, the results are compared with simplified methods and conclusion are drawn.

Keywords: spider glass, glass façade seismic behaviour, curtain wall glass façade

1. Introduction

Recent developments in science and technology allowed glass to be used in several

structural demanding applications such as façades, roofs, girders and columns, etc.

Due to glass’ brittle behaviour this calls for more refined analysis methods and greater

design detailing to ensure structural stability and safety.

Currently modern facade buildings rely on glazed curtain wall systems. These systems

include either singular aluminum alloy frame glass curtain walls or frameless glass

curtain walls. This is the case of the so called spider fixing systems, which are pointed

supported.

Although there are some research on the behaviour of glass panels under out-of-plane

loads, e.g., wind loads, the same is not factual in the in-plane loads behaviour, for

instances in what seismic actions are concerned.

In fact, seismic action brings out specific problems to designers, due to the lack of, at

least well-known, international rules or regulations about this problem.

The scope of this paper is to present the relevant aspects of the seismic loading in point

fixed glass panels. Firstly a simplified method based upon the elastic response spectrum

is introduced, then the results of a time history dynamic analyses are presented.

Challenging Glass 3

2. Seismic design of glass façades

Earthquakes cause damage to the building main structure as well as to the non-structural

components. Falling façade fragments during an earthquake poses a serious hazard to

pedestrians and occupants as well, so in earthquake prone regions its must be accounted

in the design.

Sucuoǧlu and Vallabhan refer broken window glass as the second most serious non-

structural damage, for example in the 1985 Mexico City earthquake over 50% of the

263 office observed buildings had experienced some sort of glass breakage [1].

During an earthquake two types of lateral loads are considered acting in the façade

panels: the “in plane” loads and the “out of plane” loads. Both inertial loads are caused

by the horizontal displacements of the building’s floors, yet the in-plane actions causes,

mainly shear stresses, and the others give rise to extra inertial forces due to the panel’s

bending.

The frequency content of the dynamic loads transmitted to the panels is modulated by

the building natural frequency, so if it happens that it has a value very close to the

panel’s natural frequency, resonant effects occur with an agonisingly increase of the

dynamic response, a well-known phenomena that must be avoided, otherwise structural

safety may be compromised.

As there are no design regulations for determine seismic loads in glass façades, a

laboratory test procedure according the American Architectural Manufacturers

Association recommendations [2, 3] is followed in order to evaluate the maximum

seismic drift which may cause glass brakeage and fall out of framed glass panels.

The dynamic test procedures considers a sinusoidal drift history for the procedure

testing with growing amplitudes to a maximum of 150 millimetres (Figure 1). This test

method has been applied in previous studies, like (Memari et al) [4], and is going to be

used to assess the seismic behaviour of point fixed glass panels in the Seismic

Laboratory of the University of Porto Engineering faculty.

Figure 1: Schematic of displacement time history for dynamic crescendo test [2].

Seismic Behavior of Point Supported Glass Panels

3. Simplified method to assess the seismic forces

The simplified method to assess the seismic forces transmitted to the façade panels, was

adapted by Camposinhos [5] from the work of Singh [6, 7], and is based on the response

spectra of Eurocode 8 (EC8) [8]. Figure 2 presents the elastic response spectrum

adopted in this work.

Figure 2: Response spectrum [8].

Thus the dynamic load transmitted to the façade panels (equation 1) depends on the

building’s natural vibration period and on the panel’s mass and natural vibration period.

E

EEDSfZ

EkR

MSCF

×γ×××=

40.0 (1)

where,

FEk characteristic seismic force;

CfZ seismic coefficient of the panel (depends on the building dynamic

characteristics and the position on the panel in the building);

SDS ground acceleration value;

γE importance coefficients of the panel (ranging between 1.0 and 1.5);

ME panel’s mass;

RE coefficient of performance of the panel (ranging between 1.5 and 3.5).

Prior to the application of the simplified method a parametric analysis was made to

evaluate the variation of seismic coefficient CfZ with the dynamic properties of the panel

and its position or height in the building. Two different cases were studied: in the first

case the panel is assumed to be in the last floor (m=N), and in the second, the panel was

assumed to be in placed at the penultimate floor (m=N-1). Figure 3 and Figure 4

presents the variation of Cfz with the natural period of vibration of the building for

different values of the natural period of vibration for the glass panel.

0

2

4

6

8

10

0 1 2 3 4

Sa

(m

/s2)

T (s)

Response spectrum (Seismic action type II; soil type D; seismic

region 2.1; η=1.0)

Sa

Challenging Glass 3

Figure 3: Variation of Cfz (m=N).

