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CURTAIN WALL DESIGN AGAINST
STORY DRIFT |
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Written
By Raymond Ting
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INTRODUCTION
A curtain wall system must be designed for multiple
performance functions including aesthetic, thermal
insulation, sound insulation, water tightness, tolerance
for inter-floor deflection, and structural safety
against various loads including thermal, wind, and
seismic. The recent trend in the building frame
design against seismic force calls for a much larger
amount of story drift. This trend causes extreme
difficulty for maintaining the various curtain wall
functions in the conventional curtain wall design
approaches. The purpose of this paper is to analyze
the cause and effect of seismic force on the curtain
wall performances leading to a new set of design
principles and approaches.
CRITICAL PERFORMANCE FUNCTIONS
The determination of critical performance functions
can be based on maintenance cost, safety and liability
issues. Based on maintenance cost alone, the water
tightness performance is recognized as the most
critical, due to the need of zero tolerance for
water leakage and the degree of difficulty in finding
a leakage source and in executing remedial work.
Based on the structural safety issue, the curtain
wall system must be designed with adequate safety
factors for the expected design conditions. Arising
from the liability issues from consequences of component
structural failure, it is a common practice in the
industry to specify that no component shall fall
from the wall at the theoretical ultimate load condition.
Based on the liability issue of "Sick Air Building
Syndrome", the water tightness performance apparently
becomes the most critical function. In general,
the structural safety issues can be solved by engineering,
therefore, with sound engineering, the structural
safety issues can be taken off of the critical list.
In summary, the maintenance cost and the liability
issue are the critical performance functions in
the curtain wall design consideration.
DEGRADATION OF CURTAIN WALL PERFORMANCE FUNCTIONS
It is a common understanding in the industry that
all curtain wall functions are subjected to certain
degree of degradation over time. However, the rate
of degradation and the associated cost and liability
impact vary significantly among the various performance
functions. Again, the water leakage problem is the
most troublesome, for example, the degradation of
thermal insulation value and the "Sick Air Building
Syndrome" due to mold growth caused by the wetting
of insulation are the by-products of the water leakage
problem. The following section discusses the factors
affecting the degradation of water tightness performance.
FACTORS AFFECTING THE DEGRADATION OF WATER TIGHTNESS
PERFORMANCE
In normal curtain wall design practice, impervious
material is utilized for the facing panel and the
supporting components; therefore, the sealing integrity
of the curtain wall joints governs the water tightness
performance. The sealing location can be generalized
into three categories: perimeter seal of facing
material; panel-to-panel joint seal; and panel-to-support
seal. In the effort to improve the longevity of
the joint sealing integrity, significant advances
have been made through use of more durable sealant
material and by reducing the number of locations
requiring critical seals. A critical seal location
is normally defined as a location requiring the
sealing of both air and water. However, water leakage
will still occur due to the degradation of the critical
seals caused by the aging effect, the workmanship
on the execution of the critical seals, and sealant
line stress fatigue or stress failure due to repeated
thermal, wind, and seismic loads, as well as various
other structural displacements of the building frames.
Due to the reliance on workmanship and the great
number of factors producing sealant line stress,
the water tightness performance has been unpredictable
in time and location. There are two types of sealant
line stresses, the first type is tension or compression
and the second type is shear. In most design orientations,
thermal load or wind load will cause tensile or
compressive stresses in the sealant lines while
seismic induced story drift will cause shear stresses
in the sealant lines. The seismic induced shear
stresses in the sealant lines are most critical
for the sealing integrity of the critical seals
as explained in the following section.
SEISMIC EFFECTS ON A CURTAIN WALL SYSTEM
A curtain wall system consists of multiple facing
panels, panel supporting frames, and frame connection
system secured to the edge of the floor slab or
the spandrel beam. Weather sealing methods against
air and water infiltrations are provided along the
joints between two adjacent facing panels. The thermal
load and wind load are the direct loads on the curtain
wall system. The direct loads can relatively easily
be designed with adequate safety factors. For example,
the curtain wall system is normally designed with
an adequate safety factor for a wind load caused
by the maximum wind speed with 50-year recurrence
intervals. In the event of seismic activity, however,
the curtain wall system cannot be simply designed
for a specific earthquake magnitude with 50-year
recurrence intervals. A seismic event causes the
building frame to undergo various displacements
producing relative inter-floor deflection (vertical
component of seismic motion) and inter-floor story
drift (horizontal component of seismic motion).
Since the mass of the building frames and floor
slabs are extremely large as compared to the mass
of the curtain wall system, it is impractical to
design the curtain wall system to resist the inertia
forces inherent in the relative inter-floor deflection
or inter-floor story drift. Therefore, the curtain
wall system must be designed to tolerate the seismic-induced
building frame displacements which are functions
not only of the seismic zone rating but also of
the building frame stiffness. In case of seismic
induced story drift, the curtain wall supporting
mullions are forced to tilt to one side which in
turn will create the tendency to force the curtain
wall panel to change shape. However, the in-plane
stiffness of the curtain wall panel is normally
very high against changing shape, therefore, absorption
of the amount of lateral distortion caused by the
story drift produces a significant amount of sheer
strain along the sealant lines. This type of shear
strain along the sealant line is the primary cause
of sealant line failure in the seismically active
regions. The recent trend of allowing a higher degree
of story drift in the building frame design makes
the sealing integrity design even more problematic.
CONVENTIONAL DESIGN APPROACH VS. NEW THINKING
FOR SEISMIC EFFECT
There are two primary objectives for curtain wall
design against seismic-induced building displacements.
