|
The
purposes of providing a curtain wall on a building
are to provide the aesthetic character of the building
skin and to protect the building interior from the
effects of natural phenomena including sunlight
exposure, wind, rain, earthquake, and temperature.
Various facing materials have been utilized in the
curtain wall construction such as glass, stone,
aluminum plate, aluminum composite plate, steel
plate, and composite foam panel etc.
To complete the exterior building skin, many pieces
of facing panels must be joined together and sealed
against air and water infiltration. The majority
of the curtain wall problems is related to the panel
joint design.
Due to the multiple performance functions required
for the curtain wall system design, it has been
a long struggle in the industry for trying to provide
an enduring system to fulfill all performance functions.
The curtain wall design work is further complicated
by the interactions among the various performance
functions, for example, the best solution for a
particular performance function may be detrimental
to the other performance functions.
The major curtain wall problems & their solutions
using Airloop System are listed below:
1. Problems Induced by Water Leakage:
Due to zero tolerance requirements is required
for this problem, it has been a constant struggle
in the industry to provide a system with long lasting
water tightness performance. The difficulty of providing
a long lasting watertight system is due to the difficulty
of maintaining long lasting sealant integrity at
the "Critical Seal Locations".
A "Critical Seal Location" is defined
as any location that requires a perfect seal to
prevent water leakage. The "Critical Seal Location"
is required if the location is required to simultaneously
seal both air and water. Sealant failures are caused
by sealant stress fatigue and/or sealant material
degradation. The problems induced by water leakage
include the interior water damage, the loss of insulation
value due to the wetting of the insulation material,
and the sick air building problem due to the growing
of toxic molds in the wall cavity.
Attempting to solve this problem in recent years,
methods of implementing "Rain Screen Principle"
have been widely accepted in the industry. The "Rain
Screen Principle" is an attempt to nullify
the wind force that causes the water infiltration.
However, due to the fact that the horizontal joint
cavity is blocked off at the ends by the vertical
frame or support member, the pressure equalization
mechanism would by disabled when the water running
along the surface of the curtain wall starts to
blanket over the air entry horizontal joint gap.
In addition, the member joint and fastener holes
at the horizontal/vertical juncture remain to be
"Critical Seal Locations".
Therefore, the "Rain Screen Principle"
does not completely solve the long term water leakage
problem. The long term water leakage problem is
solved by the "Airloop System" by eliminating
all "Critical Seal Locations" in the system
as explained as follows with reference to figures
1 and 2. In the ":Airloop System", all
sealant lines can be classified into two groups,
namely, 1) "Water Seal" which is located
between the exterior water path and the pressure
equalized dry outer or inner "Airloop"
and 2) "Air Seal" which is located away
from the exterior water path and along the interior
border of the pressure equalized dry outer or inner
"Airloop".
In the "Airloop System", typically each
rectangular facing panel is shop assembled and sealed
to one top "Airloop" member, two vertical
"Airloop" members, and one bottom "Airloop"
member. The "Airloop" members are miter-matched
at the corners to form a complete "Inner Airloop".
After the panels have been erected to form the curtain
wall, the cavities in the vertical panel joints
are connected with the cavities in the horizontal
panel joints forming the "Outer Airloop"
around each panel. The "Outer Airloop"
is separated into two segments.
The first segment (1st OAL on Figures 1 & 2)
is utilized for instantaneous water drainage and
air entry into the "Inner Airloop". The
second segment (2nd OAL on Figures 1 & 2) is
the dry "Outer Airloop" to protect the
panel joint and support "Air Seals". In
this arrangement, the "Water Seals" including
the miter joint seals are completely separated from
the "Air Seals" eliminating all "Critical
Seal Location" and pressure equalization mechanism
will not be disabled due to water blanketing over
the horizontal panel joint since air can freely
feed into the "Outer Airloop" from the
vertical panel joint.
Therefore, all the drawbacks of the "Rain Screen
Principle" are eliminated in the "Airloop
System" and long term water tightness performance
is ensured.