Figure 4: Variation of Cfz (m=N-1).

The results (Figure 3 and Figure 4) shows a very significant resonant effect for low

periods and a comparison between them allow to conclude that a panel in the last floor

corresponds to the most severe situation in what seismic excitation concern, with values

about 40% higher, once the coefficient, CfZ is directly correlated with the maximum

seismic force acting in the panel.

4. Case study

4.1. Field application

The search for technical innovative solutions in glass façades, namely looking for the

development and easy installing thinner and lighter glazing without compromising

safety is one of the main purpose of the FACAL – Façade Engineering Lda.

Facal is well known by the execution of famous facade glazings such as the glass

system in Casa da Música in Oporto, the double skin façade Torre H in Lisbon, the

glazing of the Spanish Pavilion in the International Fair in Zaragoza or the glass roof of

0

20

40

60

80

100

0 1 2 3 4

CfZ

T(s)

Variation of CfZ (m=N; ξ=5%)

Tpanel=0.05s Tpanel=0.2s Tpanel=0.4s

Tpanel=0.6s Tpanel=0.8s Tpanel=1s

0

20

40

60

80

0 1 2 3 4

CfZ

T (s)

Variation of CfZ (m=N-1; ξ=5%)

Tpanel=0.05s Tpanel=0.2s Tpanel=0.4s

Tpanel=0.6s Tpanel=0.8s Tpanel=1s


the Metro Station at Sá Carneiro Airport in Porto, FACAL always achieves the

resolution of the concept.

Figure 5: Examples - Torre H; Casa da Música; Spanish Pavilion; Metro Station.

Pursuiting new and innovative construction solutions a partnership study have been

develloped with Faculty of Engineering of the University of Porto in order to analyze

the behavior of different solutions of point supported glass panels systems under

seismic loads.

On the basis of this work is the newly-built office building of Bouygues Imobiliária in

Lisbon. A particular attention was given to the safety requirements of a 20 meters high

point-supported glass façade over the main entrance of the building. The post-breakage

behaviour of the laminated safety glass panels is the primary concern. This is often

neglected problem in glass façades yet of greater importance since Lisbon as many

other metropolis is prone to seismic activity.

A crucial concern on this matter is essential: The integrity of the glass façade must be

preserved and assured that there’s no risk of glass fragments fallings.

Among the various enhancement solutions the use of laminated safety glass with the

DuPont interlayer SentryGlas® provides an optimal behaviour due to its excellent post-

breakage performance with the glass fragments remaining adhered to the interlayer,

without falling down.

Figure 6: Point-supported glass façade - office building of Bouygues Imobiliária in Lisbon.

Challenging Glass 3

4.2. Geometrical and mechanical properties

This paper focus on a set of five point supported

specimens have a surface of 2350x2300 m

interlayer and distance from edges to holes (

Figure 7: Geometrical configuration of the glass panels: left) V1 and V2; right) V3, V4 and V5 (Drawings by:

FACAL).

The panels identified as V1, V2, V3 and

glass and a 1.52mm thick interlayer together with a

is made from two 12mm tempered glass sheets.

Panels V1 and V3 have a SentryGlass® film

interlayer. Furthermore a panel with the same geometrical configuration

but with SentryGlass® interlayer film was additional considered

The properties of the the PVB’s Young modulus

10], so in the panels with this type of interlayer

were considered: (i) 3.2MPa, (ii) 9.0 MPa

SentryGlas® a the value of 300 MPa was adopted for its Young modulus, as suggested

by Delincé [10].

Table 1summarises the relevant data for the

Table 1: Test specimen’s description

Glass

panel

Dimensions

[mm]

Total

thickness

[mm]

Interlayer

film

V1 2350x2300 10+1.52+8 SentryGlas®

V2(i) 2350x2300 10+1.52+8 PVB

V2(ii) 2350x2300 10+1.52+8 PVB

V2(iii) 2350x2300 10+1.52+8 PVB

V3 2350x2300 10+1.52+8 SentryGlas®

V4(i) 2350x2300 10+1.52+8 PVB

V4(ii) 2350x2300 10+1.52+8 PVB

Challenging Glass 3

Geometrical and mechanical properties

point supported laminated glass panels. All the

mm2 differing on the glass thicknesses, the

(Figure 7).

panels: left) V1 and V2; right) V3, V4 and V5 (Drawings by:

FACAL).

