Curtain Wall Design Considerations
Over at Construction Specifier, there’s an article, this month, on the basics of curtain walls. I’ve discussed curtain wall previously in A Few Notes on Glass Curtain Wall and Curtain Wall Installation. Here are some points from this read; which is available in digital form here.
2 sided SSG vs. 4 sided SSG
Structural silicone glazing (SSG) is the method of using silicone sealant to structurally adhere glass to curtain wall framing. This allows the façade to be free of mechanical covers at the edges of each piece of glass. The article notes that 2 side structural silicone glazing may be completed in the field, while four side silicone glazing should always be completed in the controlled conditions of a factory.
Storefronts
In modern terminology, storefronts are somewhat the opposite of curtain walls. Unlike curtain walls that hang (like a curtain) from the edge of floor slabs, storefronts rest on the floor slab directly. They are typically field fabricated and glazed.
Pressure walls
In a previous post, I mentioned the difference in ‘stick’ and ‘unitized’ curtain wall systems. Stick systems can also be called ‘pressure walls’ as extruded aluminum pressure plates are screwed on to compress the glass against the aluminum framing. A snap on cap typically conceals the pressure plate and its fasteners. These field assembled walls can be greatly affected by outdoor conditions. This increases the number of variables that affect the final product’s performance.
Unitized curtain walls
Unitized walls attempt to reduce the variables affecting quality by completing as much of the glazing process as possible under factory conditions. The units are then shipped to the jobsite and installed in a sequential manner. The units are interlocking in geometry with weather-stripping to create a thermal seal. The joints in each unit also accommodate expansion or contraction.
Window Walls
Like storefronts, these systems are supported on the floor slab directly. However unlike storefronts, these systems are typically factory glazed. The head and sill of these panels are set in receivers, called “starters.” Unlike curtain walls, window walls can be installed in various sequences.
Structural Integrity
Wind load criteria are key to any exterior wall design. In general, determining the wind loads is the job of the engineer of record, not the curtain wall manufacturer. Wind load criteria should be listed clearly in the construction documents.
For panels spanning less than 13.5ft deflection should be limited to L/175. For units that span an entire floor, the ideal splice location is 20 to 22 percent of the span, where the moment is near zero. Architects should also be aware of the maximum deflection for both vertical and horizontal members.
It’s important to understand how the interior finish, such as drywall is attached to these facades. Curtain walls allow for movement vertically as well as horizontally from unit to unit. Therefore the drywall should be attached in such a way to allow this freedom of movement.
Anchorage
The previous post, Curtain Wall Installation, addressed why curtain walls are typically anchored from the above. Anchorage can be specific to the manufacturer and global location of the project. Here are two common types.
Double-angle mullion anchors
These straddle each side of the vertical mullion and attach with a through bolt and pipe spacer. The pipe allows for movement of the mullions. This anchor type is typically attached to the face of the slab.
Three-way adjustable anchors
This type can accommodate in/out, up/down and side to side adjustment in the field. The anchor has a jack-bolt to provide fine levels of vertical adjustment during installation.
Movement
While aluminum has the benefit of being light weight and durable, it is quite thermally conductive. This thermal expansion results in large amounts of movement between the curtain wall and the building’s substrate. Anchors must be able to resist this movement.
Inter-story differential movement
Concrete creep and other settling over the life of the building can cause movement from one floor to the next. This movement should be quantified and considered in the wall’s design.
Seismic movement
Seismic movement is predominantly horizontal. The architect can specify elastic and inelastic design criteria based on a fixed percentage of the floor-to-floor height. Jamb and sill conditions should be carefully detailed to maintain continuity of seismic movement.
Thermal movement
In some locations, curtain wall framing can vary by 230°F over the course of the year. Under such conditions, a 10ft long piece of aluminum might experience 3/16” of expansion. If the piece is secured in such a way that did not provide for such movement, damage and buckling could result.
Other Criteria
Other concerns that often arise in curtain wall design are: blast mitigation, hurricane resistance, acoustic isolation, daylighting, shading and natural ventilation.
Mock ups
A project specific mock up testing can make economical sense on many curtain wall projects. Refer to the American Architectural Manufacturers Association (AAMA) 501-05, Methods of Test for more information on testing criteria.
2 Comments
Reader Comments (2)
Nice post and nice information too. I read your post. It's really nice and I like your post. It’s very simple to understand........Thank you for sharing...........
Marble Cleaning
I not such a fan of the unitized curtain wall. from what I have seen the only savings would be on the installation side of things. The way unitized curtain wall is designed to allow individual frames to be stacked on each other requires many more parts. On a typical curtain wall elevation you would just have head sill horizontals and vertical mullions. On unitized you have 10 times the amount. I also see issues with unitized curtain wall leaking.