2. Problems Induced by Interfloor Story Drift
due to Wind Load or Earthquake:
In the conventional "Stick" or "Unitized"
curtain wall systems, the inside parts of the sealant
lines around the facing panel (e.g. glass, aluminum
plate, stone, etc.) are also the sealant lines on
the panel supporting mullion and girt.
In the event of inter-floor story drift, the supporting
mullion which is attached to the floor structure
would be laterally shifted with the floor forcing
the panel to change from the original rectangular
shape into a non-rectangular parallelogram as shown
on Figure 3.
The inherent high in-plan shear stiffness of the
facing panel in resisting the in-plan shear distortion
would force the sealant lines to absorb the majority
of the shear distortion. Stress fatigue on the sealant
lines due to the effect of repeated inter-floor
story drifts would cause bond shear failure of a
caulked sealant line and eventually cause the sealant
material to walk out of the sealing pocket. Falling
cured caulking strings or gaskets are normally caused
by this problem.
If the sealant failure occurs at the "Critical
Seal Location", it would cause the water leakage
problem. If there is inadequate horizontal clearance
between the adjacent facing panels to absorb the
inter-floor story drift, it would result in the
structural failure of the facing panel such as panel
buckling in metal panel or broken glass falling
from the sky.
In the "Airloop System", both the structural
and sealant stress problems are eliminated as explained
as follows: Each facing panel is supported by two
screws at the top corners and structurally engaged
at the bottom against forces perpendicular to the
panel surface with no restraint against lateral
sliding and the vertical segment of the "Outer
Airloop" provides the clearance (Dimension
C in figure 2) for lateral horizontal joint sliding.
Therefore, the total story drift can be designed
to be absorbed by stepwise stress-free panel drifts
as shown in Figure 4. The panel distortion analysis
are illustrated below.
Design Story Drift = D
(mm or in)
Story Height = L (mm or in)
Stress-free Panel Drift = B (mm or in)
Outer Airloop Clearance = C (mm or in)
Panel Height = H (mm or in)
The following relationship exist is a stress-free
panel drift condition.
B/H = D/L
Solving the above equation for B yields:
B = D H / L-------------------------------------------------------------(1)
If C > B, the "Airloop System" undergoes
stress-free panel drift. It can be seen from Equation
(1) that for a known floor height, a stress-free
panel drift design can always be accomplished by
reducing H and/or D.
If C < B, the "Airloop System" would
undergo a reduced panel distortion, R, as measured
by the following equation:
R = (B – C) / H---------------------------------------------------------(2)
For a conventional "Stick"
or "Unitized" system, C = O in Equation
(2), therefore, the panel distortion in the system
is always much higher than that in the "Airloop
System". Due to the experience of earthquake
damage, in addition to the elastic story drift requirement
for the design seismic load, the U.S.
Building Codes have changed dramatically to require
a high allowable inelastic story drift for absorbing
the earthquake energy in the building design for
the factored seismic load. For example, in 1991
& 1994 UBC (Uniform Building Code), the maximum
allowable elastic story drift is L/200 (L = floor
height) for structures having a fundamental period
of less than 0.7 second. In 1997 UBC, the additional
parameter of maximum allowable inelastic story drift
is L/40.
In response to this Code change, it seems to be
reasonable to establish the following two parameters
for the curtain wall design. (1). The curtain wall
system shall be designed for no damage or permanent
deformation under the effect of the actual elastic
story drift for design seismic load. (2). The curtain
wall system shall be designed as the ultimate load
condition under the effect of the inelastic story
drift for the factored seismic load.
In the "Airloop System", the problem of
sealant stress fatigue can be easily eliminated
by designing a stress-free panel drift condition
for the elastic story drift. Under the effect of
the inelastic story drift, the curtain wall responses
in the "Airloop System" can be explained
in the following three stages.
Stage 1: Stress-free panel drift up to the
point that the outer Airloop clearance (dimension
"C") is completely exhausted.
Stage 2: Sealant stress built-up up to the
point that the inner Airloop clearance is completely
exhausted.
Stage 3: In-plan shear stress built-up within
the facing panel until the facing panel reaches
the ultimate failure point.