V4 are obtained from 10 mm strengthened

together with a 8mm also tempered glass.Panel V5

sheets.

anels V1 and V3 have a SentryGlass® film while the remaining panels have PVB

panel with the same geometrical configuration of that of V5

additional considered in the analyses.

the PVB’s Young modulus ranges from 3.2 MPa to 18 MPa [9,

interlayer three different values for this parameter

(i) 3.2MPa, (ii) 9.0 MPa and (iii) 18.0 MPa. In the case of

SentryGlas® a the value of 300 MPa was adopted for its Young modulus, as suggested

the studied glass panels.

Test specimen’s description.

Interlayer

film

Interlayer’s

Young

modulus [MPa]

Glass’

Young

modulus [GPa]

Glass’

Poisson

coefficient

SentryGlas® 300 70 0.20

PVB 3.2 70 0.20

PVB 9.0 70 0.20

PVB 18.0 70 0.20

SentryGlas® 300 70 0.20

PVB 3.2 70 0.20

PVB 9.0 70 0.20


V4(iii) 2350x2300 10+1.52+8 PVB 18.0 70 0.20

V5(i) 2350x2300 12+1.52+12 PVB 3.2 70 0.20

V5(ii) 2350x2300 12+1.52+12 PVB 9.0 70 0.20

V5(iii) 2350x2300 12+1.52+12 PVB 18.0 70 0.20

V5(iv) 2350x2300 12+1.52+12 SentryGlas® 300 70 0.20

4.3. FE model

To evaluate the structural response of the glass panels a set of numerical models were

made using commercial finite element (FE) software.

The glass panels and interlayer film were modelled with 8-node 3D finite elements. In

the interior of the panel the maximum size of the finite elements was limited to 2

centimetres, while near the supports the maximum size was reduced to half to attend the

stress concentrations near the holes to take in account the expected stress concentrations

in this regions.

Figure 8: FE mesh details - a) lateral view; b) interior of the panel; c) support region.

The degrees of Freedom (DOF) restrictions which enabled this stress concentration near

the support region and the brittle nature glass leads to a several numerical model

simulations stages until a solution that that correctly represent the real support condition

was achieved.

To assess the behaviour of the model two premises were advanced: (i) the allowance for

rotations of the panel acording the real behaviour; (ii) Stress distribution around the hole

should be compatible with reality.

The best solution lead to the implementation of an external node where the support

constraints are included. The connection to the panel was provided throughout rigid

pseudo beam elements (Figure 9).

Challenging Glass 3

Figure 9: Lateral view of the support solution.

5. Results

5.1. Simplified method

The natural frequencies of the glass panels were determined using the FE model and are

depicted in (Table 2),

The already mentioned simplified method was applied to assess the maximum seismic

effect on the façade panel and the results are presented in Figure 10.

Table 2: Glass panels natural frequency.

Glass panel Natural frequency (Hz)

V1 13.4

V2(i) 10.3

V2(ii) 11.5

V2(iii) 12.2

V3 8.1

V4(i) 6.4

V4(ii) 7.1

V4(iii) 7.5

V5(i) 8.0

V5(ii) 9.0

V5(iii) 9.5

V5(iv) 10.5


Figure 10: Maximum seismic force transmitted to the glass panels.

As it can be observed, the peak values presented in the figure, nearly 10 times the

panel’s self weight, confirm the expected resonant effects in the panels.

5.2. Time history dynamic analyses

In order to qualitatively assess the results obtained by the simplified method a set of

time history dynamic analyses has been performed, using the El Centro ground motion

record (Figure 11) appropriately scaled so that the maximum spectral acceleration was

equal to the one calculated by EC8.

Figure 11: El Centro ground motion record [11].

To verify the existence of resonant effects in the panel a structure with a natural period

of 0.12 s (very close to the natural period of that of panel V5(i)) was analysed with the

selected ground motion record. The floor accelerations obtained, Figure 12, were then

applied to the façade panel and the corresponding response was gathered, Figure 13.

0

10

20

30

0 1 2

FE

k (k

N)

Τ (s)

Seismic forces transmited to the façade panels

V1 V2(i) V2(ii) V2(iii) V3 V4(i)

V4(ii) V4(iii) V5(i) V5(ii) V5(iii) V5(iv)

-4

-2

0

2

4

0 5 10 15 20 25 30 35

üg

(m

/s2)

Time (s)

El Centro earthquake, May 18th 1940 (N-S)

Ground acceleration

Challenging Glass 3

Figure 12: Floor accelerations.

Figure 13: Response of the panel V5(i).

The maximum response acceleration of the panel, about 81 m/s2, 10 times the peak floor

acceleration, confirms the existence of resonance in the façade panel.

As the simplified method does not consider any damping effects, new dynamic analyses

were made considering the panel’s damping ratio (ξ) ranging from 0% to 5%.