The first objective is to maintain long term sealing
integrity for water tightness performance in seismic
events. The second objective is to assure that no
curtain wall component falls off the wall at the
theoretical ultimate story drift condition.
1. Sealing Integrity Design Against Seismic Induced
Story Drift
In the conventional design approach, in order to
assure long lasting critical seals, efforts have
been concentrated on the items listed below:
(1). To make most critical seals in the shop where
best quality control can be executed so that the
number of field-executed critical seals is minimized.
(2). To use skilled workers to execute all critical
seals.
(3). To use the best available sealant material
with maximum ultimate shear strain for making the
critical seals.
(4). To stiffen the intersecting corners of support
members such that the corner angle distortion can
be minimized within reasonable cost.
The new thinking includes the items listed below:
(1). To completely separate the air sealing function
from the water sealing function such that no critical
seal is required. The author has coined the term
Airloop Principle for this technology (see Figs.
1 and 2 of Reference 1).
(2). To unitize each facing panel with the panel
frame around the perimeter of the facing panel,
to release the fixity of the intersection point
between the panel frame and the support member,
and to allow lateral stress-free sliding at the
sealing joint between two adjacent panels. This
arrangement allows the story drift to be absorbed
by stages of panel drift without producing sealant
line stress (see Fig. 4 of Reference 1).
(3). To use non-bonding type of sealing material
such as foam tape, to allow stress-free relative
contact sliding between two sealing components,
for example, in-plane sliding at horizontal panel
joints and between the panel jamb frame and the
vertical mullion support.
2. Sealing Integrity Design Against Seismic-Induced
Vertical Inter-floor Displacements
In the conventional design approach, the splice
joint of the supporting mullion and a corresponding
curtain wall panel joint must be designed to absorb
the total amount of seismic-induced and live load
induced vertical inter-floor deflection. This design
approach often causes great difficulty in maintaining
sealing integrity. The recent design trend of reducing
the rigidity of the building frame makes the sealing
integrity design even more difficult.
The new thinking is to control the maximum vertical
curtain wall joint movement to an amount significantly
less than the maximum vertical inter-floor deflection.
This design is explained below:
(1). Figure 1 shows typical mullion connection details.
With reference to Figure 1, the erection procedures
are explained in the following steps: Step 1: Pre-set
the anchor bolt, AB, into the floor slab, FS Step
2: Place mullion 1, M1, mullion clip, MC, mullion
clip washers, WS, and mullion bolt, MB, into position.
Step 3: After three way (up-and-down; left-to-right;
in-and-out) positioning adjustments, install the
dead load plate, DLP, and tighten the nuts on AB
and MB to secure the installed position of M1. Step
4: The mullion splice tube, MST, is secured to M1
using a bolt, TB1. Engage the top mullion, M2, with
MST and apply the bolt, TB2, at the center of the
slotted hole on M2.
(2). In order to show the mechanism of the mullion
splice joint movement and the floor slab movement,
Figure 2 shows the side cross-section of the mullion
connection taken along the web surface of M1 and
M2. This is the view of the initial installed position.
The slotted hole, SH1, on M1 is provided for the
vertical positioning adjustment plus the anticipated
maximum floor deflection, Df. The slotted hole,
SH2, on M2 is provided to control the maximum downward
movement, Dm1, of M1 and the maximum upward movement,
Dm2, of M1. The distance of Df can be designed to
be much larger than Dm1 or Dm2. At the installed
position, the dead weight of M1 including the associated
curtain wall is carried by the bearing of DLP on
MB. When the floor, FS, starts to deflect downward,
M1, MST, and TB2 will move in tandem, due to the
dead weight. At a deflection of Dm1, TB2 will bottom
out on slotted hole, SH2. When the floor deflection
exceeds Dm1, MB which is connected to FS with MC
must continue to deflect with FS. Since TB2 has
bottomed out on SH2, there will be no more relative
movement between M1 and M2. Therefore, MB will lose
contact with DLP and continue to go down along the
slotted hole, SH1. In this condition, the dead weight
of M1 is hanging on M2 by the bolt, TB2. Similarly,
in case of upward movement of FS, the maximum mullion
joint movement is controlled to be Dm2. For an upward
floor deflection larger than Dm2, TB2 will top out
on the slotted hole, SH2, and the dead weight of
M2 will be seating on top of M1 by the bearing of
M2 on TB2. Since the curtain wall panels are fastened
to the mullions, limiting the mullion joint movement
will limit the corresponding curtain wall joint
movement. Since the design allows the control of
the curtain wall joint movement to become independent
of the floor deflection, the sealing integrity of
the curtain wall system can be assured regardless
of any large upward or downward floor displacements
in a seismic event.
CONCLUSION
The recent advance in building design technology
against seismic events calls for a more flexible
building frame structure. As a result, serious structural
safety and functional-damage liability issues for
the curtain wall design become a very difficult
challenge in the conventional design approach. It
becomes apparent that new thinking is needed. In
response to this need, this paper discusses the
new logic, often in the opposite direction of the
conventional design approaches, and provides a set
of solutions in design. It is hoped that this will
generate more discussions leading to better solutions.
List of References:
1. "SOLUTIONS TO CURTAIN WALL PROBLEMS USING AIRLOOP
SYSTEM" by Raymond Ting, Proceedings Vol. 2, Page
257, International Conference on Building Envelope
Systems and Technologies, 2001.
*To be presented at ASCE 2004 Structures
Congress, Nashville, TN |
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| Fig.1 Mullion
Connection Details |
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Fig.2 Floor
Deflection, Df vs. Curtain Wall Joint Movements,
Dm1 or Dm2 |
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