1. Identification of basic CW components.
2. Identification of CW performance functions.
3. Identification of causes affecting CW performance functions.
4. Identification of effects due to causes.
5. Functional Isolation Concept (FIC).
6. Application of Functional Isolation Concept.

Identification of Basic Curtain Wall Components
The basic CW components include the following items.

1. A CW support system (CWSS).
2. A support connection system (SCS) securing CWSS to the edge of building floor.
3. A facing panel system (FPS) consisting of one or more facing materials.
4. A panel connection system (PCS) securing FPS to CWSS.
5. A weather sealing system including the following options.

a. Single sealing system (SSS) sealing all FPS joints.
b. Double sealing system (DSS) utilizing the following sealing components.
(1) A joint weather sealing system (JWSS) between FPS and CWSS.
(2) A panel weather sealing system (PWSS) around and between panel
edges of FPS.
(3) A water drainage system (WDS) for draining partially infiltrated water
between JWSS and PWSS.

Identification of Curtain Wall Performance Functions

1. Structural Integrity

a. PF-1: Safety against stress fatigue in annual loading cycles in combination with dead load.
b. PF-2: Safety against extreme loads in combination with dead load (e.g. 50- yr recurrence interval wind and seismic loads).

2. Sealing Integrity

a. PF-3: Air and water-tightness after various annual load cycles.
b. PF-4: Air and water-tightness after spandrel beam deflection cycles (including dead load strain creep and live floor load deflection) in building service condition.
c. PF-5: Water-tightness at the maximum design inter-floor deflection.
d. PF-6: Water-tightness in extreme rainstorm (e.g. 50-yr recurrence interval).
e. PF-7: Air and water-tightness after extreme seismic event (e.g. 50-yr recurrence interval).

3. Thermal Insulation Integrity
For a given thermal insulation design, the aging effect on the insulation material normally has a minor impact on the thermal insulation integrity. The major impact on the thermal insulation integrity is normally due to excessive air leakage and/or water leakage (wetting of insulation material). Therefore, “thermal insulation integrity” can be considered to go hand-in-hand with “sealing integrity”.

4. Performance Functions excluded from the scope of this paper: aesthetic durability and sound insulation.

Identification of Causes Affecting Curtain Wall Performance Functions

Except the aging effect on material, the causes affecting curtain wall functions can be categorized into the following two groups.

1. Direct Causes include the following items.

C-1: Wind Load.
C-2: Seismic Load.
C-3: Thermal Load.
C-4: Dead Load.

2. Indirect Causes include the following items.

C-5: Spandrel Beam Deflection.
C-6: Story Drift.
C-7: Differential Thermal Load between Curtain Wall and Building Frame.

Identification of Effects due to Causes
In the following list of effects, the preceding * indicates that the effect is significantly reduced or almost eliminated by the application of FIC presented in the last section of this paper.

1. Effects due to Direct Causes

E-1: Bending/shear stresses and deflections on facing pane, sub-frame, and
support frame caused by wind load (C-1).
*E-2: Edge compressive or tensile stresses on facing pane, sub-frame, and support
frame due to thermal load (C-3).
E-3: Edge compressive or tensile stresses on facing pane due to dead load (C-4).
*E-4: Edge compressive or tensile stresses on facing pane due to seismic load
(C-2).
*E-5: Loosening of fasteners due to repeated cycles of tensile or shear stresses on
fasteners caused by dynamic wind load (C-1).
E-6: Tensile or shear stresses on fasteners caused by dead load (C-4).
E-7: Tensile or shear stresses on fasteners caused by seismic load (C-2).
E-8: Sealant line stresses (tensile, compressive, and shear) caused by wind load
(C-1).
*E-9: Sealant line stresses (tensile, compressive, and shear) caused by dead load
(C-4).
*E-10: Sealant line stresses (tensile, compressive, and shear) caused by seismic
load (C-2).
*E-11: Sealant line stresses (tensile, compressive, and shear) between two
adjacent facing panes caused by thermal load (C-3).

