Assessing the In-Plane Racking Performance of Glass Façade Systems

S. Sivanerupan*, J. L. Wilson* and E. F. Gad*

*Swinburne University of Technology, Hawthorn, VIC, 3122, Australia (E-mail:

ssivagnanasundram@swin.edu.au)

ABSTRACT Glass façade systems in buildings may be subjected to racking action due to inter storey drift caused

by earthquake and wind actions. The performance of façade systems is dependent on the amount of

drift and the interaction of the glass panels with the façade structural support frames. The glass

façade systems can be classified into two types namely, framed glass façade system (FGFS) and

point fixed glass façade system (PFGFS). It was observed that the damage to glass façade systems

resulting from in-plane racking actions mainly from earthquakes is increasingly common and yet

there have been limited number of laboratory tests and detailed analyses undertaken. The research

conducted to date mainly focused on traditional FGFS.

However the seismic performance of PFGFS is likely to be quite different from conventional

framed systems. If the system does not have enough in-plane drift capacity it will be vulnerable in

racking actions mainly during earthquakes and wind actions. A unique real scale in-plane racking

laboratory test on a typical PFGFS was conducted. The major aim of the project is to assess the in-

plane racking performance of PFGFS. In this paper, the laboratory test setup and the experimental

results are discussed together with confirmatory analytical studies.

KEYWORDS Glass façade systems; Earthquake loading; In-plane drift capacity, point fixed glazing

INTRODUCTION

Glass façade systems provide the interface between the internal and the external environments and

therefore have significant impact on the building aesthetics. Conventionally, the glass façade is

framed using aluminium mullions and transoms. A relatively new contemporary glass façade

system is available which provides greater transparency and improved aesthetics, known as point

fixed or bolt fixed glass façade systems. In the structural design of glass façades both out-of-plane

and in-plane actions are considered by the façade engineer. Self-weight, thermal expansion,

spandrel beam deflection and in-plane building movements due to wind and seismic loadings are

considered for in-plane design whilst wind load on glass panel is the main design action for out-of-

plane performance. From a seismic design perspective, glass façade systems are considered to be

drift sensitive non-structural elements and the performance is dependent on the in-plane drift

capacity of the system which should be greater than the in-plane drift demand.

In-plane drift provisions in standards are recommended for serviceability and ultimate limit states.

AS 1170.4 (2007), clauses 5.4.4 and 5.5.4, specify that, “the inter-storey drift at the ultimate limit

state, calculated from the forces determined according to strength and stability provisions shall not

exceed 1.5% for a 500 year return period (RP) event of the storey height for each level and “the

attachment of cladding and façade panels to the seismic-force-resisting system shall have sufficient

deformation and rotational capacity”. The New Zealand Standard “Earthquake actions”, NZS

1170.5 (2004) specifies in clause 7.5 that, “a maximum inter-storey drift limit of 2.5 % is applicable

for the ultimate limit state of 500 year RP event. In the case of a 2500 year RP near fault event, this

limit has to be increased to 3.75%. From discussions with façade design experts, the glass façades

are generally designed for in-plane racking performance due to wind loading which is usually

H/500 (0.2%) for serviceability conditions.

ASCE- 10 (2010) provides a general expression for assessing conventional FGFS under in-plane loading as expressed by Equation 1. The drift capacity (∆fallout) of a framed system is to be greater than the drift demand which is a function of inter-storey drift from the building (Dp) and the occupancy importance factor (I).

1.25 or13mm whichever is greaterfallout p

ID∆ ≥ (1)

Exceptions are recommended by ASCE7-10 (2010) for framed glass façades with sufficient glass-

to-frame clearance such that physical contact between the glass and frame will not occur at the

design drift demonstrated by Equation 2.

2

1

1

1.25 ; and 2 1p

clear p clear

p

h cD D D c

b c

≥ ≥ +

(2)

Where hp = height of rectangular glass; bp = width of rectangular glass, c1 = clearance (gap) between the vertical glass edges and the frame; and c2 = clearance (gap) between the horizontal glass edges and the frame. The mechanism of contact between the glass and frame is explained by Sucuoglu and Vallabhan (1997), considering rigid body movement and rotation of the glass panel. In FGFS, if the exceptions are not satisfied, mock-up tests can be carried out to evaluate the drift in-plane drift capacity. American Architectural Manufacturers Association (AAMA) specifies the laboratory test methods for both static and dynamic testing to evaluate the in-plane drift capacity of FGFS (AAMA, 2001a, 2001b).

However, the seismic performance of PFGFS is different from the conventional FGFS and there are

no standards or design guidelines available to evaluate the in-plane drift capacity of such systems.

