This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Challenging Glass 3 – Conference on Architectural and Structural Applications of Glass,
Bos, Louter, Nijsse, Veer (Eds.), TU Delft, June 2012.
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.
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
Seismic Behavior of Point Supported Glass Panels
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
Seismic Behavior of Point Supported Glass Panels
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