2. Effects due to Indirect Causes

*E-12: Edge compressive or tensile stresses on facing pane due to spandrel beam
deflection (C-5) caused by live load on the floor.
*E-13: Edge compressive or tensile stresses on facing pane due to spandrel beam
deflection (C-5) caused by column shortening due to strain creep in column
material (e.g. concrete).
*E-14: Edge compressive or tensile stresses on facing pane due to spandrel beam
deflection (C-5) caused by strain creep in beam material (e.g. composite beam)
due to dead load and/or long term live load.
*E-15: Edge compressive or tensile stresses on facing pane due to differential
thermal expansion or contraction (C-7) between curtain wall and building frames.
*E-16: Diagonal compression and tension forces on facing pane due to inter-floor
story drift (C-6) caused by wind load or seismic load.
*E-17: Sealant line stresses due to spandrel beam deflection (C-5) caused by live
load on the floor.
*E-18: Sealant line stresses due to spandrel beam deflection (C-5) caused by
column shortening due to strain creep in column material (e.g. concrete).
*E-19: Sealant line stresses due to story drift (C-6) caused by wind or seismic
load.
*E-20: Sealant line stresses due to spandrel beam deflection (C-5) caused by
strain creep in beam material (e.g. composite beam) due to floor dead load and/or
long term floor live load.
*E-21: Sealant line stresses due to differential thermal expansion or contraction
(C-7) between curtain wall and building frames.
*E-22: Compressive or tensile stresses on sub-frame and support frame due to
spandrel beam deflection (C-5) caused by live load on the floor.
*E-23: Compressive or tensile stresses on sub-frame and support frame due to
spandrel beam deflection (C-5) caused by column shortening due to strain creep
in column material (e.g. concrete).
*E-24: Compressive or tensile stresses on sub-frame and support frame due to
spandrel beam deflection (C-5) caused by strain creep in beam material (e.g.
composite beam) due to floor dead load and/or long term floor live load.
*E-25: Compressive or tensile stresses on sub-frame and support frame due to
differential thermal expansion or contraction (C-7) between curtain wall and
building frames.
E-26: Lateral bending on sub-frame and support frame due to inter-floor story
drift (C-6) caused by wind load or seismic load.

Functional Isolation Concept (FIC)
The causes and effects on all curtain wall performance functions are summarized in the following table.

TABLE 1: List of Functions, Effects, Causes

It can be seen from the above table, one cause produces multiple effects on multiple functions. The degradation of one performance function often leads to the degradation of other performance function or functions. To obtain durable performance functions, it becomes apparent that it is desirable to isolate the function or functions of each curtain wall components (FIC). In the above table, the preceding * on the effect indicates that the effect is significantly reduced or near eliminated after the application of FIC as presented as follows. The possibilities of FIC are discussed as follows.

As shown in TABLE 1, a single cause often produces multiple effects on multiple structural components and elements between components. To reduce the intertwined effects, the concept is to design the system with either insignificant or no relative movement between two components or stress-free movement between two components. This concept leads to the following FICs.

FIC-1: Each facing pane must be unitized with perimeter supporting panel frame and weather seals in between. (i.e. insignificant or no relative movement between facing pane and panel frame).
FIC-2: The panel frame must be separated from CWSS to allow the possibility of stress-free relative lateral movement in between to eliminate E-16 and E-19.
FIC-3: Open panel joints must be provided to allow the possibility of stress-free relative movement between two adjacent panels to eliminate E-2, E-3, E-4, E-9, E-10, E-11, E-15, E-21, and E-25. Past experiences indicated that a major cause of degradation of sealing integrity could be attributed to E-5. Therefore, it is desirable to eliminate E-5 using the following concept.
FIC-4: Use mechanical engagement between panel frame and supporting mullion for transferring wind load into the mullion. Also, use mechanical engagement for connecting the horizontal panel frame to the vertical panel frame. The panel fasteners are used for supporting the panel weight only, therefore, E-5 can be eliminated.

The performance functions PF-3 to PF-7 (air and/or water tightness) deals with the interior environmental condition of daily concern in addition to the 50-year recurrence interval rainstorm condition. The impact of air leakage is the loss of energy which is a daily operational concern and is often more tolerable than the water leakage problem due to the fact that air leakage normally constitutes a small percentage of the total energy loss of the building. The water leakage is an occasional problem (i.e. windy rainstorm condition), however, it has very low tolerance level. Therefore, the durability of water-tightness performance is vitally important to the building owner. It is well known that a critical seal is defined as a seal being used to seal off both air and water or water with a water head (e.g. bottom of a gutter). To prevent water leakage, the critical seal must be perfect. However, adequate durability of the critical seal is highly
questionable due to the effect of various structural movements of curtain wall components and the aging effect on the sealant material. Therefore, the following FICs are formed to secure durable PF-3 to PF-7.

FIC-5: To eliminate the critical seals, isolate the air sealing function from the water sealing function by using two independent types of sealant lines, namely, water seal lines with pressure equalized joint air space behind it and air seal lines with dry pressure equalized joint air space in front of it.
FIC-6: Isolate air entry from water entry into the pressure equalized joint air space using a rain screen member with hidden air entry gap or holes.
FIC-7: In the process of air entry, some incidental water can be flown over the rain screen member, therefore, it is necessary to isolate the front section of the pressure equalized joint cavity to create a gutter space behind the rain screen member. To eliminate the water head problem, use a gutter connected to a vertical groove without gutter end dams for instantaneous drainage.
FIC-8: Isolate the water drainage area from a dry section in the pressure equalized joint cavity by providing a water seal line behind the water drainage area and in front of the air seal line.

