Window Wall System and Precast Concrete Panel System; Two Bodies Side by Side Have a Story to Tell About their Connection
Precast Concrete Panel System: (PCPS)
Precast cladding or curtain wall is the most common use of precast concrete for building envelopes. Precast concrete wall systems are available in a wide variety of shapes, colors, textures, and finishes. Typically, each precast panel is independently supported to the buildings main structure using an assemblage of metal components and anchors.
Window Wall System: (WWS)
Similar to PCPS, the window wall system is attached to the main concrete structure and in some cases to the adjacent building elements such as PCP, concrete block walls, metal columns, structural studs and others. Ideally, these building cladding systems (PCPS & WWS) are most often constructed as a curtain wall or veneer in which no building loads are supposed to be transferred to any of the systems. Any uncalculated or additional imposed loads on either systems may compromise both & may lead to undesirable results.
Unfortunately, this is not the case for some buildings where site observations/investigations confirmed violation of the above rule. Nothing is more disruptive or possibly fatal, than a building failure. Building failures are usually the result of poor construction & coordination, which occurs when there is a lack of quality control during the construction phase. Details are either missing or overlooked, and the actual site conditions, tolerance for fabrication and erections are ignored or omitted. These factors can contribute to cracks, breakages and leakages resulting in building failures.
In some cases, the evidence shows that the window wall system gets attached to the surrounding building elements. This includes precast concrete panels, brick veneer & non structural members without being designed for such support. This would sometimes be done without acknowledgement of the precast concrete design engineer.
It is crucial for the designer to review & consider the major variables that will affect the wall assembly performance including but not limited to:
- Durability & serviceability
- Air/vapor barrier systems
- Rain penetration control/management system
- Load resistance
- Thermal expansion
Consideration should be given to all the loads on the wall system. In fact, each system must resist the lateral loads directly imparted on it independently, such as from wind and earthquakes, as well as vertical loads resulting from the self-weight (dead load) through the wall system. The design of any system should be in correlation with the wind study analysis calculated to project specific details.
Another important factor is the thermal expansion of control & expansion joints. The thermal expansion of the window wall system is different from the precast concrete panel. Metal and concrete have two different characteristics that allow them to react and expand differently at various weather conditions. The thermal expansion coefficients vary for various materials. For instance, the coefficient of linear expansion (at 20ºC) for Aluminum is 23 (10-6/ºC) whereas for concrete/steel is at 12 (10-6/ºC).
Allowance must be made for the considerable expansion and contraction of the cladding, for it will be subjected to a full range of air temperatures plus the effect of solar radiation.
Therefore, these two bodies that are resting on the building structure cannot be anchored or attached to each other unless the system is designed for such connection to allow an adequate movement. If the design does not incorporate thermal and or movement expansion joints and connection; the situation become complicated and any minor deviation from the original design may lead to potential problems.
This phenomenon could affect the performance of air/vapor barrier and may contribute to distress conditions, panel cracking, displacements and joint movement failure which may eventually lead to deterioration or water penetration into the occupied areas. This could also present serious safety concerns that may cause steel components to rust, window system to bulge, glass to crack, window anchors to fail, concrete to spall at lateral support locations or structurally-reinforced concrete to be exposed to freezing and thawing conditions.
Every manufacturer is responsible for its own product and quality; however, in some cases the lack of coordination between different manufacturers, whose products are installed side by side, may create potential problems and serious consequences.
Could Failures be Solved or Even Avoided in the First Place?
The manufacturer designs the cladding for the specified erection loads, connection details and provides for the weatherproofing, performance and durability of the cladding itself. Based on drawings review and personal site investigation, it was noted that some manufacturers such as Window, Railing and evenVent Boxes allow their products to be attached/secured to the adjacent concrete precast panel cladding without the consent of the precast panel engineer.
Under other circumstances, it was noted that window or railing shop drawings were shown to be attached to precast concrete panels without the approval or review of the precast designer.
The warranty of the precast panel may become void once the manufacturer is aware that other building components are attached to their product without their consent, consult or calculation of certain factors. This is a fair judgment, since their system was not designed to take into consideration any additional loads and since the precast concrete system is not a part of the main supporting structure.
McIntosh Perry has been involved in the investigation of building defects, problems and failures for over 40 years. The team, who has been working as consultants and litigation experts for disputed faults, asserts that these types of building failures can be solved or even avoided in the first place if the necessary technical knowledge and good judgment were used in the design/construction phase:
Consideration should be given to some procedures/techniques that may minimize building failures and contribute to better building envelope performance:
- Promoting sustainable design.
- Allowing enough time for design development.
- Analyzing various building cladding and evaluating system performance.
- Adhering to standards, building codes & good engineering practices.
- Maintaining coordination between architect, manufacturers & engineers.
- Coordinating the shop-drawing reviews:
- Designing proper connections
- Allowing for thermal expansion
- Providing flexible joint movement
- Performing mock-ups for review & approval.
- Endorsing a quality control program for site installation/reviews.
When the connection is coordinated, the story will have a happy ending.