The success of any façade is measured by many factors, but by far the most overlooked is thermal movements. Thermal movement in architecture is the expansion and contraction of materials at an atomic level, based on temperature change. In short, all metal surfaces expand when the temperature rises. When the temperature dips, metal surfaces contract. The concept is very simple but in practice it can be difficult to accommodate. Here are six top issues to consider:
- Everything moves. Everything around us is in a constant state of movement due to temperature changes. Thermal movement is one of several important aspects to understand when creating any architectural system. When designing a metal façade or roof, thermal movement ranks up there with joinery methods, constructability, water shedding, and the behavior of light on the surface. These are some of the primary constraints for designing any large architectural system. (See Table1)
TABLE 1
Using this chart, you can easily determine how various metal systems will perform and what kind of allowance for the anticipated Delta T (temperature differentiation) your design should employ.
For instance, a length of metal, 120 inches (roughly 3 meters), when installed outdoors can experience a temperature differential of as much as 100 F (38 C). When this occurs, the metal will increase in length the amount indicated for that metal. If the metal is to be subjected to a higher temperature range, then you must allow for additional expansion. Metal surface can easily exceed the ambient temperature as the metal will experience solar gain, particularly so on a hot and sunny summer day. For instance, the flat lap seam detail below (See Figure A.) should be designed to accommodate the thermal movements.
The fixed end of the panel will remain fixed, and all the movements will occur at the roller end. A section through the panel is analogous to a bridge beam: One end is pinned (fixed) and the other end on a roller (free) to allow the beam to move without buckling.
- Noise. Snapping and popping sounds, and sudden loud bangs emanating from your roof or façade are often associated with the intermittent or sudden release of metal surfaces that have built up stress due to improperly designed movement joints. This is caused by the surfaces having too much friction between them to allow for free movement.
At a flat lap seam, make sure the joints are loose enough so that the metal can move without touching each other. You should also use two fasteners at each clip to prevent rotation of the clip which could cause friction. At structural wind load connections of a mullion or girt, the connections should allow for the surfaces to slide against each other without hinderance. The use of slip pads are generally employed to reduce friction. If your anchors are aluminum, you can also use anodized surfaces to reduce the friction. (See Figure B.)
- Three-to-one rule. Return leg movement joints are often very visible and are essential to a properly functioning rainscreen plate panel façade. A good rule of thumb is to design joints to accommodate at least three times the anticipated movement plus the fabrication and erection tolerances. This will help mask the differences in joint widths when the wall is experiencing thermal loads. (See Figure C.)
- 4. Fixity to allow for movements. In standard façade designs, the diamond pattern is inherently strong. By using interlocking plates of the surrounding panels, the load is taken out at the seam. Correctly installed, diamond pattern systems have shown centuries of performance due to the inherent strength and the reduction of stress maintained by the overlapping pattern.
Dimensional changes of the panel elements caused by thermal effects are handled efficiently with the diamond pattern. The top edges of each panel are fixed, usually with clips; and the bottom edges interlock into the single-lock seam along the top edge of the row of panels below. Expansion and contraction are away and toward the clips, effectively sliding over the single lock of the panel element below. (See Figure D and Figure E.)
5. All buildings are defined by edges. Buildings are recognized by their silhouettes and edges. As surfaces undergo thermal movements, the alignment of these edges can be drastically affected and cause misalignment. Stair stepping of the panel edges can occur and will disrupt the “smooth and continuous” edge profile.
To alleviate this issue, make the edges the fixed point with expansion joints away from the edges to a less visible area. Or create a starter panel that defines and fixes the edges in a unique way. The Peter B. Lewis Building at the Weatherhead School of Management at Case Western Reserve University in Cleveland is a great example of this technique to ensure the edges are aligned. Each edge is fixed and is comprised of a “starter strip” used to fix the edge and provide as template to properly set the pattern on the dual curved surface. (See Figure F.)
- Oil canning. Oil canning is a metaphorical term used to describe the characteristic of flat metal surfaces to show variations in relative reflectivity. These variations are caused by very slight undulations r waves of the metal surface. A surface of metal intended as a flat plain or constant curvature can possess regions of relative variations to exist or appear.
These variations develop from differential stresses across the thin sheet of metal. These waves are often impossible to feel or measure. It’s a visual phenomenon that makes metal panels look wavy or somewhat distorted, especially in the broad, flat areas of a metal roof or wall system. (See Figure G.)
Many factors can contribute to this phenomenon, starting with the initial casting down to the installation techniques and there is a method to control them. However, these internal stresses can be increased if the metal is not allowed to expand and contract.
Even with a properly designed joint, the surface may experience oil canning until the entire surface has gone through several thermal cycles. The panels will then find their neutral state and the waviness will dissipate over time.
The Bank of Oklahoma Center’s façade is clad with over 33,000, 22-gauge Angel Hair finished stainless steel flat seam panels. The joints were sized to accommodate the thermal movements and were installed over a curved and continuous ZEPPS panel covered with a waterproof membrane. The erection duration was over nine months and thus the panels were erected at different temperatures which induced internal stresses. Oil canning was visible in some areas. It took about a year for most of the oil canning to dissipate. Sometimes you just need to be patient. (See Figures H and I.)
Thermal movement is one of the big considerations in developing a metal panel system which lays flat and doesn’t buckle or oil can. Other considerations include metal thickness (and thus its tendency to bend or bow), reflectivity (highly reflective surfaces reveal more discrepancies in flatness), and wind forces.
Anthony Birchler, PE, is vice president of sales and engineering at A. Zahner Co., Kansas City, Mo. To learn more, visit www.azahner.com. Photos courtesy A. Zahner Co.