For comparison purposes, the following numerical
calculations are provided using 1997 UBC with
the assumption of L/200 for the elastic story
drift and L/40 for the inelastic story drift.
| Item |
Stick
or Unitized |
Airloop
System |
| C
= 8 mm |
C=20mm*
(1) |
| L |
3600 mm |
3600 mm |
3600 mm |
| D (elas.) |
18 mm |
18 mm |
18 mm |
| D (inelas.) |
90 mm |
90 mm |
90 mm |
| H |
1500 mm |
1500 mm |
1500 mm |
| C |
0 mm |
8 mm |
20 mm |
| B |
37.5 mm |
37.5 mm |
37.5 mm |
| R (Stage 1) |
0.002 |
0 |
0 |
| R |
0.005 |
0.0028*(2) |
0 |
| R |
0.025 |
N.A. |
0.012*(3) |
* (1). Outer Airloop clearance = 8 mm;
Inner Airloop clearance = 12 mm;
Total Airloop clearance = 20 mm.
* (2). Sealant stress condition.
* (3). Panel stress.
The above example illustrates that the "Airloop
System" can be easily designed for a sealant
stress-free condition for the elastic story drift
and the panel distortion in the ultimate condition
is much less than that in the "Stick"
or "Unitized" system.
3. Problems Induced by Cyclic Wind Loads:
In a conventional curtain wall system ("Stick"
or "Unitized"), the load transferring
mechanisms for the wind loads perpendicular to the
panel surface are explained as follows.
There are three steps in load transferring mechanism
for the positive wind loads as explained as follows:
(1). The loads on the panel surface are distributed
into the horizontal and vertical supports by bearing
reactions around the panel edges.(2). The bearing
reactions on the horizontal supports are transferred
into the vertical support through fasteners at the
ends of the horizontal support. (3).
The total loads on the vertical support are then
transferred into the building floor structure through
the floor connection system. There are also three
steps in load transferring mechanism for the negative
wind loads as explained as follows.
(1). The loads on the panel surface are transferred
into the exterior panel frame members through fasteners
connecting the panel frame members to the vertical
and horizontal supports. (2). The loads on the horizontal
supports are transferred into the vertical support
through fasteners at the ends of the horizontal
support. (3). The total loads on the vertical support
are then transferred into the building floor structure
through the floor connection system.
In Step (2) for positive wind and steps (1) &
(2) for negative wind, screws are commonly used
for the connection. Under the effects of repeated
positive and negative wind load cycles, the screws
would eventually become loose to induce the water
leakage problem if the screw holds are located at
the "Critical Seal Locations".
In more severe condition, the screws could be totally
disengaged causing major structural failure of flying
curtain wall phenomenon. In the "Airloop System",
the above problems are eliminated by the system
design arrangements as explained as follows (see
Figure 5). (1).The panel is structurally captured
inside the vertical support as shown in Figure 5.
Therefore, both positive and negative wind loads
on the facing panel are directly transferred into
the vertical support without going through the panel
fastener. To erect the panel, the panel can be easily
placed into the engaged position by wiggling actions.
(2). All fastener holes are not in the "Critical
Seal Location" and in fact, they are connecting
between two pressure equalized spaces. (3). In this
manner, the fasteners are being utilized to support
the panel weight only.
4. Workmanship Problem:
In the conventional "Stick" or "unitized"
system, the panel joint formation and sealing details
varies with the facing panel material plus the variations
of horizontal and vertical joints, the joint formation
and sealing procedures involves many variations.
For example, if three facing materials (e.g. vision
glass, spandrel glass, and aluminum plate) are used
on a building, there are a total of twelve different
joint formation and sealing procedures (6 for horizontal
joint and 6 for vertical joint) for a simple flat
curtain wall. Regardless of whether the joint formation
and sealing details are executed in the field (Stick
System) or in the shop (Unitized System), unintentional
mistakes are likely to occur due to the worker's
lack of memory in so many different execution procedures.
In the "Airloop System", the above problem
is eliminated due to the following three reasons.