Spider arms are used to connect the glass to the support structure in PFGFS and the glass to the

spider arms are connected using special bolt fittings (Figure 1). Despite its growing popularity,

there is very limited published research on the behaviour of PFGFS under in-plane racking action.

Figure 1 Spider arms for single bolt connection and bolt fittings

A research project has just been completed by the authors to evaluate the racking performance of

PFGFS. This involves laboratory experimental study and analytical modelling of PFGFS with

toughened glass panels. This paper provides an overview of the laboratory experimental testing and

analysis of a typical PFGFS currently used in Australia. Details of the PFGFS configuration and

experimental test set-up, instrumentation and test results are presented. The racking performance of

Countersunk Button head

Swivel

Spider arms

M16 bolts to connect

spider arm

the PFGFS was also simulated using ANSYS finite element software with some results presented

herein.

EXPERIMENTAL TEST

Specimen description

The test was conducted on a typical PFGFS as shown in Figure 2, which consisted of four 1200mm

x 1200mm toughened 12mm thick glass panels joined with 8mm thick silicon weather sealant.

There are four different types of bolt fittings available in the market to connect the glass-to-spider

arms namely, countersunk, button head, swivel button head and swivel countersunk as shown in

Figure 1. Countersunk bolt fittings and spider arm with single bolt connected to the support

structure were used in this test which allows in-plane rotation of glass panels at the spider arm-to-

support structure connection (Figure 1).

The glass bolt hole details were prepared according to the location of the glass to spider arms

connections and the dimension of the countersunk bolt fittings. A support structure was designed to

support four 1200mm x 1200mm glass panels and fabricated using 180PFCs as shown in Figures 2

and 3. The in-plane racking performance of PFGFS is dependent on the glass panels, the connection

details (between the glass to spider arm and between the spider arm to support structure) and the

support structure. In this test the support structure was articulated so that the racking performance of

the glass panels and the connection details could be assessed.

Detail of the test setup

The support structure (blue frame) was assembled into the test rig (yellow frame) as shown in

Figure 2a. Three vertical and two horizontal PFC members were pin connected and snug tightened

using M24 bolts to allow the frame to rack as a mechanism. The setup was capable of imposing a

100kN in-plane lateral load and up to 150 mm in-plane displacement.

The support structure was prevented from moving in the out-of-plane direction by four sets of

rollers mounted at the top (Figure 2a). The rollers ensured that the support structure was aligned

with the loading direction. Once the support structure was assembled glaziers fixed the spider arms,

glass panels and weather sealant (Figure 2b). A special transparent adhesive film was applied to the

glass panels to secure the glass fragments after any glass fracture. A hydraulic jack was mounted on

the reaction frame (yellow) shown in Figure 2 & 3 and used to laterally load the support structure

(blue frame) and the façade system.

(a)

(b)

Figure 2 (a) Support structure assembled into the test rig and (b) Glass panels installed

Test procedure and Instrumentation

The test procedure and instrumentation were as follows:

• The specimen was pulled at the top right hand corner with displacement increments until

failure.

• Two systems of measurements were adopted to maximise the outcomes from this test:

o Displacement measurements with LVDTs (Linear Voltage Displacement

Transducers) and

o Photogrammetry measurements

• Deflections were measured at 11 locations (horizontal, vertical and out-of- plane) with the

LVDTs (Figure 3 and the applied load was measured using a load cell (Figure 3).

• Photogrammetry provided displacement data for the target points that were tactically

positioned (Figure 4). Photographs of the targets were taken before and after each load step

and the relative movement in their positions were interpreted using software based on the

principles of triangulation.

GROUND FLOOR

180PFC

18

0P

FC

20

0

3300

90

x9

0x1

0 E

A

A A

B

100 x16 Flat Bar

300

550

1

8

2

4

3

56

10

9

7

11

Hydraulic Jack

Load Cell

Glass Panels

1- Top displacement

2- Out-of-plane deformations of the glass

panel

3- In-plane lateral relative movement of glass

panels

4- In-plane vertical relative movement of

glass panels

5- Spider arm vertical movement

6- Corner spider arm vertical movement

7- Internal central spider arm movement

8- Lateral movement of the test frame at the

bottom

9- Vertical movement of the test frame at the

bottom

10- Movement of the reactive frame at the top

11- Out-of-plane deformations of the glass

panel

Figure 3 General layout of test setup

EXPERIMENTAL TEST RESULTS AND RACKING MECHANISIM

The pushover curve measured at the top of the support structure (LVDT No. 1 in Figure 3) from the

test is shown in Figure 8. It indicates that the structure performed almost linearly up to failure. A

maximum displacement of 58 mm was measured with a corresponding 16kN racking load before

failure. Surprisingly this corresponds to a maximum of 2.1% in-plane drift capacity for the system

with only minor damage to the sealant. However, yielding of the spider arms was observed before

the catastrophic failure of one of the glass panels. The failure of the glass panel is shown in Figure