The curtain wall is supported on the edge of each floor slab or spandrel beam. The spandrel beam deflection (same as floor slab edge deflection) will cause the curtain wall joints to move accordingly. In the modern building design, the spandrel beam is commonly designed for a maximum live load deflection of ¾”. It is difficult to maintain the sealing properties if the curtain wall joints are subjected to repeated joint movements of ¾” (problem with PF-4). In addition, rainstorm can happen at the maximum spandrel beam deflection of ¾” and maintaining the water tightness at ¾” curtain wall joint movement is extremely difficult (problem with PF-5). To solve this water-tightness problem for building in-service condition and durable performance, the following FIC is required.

FIC-9: Isolate the maximum curtain wall joint movement from the maximum spandrel beam deflection such that the maximum curtain wall joint movement is within the tolerable range for maintaining PF-4 and PF-5. In order to significantly reduce the effects of C-6, the following two FICs are possible.
FIC-10: Isolate the lateral floor displacement from the support connection system (SCS) by providing laterally sliding joint between the connecting part on the slab and the connecting part on the mullion such that inter-floor story drift will not cause the mullion to tilt to one side. This FIC has been used in Point-Supported Glass system and is generally considered to be expensive.
FIC-11: Use fixed SCS and in conjunction with FIC-2, isolate the tilting movement of mullion from the movement of the panel frame such that inter-floor story drift can be absorbed by inter-panel drifts with little or no distortion to the
panel frames. In order to maintain the sealing integrity in the application of this FIC, the following four FICs are required.
FIC-11A: The top panel frame must be fastened to the mullion to support the dead weight and the bottom panel joint must be a laterally sliding joint.
FIC-11B: A lateral free gap must be provided between the panel jamb frame and the mullion to allow stress-free panel drifting.
FIC-11C: At the maximum panel drift position, the air seal line between the panel frame and the mullion must be maintained by providing adequate contacting surface in between.
FIC-11D: Non-tearing type of sealant material (e.g. one-sided adhesive foam tape) must be used in the panel joints and in the joint between the panel jamb frame and the mullion to allow relative component movement without sealant line distortion or stress.

Application of Functional Isolation Concept
A curtain wall system known as Airloop System1 is the result of the application of the FICs presented in this paper. With reference to the attached figures, the applications of FICs are summarized in TABLE 2 below.

TABLE 2: Applications of FICs

It can be seen from TABLE 2 that the FICs discussed in this paper have been incorporated into the Airloop System resulting in drastically reduced number of affecting parameters in TABLE 1 (i.e. eliminating those preceded with *). The durability of curtain wall functions is greatly enhanced.

Final Notes
1. The most important issue is the durability of curtain wall performance functions. However, the current curtain wall test methods are inadequate to evaluate the performance durability issue 2,3. Research efforts are needed to develop new practical and economically feasible test method.

2. All system designs need the proper executions of quality control procedures in fabrication and erection to achieve the intended design functions. However, the effectiveness of the quality control is often a function of the design feature. For example, it is very difficult to execute an effective quality control on a system requiring critical seals since the degree of perfection on a seal can’t be judged by eye inspection. On the other hand, it is very easy to execute an effective quality control by eye inspection on a system without critical seal.

3. For water leakage problem, any system with quality control errors would likely to leak in the first annual weather cycle regardless of how good the design is. When the method of rating curtain wall systems has been developed based on the durability issue, the following logical insurance policy is recommended.

(1) As in usual practice, the insurance premium is a function of the rating.
(2) High Self-Insured Retention on the first year for all systems to encourage proper execution of quality control.

4. Once curtain walls can be rated based on the durability of performance functions, the lifetime cost including maintenance cost can be confidently evaluated in the system selection process. This will be a huge direct benefit to the building owner.

List of References
1. Ting, R., “Solutions to Curtain Wall Problems Using Airloop System”, 2001 International Conference on Building Envelope Systems and Technologies, Ottawa, Canada. Proceedings Vol. 2, Page 257.
2. Ting, R., “Rating Curtain Walls on Performance”, The Construction Specifier, March, 2005.
3. Ting, R., “Rethinking Curtain Wall Ratings”, Construction Canada, September, 2005.

List of Terminologies
CW = Curtain Wall.
CWSS = Curtain Wall Support System.
DSS = Double Sealing System.
FIC = Functional Isolation Concept.
FPS = Facing Panel System.
JWSS = Joint Weather Sealing System.
PCS = Panel Connection System.
PWSS = Panel Weather Sealing System.
SCS = Support Connection System.
SSS = Single Sealing System.
WDS = Water Drainage System.