Table 3: Differences in the panel V5(i) response for different damping ratio.

Damping ratio [%] Maximum deflection on the panel [mm]

0 16.7

2 11.8

5 8.58

Table 3 shows that there is a decay of almost 30% in the maximum deflection when a

2% damping ratio was considered.

Thus in buildings with natural vibration frequencies that might induce resonance in the

façade panels an energy dissipation device should be applied to reduce the dynamic

response of the panel.

-10

-5

0

5

10

0 5 10 15 20 25 30 35

Acc

eler

ati

on

(m

/s2

)

Time(s)

Floor accelerations

Floor accelerations

-100

-50

0

50

100

0 5 10 15 20 25 30 35

Acc

eler

ati

on

(m

/s2

)

Time (s)

Response of the panel V5(i)

Panel acceleration


5.3. Numerical simulation of the test procedure according to AAMA recomendations

Section 2 briefly described a test procedure to assess the maximum seismic drift causing

glass breakage and fallout in framed glass panels. This test procedure was numerically

simulated using the same FE models previously developed to determine the panel’s

dynamic properties.

Due to stress concentrations in the support region, material rupture was reached for a

drift less then10 mm (Figure 14).

Figure 14: Example of results for the AAMA dynamic test.

The low drift level that causes material failure determined by the numerical test lead to

the formulation of the hypothesis that AAMA 501.6 dynamic test aims to determine the

maximum drift that causes panel’s detachment from the support rather than the material

rupture.

6. General conclusions

In a FEM analysis the importance of an adequate modelling of the support conditions in

the point supported glass panels is mandatory in order to control the stress concentration

phenomenon.

The simplified method to determine the seismic forces transmitted to façade panels

seems to be able to capture the relevant aspects of the whole issue regarding resonance

effects as well.

The numerical simulations using time history analysis confirmed the hypothesis of

resonance effects induced by the building’s natural frequency and are in agreement with

the peak values determined by the simplified method.

0

100

200

300

400

500

600

700

800

900

0 5 10 15 20 25

σ(M

Pa)

drift (mm)

Results for the AAMA dynamic test (Panel V1)

Maximum Tension in the support Tension at 5mm from support

Tension at 10 mm from support σf annealed glass

σf heatstrenghtned glass σf tempered glass

Challenging Glass 3

Tests showed up that the problem’s sensibility to damping is relevant. In fact a 2%

damping ratio for the panels lead to a 30% decrease in its maximum deflection.

It must be emphasized that in earthquake prone regions façade panels without energy

dissipation devices could be seriously and dangerously excited into non acceptable

limits.

7. References

[1] Sucuoǧlu, H. and C.V.G. Vallabhan, Behaviour of window glass panels during earthquakes.

Engineering Structures, 1997. 19(8): p. 685-694.

[2] AAMA, AAMA 501.6-09: Recommended dynamic test method for determining the seismic drift causing glass fallout from a wall system. 2009, AAMA.

[3] AAMA, AAMA 501.4-09: Recommended static testing method for evaluating curtain wall and storefront

systems subjected to seismic and wind induced interstory drift. 2009, AAMA. [4] Memari, A.M., R.A. Behr, and P.A. Kremer, Seismic behavior of curtain walls containing insulating

glass units. Journal of Architectural Engineering, 2003. 9(2): p. 70-85.

[5] Camposinhos, R.d.S., Revestimentos em pedra natural com fixação mecânica dimensionamento e projecto. 2009, Porto: Edições Sílabo. 199.

[6] Singh, M.P., et al., Seismic design forces. I: Rigid nonstructural components. Journal of Structural

Engineering, 2006. 132(10): p. 1524-1532. [7] Singh, M.P., et al., Seismic design forces. II: Flexible nonstructural components. Journal of Structural

Engineering, 2006. 132(10): p. 1533-1542.

[8] CEN, Eurocode 8 - Design of structures for earthquake resistance Part1: General rules, seismic actions and rules for buildings. 2010, IPQ: Caparica. p. 230.

[9] Chen, J., Q. Zhang, and B. Xie, Nonlinear finite element analysis on laminated glass panel based on

APDL. Computer Aided Engineering, 2010. 19(Copyright 2011, The Institution of Engineering and Technology): p. 22-6.

[10] Delincé, D., et al. Post-breakage behaviour of laminated glass in structural applications. in Challenging

Glass. 2008. [11] http://www.vibrationdata.com/elcentro.htm. Access Date: 18/05/2011

Documents

Seismic Behavior of Point Supported Glass Panels