(1). Each type of facing material has one framing
and sealing procedures in the shop while there is
only one field joint formation and sealing procedures
for all different facing materials (universal field
joint). (2). There is no "Critical Seal Location"
in the "Airloop System". (3). The load
transferring mechanism in the "Airloop System"
relies on mechanical interlocks rather than fasteners
applied by the worker.
5. Long Term Aesthetic Performance Problem:
In a wet sealed system using exposed silicone
caulking, the problems of streaking and dirt collection
along the caulked joints have become a serious concern
in the industry. In an internal gutter system, water
stains would form below the drainage holes due to
the delayed drainage mechanism. In the "Airloop
System", this problem is eliminated since all
the panel joints are open without caulking and the
water drainage mechanism is instantaneous.
6. Problems with Exterior Fixtures:
There are many exterior fixtures desirable to
the design architect or building owner such as decorative
vertical or horizontal fins, sun screen, window
washing hocks, building sign or symbol, and special
ornament. These exterior fixtures are protruding
out of the curtain wall surface and must be structurally
supported. In the conventional "Stick"
or "Unitized" system, the structural support
for the exterior fixtures often involves penetrations
of the curtain wall and/or disruptions of the water
sealing details. Therefore, the exterior fixtures
often cause undesirable structural effects or water
leakage problem. In the "Airloop System",
the above problems are eliminated as explained as
follows. An extruded connection clip can be secured
to the "Airloop Mullion Head" anywhere
along the length of the mullion to provide an anchoring
point for any type of support system for the exterior
fixtures as show in Figure 6. In this manner, there
is no penetration of the curtain wall, no effect
on the sealing integrity of the curtain wall.
7. Problems with Curtain Wall Renovation:
The idea of a curtain wall renovation project
is to improve the performance parameters including
aesthetic, structural, thermal, air/water tightness,
or the combinations thereof. Therefore, the system
details for the renovation are normally different
from the existing curtain wall. For a conventional
"Stick" or "Unitized" system,
a renovation project normally requires a complete
stop of building interior operations due to the
following reasons. (1). The scope of work requires
the removal of the existing curtain wall panels
and supports and the installation of new supports
and panels, therefore, at the end of each field
working day, there will be a building hole and the
problem of protecting the building hole against
weather and vandalism during the non-working hours
or days becomes a major project killing factor.
(2). In most cases, the interior wall and many interior
fixtures are attached to the existing curtain wall
supports, therefore, replacing the existing curtain
wall supports with new supports would require massive
restoration of interior wall and fixtures. In most
cases, the cost becomes the prohibitive factor for
the renovation project. (3). Even if the above two
problems are overcome with money and phased renovations,
the curtain wall erection must be proceeded in a
predetermined sequential direction. This requirement
might become impossible for keeping continuous interior
operations.
In the "Airloop System", the above
problems are eliminated as explained as follows.
(1). Uniquely designed adapting extrusions for
any specific existing mullion can be secured to
the sides of the existing mullion to convert the
existing mullion into the "Airloop Mullion",
therefore, there is no need for replacing the
existing mullions and the problems of building
hole and massive interior restoration are eliminated.
(2). The "Airloop Panels" are placed
into position by wiggling action between two mullions,
therefore, the erection is non-directional and
the renovation can be executed in any non-sequential
manners.
A major curtain wall renovation project for the
IBM plant in Rochester, MN with an estimated 800,000
square foot of curtain wall has been awarded using
"Airloop System" and tentatively scheduled
to be phased in 10 years with the first phase
to be completed in October 2000. The cross-section
of the renovated mullion is shown in Figure 7.
Aluma Tech / Pacific of Tualatin, Oregon is the
"Airloop Panel" supplier for the project
and Ford-Metro Glass of Rochester, MN is the erector
for the project.
8. Problems caused by Vertical Structural Movements:
The vertical structural movements include thermal
expansion or contraction of the curtain wall components
and the differential inter-floor deflection (DID).
In most cases, the DID is a major design consideration
for the curtain wall system. Similar to other wall
systems, the "Airloop System" can be used
with a dead-loaded mullion with slotted mullion
connection clip to absorb the DID (normally in low-rise)
or a floor hanging mullion system (normally in high-rise)
with flexible panel joint and mullion spliced joint
to absorb the DID.