4. The adhesive films protected the glass fragments from falling.

It was observed during the experimental test that the glass panels and the spider arms, all translated

and rotated as rigid bodies whilst the sealant deformed at the interface. The spider arms used in this

experiment had a frictional moment (torsion) capacity beyond which rotation would occur. A

simple truss analysis was carried out to determine the loading actions (tension or compression) in

the panels as shown in Figure 5a. The initial (blue) and the final (red) locations of the panels are

shown to scale in Figure 5b and these represent the translations that occurred in the glass panels

before failure. The weather sealant initially prevented the rotation of the spider arms however,

within beyond a 3mm racking displacement, rotation in the perimeter arms was observed (Figure

6a).

Figure 4 The test specimen after failure of a glass panel

A significant amount of rigid body translation followed by bearing action in both the horizontal and

vertical directions was observed at the bolted connections where the built-in standard gaps allowed.

The weather sealant offered some resistant against tensile, compressive and shear actions and

resisted some relative movement of the glass panels in both in-plane and out-of-plane directions.

The applied load was eccentric to the bolted glass panels, which created differential out-of-plane

deformations on the spider arms resulting in the glass panels displacing in the out-of-plane direction

relative to each other. A significant amount of out-of-plane movement was observed between arms

PBB4 (Panel PB spider arm B4) to PDB2 and PAB3 to PCB1 (Figures 8a & 9b) with a maximum

differential movement of approximately 8.5mm, which induced combined local bending and tensile

stresses particularly around the bolt to glass panel (Bolt PBB4) connection resulting in the initiation

of cracking and catastrophic failure of the bottom right hand panel as shown in Figure 4.

COMPRESSION TENSION

B1

B3 B4

B2

Glass panel (PA)

B1

B3 B4

B2

Glass panel (PB)

B1

B3 B4

B2

Glass panel (PD)

B1

B3 B4

B2

Glass panel (PC)

(a)

Translation of the glass panel at the drill holes

0

500

1000

1500

2000

2500

0 500 1000 1500 2000 2500

Width (mm)

He

igh

t (m

m)

(b)

Figure 5 (a) Glass panels and spider arms labelled including the rotational directions (b)

Translation of the spider arms and glass panels

-25

-20

-15

-10

-5

0

5

10

15

20

0 10 20 30 40 50 60

Lateral displacement (mm)

Vert

ical d

isp

lacem

en

t (m

m)

PAB1PDB4PBB2PAB2PAB3PAB4PBB4PDB3PCB3

(a)

0

1

2

3

4

5

6

7

8

9

0 10 20 30 40 50 60

Lateral displacement (mm)

De

form

ati

on

(m

m)

PDB2 to PBB4

PCA3 to PCB1

(b)

Figure 6 (a) Displacement of the spider arms in the vertical direction (spider arm rotation)

(b) Differential out-of-plane movement of the spider arms against the system displacement

The application of a racking load to the structural support frame resulted in diagonal forces

developing in the glass panels transmitted from the spider arms bearing on the glass bolt holes as

illustrated in Figure 5a. The net force transmitted from the spider arm connection to the glass panel

was approximately along the diagonal direction but eccentric resulting in one arm moving out wards

from the structural support frame and the other arm moving inwards causing relative out-of-plane

movement of the adjacent glass panels. The experimental results revealed that the applied racking

displacement was accommodated by three mechanisms:

o In-plane rigid body rotation of the spider arms;

o Rigid body translation facilitated by the built-in standard gaps between the bolts and

holes with in the spider arm and structural support frame connections; and

o Deformations and distortions of the spider arms.

ANALYTICAL MODELLING

Based on the experimental results and racking mechanism observed a detailed three-dimensional

non-linear finite element model (FE model) was developed using ANSYS finite element software to

replicate the laboratory tests. The FE model is shown in Figure 7.

Features of the model

The model was created with a number of features to represent the racking behaviour, including;

• The structural support frame, spider arms and M10 bolt to connect the spider arm to glass

panel were modelled using beam elements,

• Allowance for the spider arm to rotate when it overcomes the internal frictional force at the

structural support frame connections. This action was modelled using non-linear springs

with real constants assigned to represent the frictional torsional moment versus rotation at

the spider arm connection,

• Glass panels were modelled using shell elements and finely meshed around the glass holes

to determine the failure stresses,

• Conservatively, the countersunk bolt head was modelled as a 20mm diameter cylindrical

head and modelled using shell elements,

• The translations between (a) bolt fittings and spider arm and (b) spider arm and support

structure were modelled using non-linear springs, and

• Silicon sealant was modelled using a material model specified in the ANSYS called Blatz

and Ko (Bondi and McClelland, 2009) which is a one parameter model to represent hyper

elastic behaviour.