To prevent the failure of the sealing properties
(air and/or water at the mullion spliced joint,
it is desirable to limit the DID as much as possible.
A maximum spandrel beam deflection of + 3/8"
is found to be practical for most curtain wall systems
including the "Airloop System".
In case of thermal movements, the "Airloop
System" has a unique advantageous behavior
as explained a follows: (1). The engaged horizontal
panel joint has a gap which is more than adequate
to allow unrestrained vertical thermal movements
of the panel. (2). The "Airloop Panel"
is fastened to the mullion near the interior face
which is subjected to only minor temperature changes
while the outer face of the panel vertical frame
can undergo near unrestrained rotation and horizontal
movements within the "Outer Airloop" space
to absorb the horizontal thermal movements of the
facing panel.
Therefore, the common problem of face wrinkling
of long metal panel in the conventional "Stick"
or "Unitized" system is eliminated in
the "Airloop System".
The Phase I of a project named Omega Corporate Center
(about 80,000 square foot of curtain wall) near
Pittsburgh, PA has been completed since October
1999 with "Airloop Panels" of exposed
frame vision glass, exposed frame spandrel glass,
and hidden frame aluminum plate system. Many long
aluminum plates (max. 3' 8" by 26') are utilized
and excellent surface flatness is maintained in
cold or hot under the sun (see Photos 1 to 3).
The Phase II project is now under construction.
The project owner is Kossman Development Company,
Pittsburgh, PA and the panel supplier & erector
is EPI Wall Systems, Pittsburgh, PA.
9. Maintenance Problems:
In the conventional "Stick" or "Unitized"
system, replacing facing material such as glass
or plate often require to work form both outdoor
and indoor and very time consuming and replacing
panel frame members damaged by air born missiles
is even more difficult.
In the "Airloop System", replacing damaged
glass can be done easily from the indoor for a vision
glass and from the outdoor for a spandrel glass
and each individual panel can be replaced with minimum
effort by dropping two panels below to provide room
for the replacement work.
In the case of the March 2000 massive tornado damages
to the curtain walls in downtown Fortworth, TX,
should the curtain wall be the "Airloop System",
the required time to repair the curtain wall is
estimated to be less than 50% of the time required
for the conventional "Stick" or "Unitized"
system.
10. Problem of Interior Water Condensation:
When the interior surfaces temperature of a
curtain wall component is below the dew point of
the interior air environment, water condensation
will form on that interior curtain wall surface.
Similar to other curtain wall systems, to improve
the condensation resistance factor (CRF), various
thermal break methods can be utilized in the panel
frames of the "Airloop System" as shown
in Figure 8.
Due to the fact that the ratio of the interior mass
to the exterior mass of the "Airloop Mullion"
as shown in Figure 8 is much larger than that of
the panel frame, the CRF of the "Airloop Mullion"
is expected to be higher than that of the "Airloop
Panel Frame" without thermal break, therefore,
it is preferred not to use thermal break on the
mullion if its CRF is adequate since the use of
thermal break would reduce the structural strength
of the mullion. However, if it is necessary, thermal
break can be utilized in the "Airloop Mullion"
as shown in Figure 9.
It is believed that the unusually high volume of
air trapped in the joints of the "Airloop system"
would provide additional insulation value to increase
the CRF value as compared to the conventional "Stick"
or "Unitized" system. This has been indicated
by the result of the CRF test on the "Airloop
System" conducted at Architectural 測試成果
Inc., York, PA as shown below. Test Temperatures:
Indoor = 67.9F; Outdoor = 18F CRF Values: Frame
= 58; Glass (1" insulated) = 45.
11. Problem with Long Term Air Tightness Performance.
The degradation of the air tightness performance
is due to the loosening or bond failure of the air
seals under the effect of various cyclic structural
movements (most critical in the story drift movement)
within the curtain wall system. This problem is
typical in the conventional "Stick" or
"Unitized" system. In the "Airloop
System", this problem is either eliminated
or minimized due to the following two reasons: (1).
As explained in Item 2 above, the story drift movement
does not produce sealant stress in the "Airloop
Panel", therefore, loosening of the panel air
seal is unlikely. (2). Compressive type of dry air
seal is used for the support air seal to allow free
sliding between the panel frame and the support,
therefore, the problem of sealant bond failure is
eliminated.
The extraordinary water tightness performance
of the "Airloop System" has been repeatedly
demonstrated in two very unusual mock-up tests
conducted at Farabaugh Engineering & 測試成果,
Inc., Turtle Creek, PA as summarized below.
(1). Kossman Mock-up: two story high; mixed horizontal
and vertical support systems; three types of panels
(exposed frame vision glass, exposed frame spandrel
glass and hidden frame aluminum plate); mixed
ASTM E-283 air test, ASTM E-331 water test, ASTM
E-330 structural test, and building side sway
test.
a. Over more than a one year period, the
same mock-up has been tested ten time with the
mixed tests and the very unusual water test procedures.
The mock-up has not been repaired or modifies.
The result was no water leakage each time.
b. Test duration up to 45 minutes as compared
to the standard 15 minutes.
c. Test pressure up to 20 psf compared
to the AAMA recommended maximum of 12 psf.
d. During the water test, a marriage seal
was intentionally destroyed to demonstrate the
tolerance of imperfect seal without causing water
leakage.
The high degree of long term air tightness performance
has also been demonstrated in the test results
of this mock-up. The air leakage rate at 12 psf
pressure varied within a narrow range with a mean
of about 0.01 cfm per square foot of wall (which
is much better than the AAMA recommended maximum
air leakage rate of 0.06 cfm per square foot of
wall) among the ten test indicating no performance
degradation under repeated structural test, over
more than a one year period.
(2). IBM Mock-up: two story high; vertical support
system with existing mullions and renovation details;
two types of panels (exposed frame vision glass
& exposed frame aluminum composite plate with
thermally broken frame); mixed ASTM E-283 air
test, ASTM E-331 water test and ASTM E-? water
test.
a. Over more than three a month period,
the same mock-up has been tested three time with
the mixed tests and the very unusual water test
procedures. The mock-up has not been repaired
or modified. The result was no water leakage each
time.
b. In order to demonstrate the extraordinary
high degree of imperfect seal tolerance, severe
air leakage rate (0.264 cfm per square foot of
wall area at 5.2 psf) was intentionally introduced
in the erection of the mock-up.
c. Test duration up to 30 min. as compared
to the standard 15 min.
d. Test pressure up to 15 psf as compared
to the AAMA recommended maximum of 12 psf.
In order to demonstrate the structural strength
of the "Airloop System" against the
negative wind load, a separate structural mock-up
test using the system for the IBM renovation project
was conducted. The mock-up used a vision glass
panel of 4 foot by 4 foot and an Alpolic panel
of 4 foot wide by 7 foot high. An ultimate negative
load of 150 psf was recorded with the failure
mode of Alpolic panel being pulled out from the
gazing pocket along one of the 7 foot long side.
A fastener or "Airloop" frame distress
was observed. This proved out the mechanism of
direct load transfer into the mullion with no
connection stress.
In summary, the "Airloop system" offers
an unprecedented degree of imperfect seal tolerance
without causing the water leakage problem ensuring
a long term water tightness performance. In addition,
the improvements in the various structural performance
parameter are also very significant. It is believed
that there exists a limit between the water tightness
performance and the degree of imperfect seal.
However, this limit in the future research by
increasing the water test pressure and/or the
air leakage rate in the system.
Even though the "Airloop System" seems
to have provided solutions to most of the curtain
wall problems, nothing is perfect in this world.
It is hoped that this paper would generate more
discussions in the industry to further the "Airloop"
technology. For reference to the inception of
the "Airloop" technology, the following
papers published by this writer would be helpful.
1. "Evolution of Curtain Wall Design Against
Water Infiltration" Metal Architecture, January
& February 1997.
2 "Evolution of Curtain Wall Design Against
Water Infiltration", Proceedings, International
Conference on Building Envelope Systems &
Technology, Bath, U.K., April 1997.
3. "Curtainwall Design Evolution" Glass
Magazine, April 1998.
|
|