(a)

(b)

Figure 7 (a) Front view of the model, showing glass, sealant and bolt heads and (b) side view of the

model, showing the support structure, spider arms and bolt fittings.

The results obtained from the FE models were benchmarked against the test data including,

pushover curve, failure stress and out-of-plane deformation of glass panels. A good correlation was

found between the experimental and analytical results as shown in Figure 8. Further detailed FE

analyses were conducted to evaluate the racking capacity from each mechanism. The analyses

showed that a significant amount of the drift capacity was contributed from the rigid body

translation facilitated by the built-in standard gaps between the bolts and holes within the spider

arm and structural support frame connections (1.1% drift). The estimated drift contributions from

each mechanism using the FE models are summarised as follows:

a) In-plane rigid body rotation of the spider arms – 0.4%.

b) Rigid body translation facilitated by the built-in standard gaps between the bolts and

holes with in the spider arm and structural support frame connections – 1.1%.

c) Deformations and distortions of the spider arms – 0.5%.

Figure 8 Pushover curve for the test specimen

CONCLUDING REMARKS

In-plane racking test on a typical PFGFS currently used in Australia was conducted. A maximum

in-plane lateral displacement of 58 mm was measured with a corresponding 16kN racking load

before failure. This translates to a 2.1% in-plane drift capacity for the system with minor damage to

the sealant and yielding of a perimeter spider arm, before catastrophic bending failure of one of the

glass panels. The bending stresses were induced from an approximately 8.5mm relative out-of-plane

movement at the adjacent glass bolted connections. It was verified from the experimental and

analytical results that the applied racking displacement was accommodated by three mechanisms.

The major contribution was from the rigid body translation (1.1% drift) at the built in standard gaps

provided in the connections. Conservatively, the in-plane racking capacity of the PFGFS from only

the rigid body translation at the built in standard gaps could be used as the design in-plane drift

capacity. The drift capacity could be increased further by introducing additional articulation

tolerances at the bolted connections at the spider arms or façade structural support frame. Simple

trigonometric equations can be developed to quantify the drift contribution from the built in gaps.

The seismic assessment of glass façade systems requires an estimate of the likely drift demand from

the building. Codified provisions for in-plane drift limits on glass façade systems can be used as a

conservative option. However, analysis results indicated that the inter-storey drift could be much

smaller than the 1.5% limit in AS1170.4 (2007) for most buildings in Australia for the 500 year RP

event except for soft storey structures. Therefore, standard seismic analysis procedures could be

used to calculate the drift from the buildings to optimise the glass façade design. Further

experimental tests and analytical studies on PFGFS are underway by the authors.

ACKNOWLEDGEMENT

The authors are very grateful to AEES for awarding the research scholarship for 2010 which

assisted with financing the test. We are also very grateful to Bill Vun, Jon Yan and Leonard Tan

from Australian Glass Assemblies and Lynton Wombwell from Viridian World Glass for supplying

the spider arms fittings and the glass panels. We also acknowledge the University of Melbourne for

providing access to the laboratory and Melbourne Testing Services in particular Rodney Wilkie for

the active test support. Our special thanks go to PhD students, David Heath from the University of

Melbourne, Deepti Wagle and Bara Baraneedaran from Swinburne University of Technology for

their assistance.

REFERENCES AAMA 2001a. Recommended static test method for evaluating curtain wall and storefront systems

subjected to seismic and wind induced inter-story drifts. Publication No. AAMA 501.4-01,,

Schaumburg, III.

AAMA 2001b. Recommended dynamic test method for determining the seismic drift causing glass

fallout from a wall system. Publication No. AAMA 501. 6-01,, Schaumburg, III.

AS1170.4 2007. Structural design actions, Part 4: Earthquake Actions in Australia. Australian

Standard, Standards Australia, 1 The Crescent, Homebush, NSW 2140.

ASCE7-10 2010. Minimum design loads for buildings and other structures. 1801 Alexander Bell

Drive, Reston, Virginia 20191-4400: The American Society of Civil Engineers.

BONDI, S. & MCCLELLAND, N. 2009. Capturing structural silicone non-linear behaviour via the

finite element method. Glass processing days 2009, Tampere, Finland, 183–185.

NZS 1170.5 2004. Structural design actions, Part 5: Earthquake actions. New Zealand Standard.

SUCUOGLU, H. & VALLABHAN, C. V. G. 1997. Behaviour of window glass panels during

earthquakes Engineering Structures, 19, 685-694.