Fundamentals of Structural Curtainwall Design

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Successful structural design relies on the harmonious integration of various elements. The primary building structure often comes to mind first when thinking of the structural integrity of a structure, but many other elements are critical to the design. The building facade is one such element, with curtainwall being a further subset.

The goal of the engineer is ultimately to protect the “health, safety, and welfare” of the public as dictated by the engineering code of ethics. While individually, one can complete a scope and achieve this goal, the greater goal of achieving this for a building is only satisfied when this is accomplished for all structural trades. It is at these crossroads where critical junctions occur, and proper coordination is essential for successful design.

Curtainwall is a type of exterior building cladding commonly unitized to facilitate rapid installation on large projects. Modular sections, known as units, are prefabricated in a factory and shipped to the site for fast installation and better quality control. The units are lifted into place and hung from the structure with perimeter anchors (Fig. 1 and 3). The units interface with one another to provide a weather-tight seal via gasket engagement and minimal field-applied sealant.

A typical unit is rectangular with a transparent vision area and an opaque spandrel area, collectively known as the infill, but limitless geometric configurations exist. The vision area permits occupants to view the exterior while the spandrel area covers the slab and plenum space. Vertical framing components, known as mullions, interface with horizontal elements, called transoms (Fig. 4). The transoms frame into the mullions and the mullions engage with the units above and below for structural continuity.

Curtainwall must resist all applicable loads as dictated by their use case, but common loads are self weight, wind pressure, loads induced from seismic acceleration, and live loads from building maintenance units (e.g. window washing platforms). The common standards governing the designs are the adopted versions of ASCE 7 (loads), Aluminum Design Manual (ADM) (aluminum), ASTM E1300 (glass), AAMA TIR-A9 (fasteners), AISC-360 (steel) and ACI 318 (concrete anchorage), in addition to any unique materials used in the facade. Curtainwall is commonly still designed according to allowable stress design (ASD). Wind pressures are frequently the type of load governing the design of curtainwall. A typical unit will resist the resultant force of the building internal pressure and the external pressure imparted from wind events. Zone 4 (interior wall) and 5 (edge/corner wall) component and cladding wind pressures from ASCE 7 Chapter 30 are of interest, which consider the localized “hot spots” on a structure during a wind event. Negative wind pressures (suction) at the corners of the building often govern cladding design as they can be nearly double those of interior wall pressures. Wind pressures are resisted by the framing infill, commonly glass or metal panel, transferred to framing members, and then to anchorage into the primary structure. See Figure 5 for an example load path and free body diagram. For complex buildings, a wind tunnel study is frequently commissioned since the often lower-than-code-prescribed pressures can result in net savings.

The engineer of record will naturally have to make assumptions about the weight of the facade and loading imparted to the primary structure prior to the engagement of the facade contractor. For critical interfaces or incipient design where conservative assumptions are warranted, one should take due care for the determination of loads. However, the following may be considered as general guidance: metal panel cladding typically does not exceed 10 psf; typical glass units, 15 psf; or extra thick glass, 30 psf. Facades with stone or other cementitious products can greatly exceed this value. One can determine an approximate facade weight by multiplying the thickness of the predominant infill material by the appropriate specific gravity. For the case of an all-glass facade unit, most of the weight is the glass, so an approximate determination can be carried out if the thickness of glass is known.

Unitized curtainwall designers often leverage the interface of the units by using the unit above and below for stability and load transfer, thus requiring two anchorage points to the structure per unit. The mullions are the primary wind load resisting members since the horizontals frame into them. If horizontal members (transoms) are beams, then mullions are girders. The mullion can be conservatively analyzed as a simple beam loaded based on its tributary width with determinate boundary reactions. Mullions of unitized curtainwall mate together as the units are installed and will share tributary out of plane load based on relative stiffness since they are constrained to deflect together. In addition to aluminum stress and buckling checks, mullions must be designed for deflection, which is commonly L/175 for spans less than 13 feet-6 inches and L/240 + ¼ inches for greater spans, where L is the clear span between supports. Framing members supporting glass must be limited to L/175 along the length of the glass infill for the edge to be considered firmly supported, which dictates which edges may be considered supported for glass analysis.

The wind load imparted to the primary structure at discrete anchorage points can accurately be determined by statics of an individual unit or approximated by the tributary wind of the adjacent unit on either side of the anchor multiplied by the tributary height. Reactions can exceed this simplification at the lower floor of a curtainwall run, the upper floors of a run, and at parapet conditions, and should be considered on a case by case basis accordingly.

Unitized curtainwall is commonly hung from the perimeter of the slab. The self weight of the unit is imparted at the slab edge, inducing eccentricity in the primary structure. A common note in structural drawings and specifications is that the facade shall not impart any torsion to the perimeter of the structure, which is not feasible. To be more accurate and avoid inherently impractical requirements, construction documents and specifications should state that “the facade engineer of record shall submit a diagrammatic representation of the loads imparted to the primary structure and associated eccentricities,” for proper consideration in the design of the structure. For unique or critical load coordination locations, the engineer of record should require that a loads-imposed coordination document be sealed by the facade engineer of record.

Coordinating loads imposed is almost always challenging due to the varying manner in which loads can be conveyed and engineers’ attention to detail on the providing and receiving end. The facade is most often designed based on allowable stress design, and the structure is likely designed according to Load Resistance and Factor Design (LRFD). A loads-imposed document could indicate factored ASD loads, unfactored loads, factored LRFD, and so on. The document must convey the loading category, factors to the loads, how load combinations are considered, and eccentricities. The facade engineer must discern how to concisely provide reactions at facade anchorage points in a document with sufficient accuracy. The document must encompass the varying conditions while striving for brevity to avoid providing loads for each individual anchorage condition. Additionally, he or she must also discern what buffer to incorporate into the loading to account for conservatism and the potential for the design to change that could alter the loading. The author encourages engineers of record to tell the facade engineer how to communicate loads, since they are the ultimate recipients of such information.

The facade engineer should provide curtainwall shop drawings and/or calculations that clearly convey the loads imposed. Elevation drawings should have graphics of wind load and dead load anchors of the facade, and calculations and/or detail drawings should convey facade anchor reactions to clarify how load is being imparted to the structure. The coordination of loads at the interface of engineering trades is one that requires careful consideration and communication to avoid additional provisions for facade reactions that are higher than anticipated.

The final consideration for this article is the coordination of building and facade movements. The facade will move, and the building will move. Their movements must be compatible in order to avoid a clash. Any such clash will almost certainly result in a failure of the facade in the form of glass breakage, facade damage, and potentially dislodgement of the facade from the building. The impetus is on the facade engineer to facilitate the proper design of the facade for movement consideration; however, the engineer of record must also provide values of the primary structure movement for proper coordination. While the facade engineer must take due care in providing loads with adequate specificity and conservatism, so too must the engineer of record when providing building movements. Common structure movement considerations are structure creep, slab edge deflection prior to curtainwall installation, slab edge deflection due to curtainwall self weight, slab edge live load deflection after curtainwall installation, structure settlement, construction tolerances, and service/ultimate level drifts for wind and seismic building movements. Additional unique movements should be provided as needed.

The facade is effectively always set in a theoretical position, regardless of the position of the primary structure. This is required to ensure the aesthetic of the facade. Therefore, unitized curtainwall units must accommodate movements in several ways. Movements prior to the installation of the facade can be accommodated via 3-way adjustability of the facade anchorage. Thus, the magnitude of these movements needs to be understood for provisions in the anchorage design. Further, smaller erection tolerances will result in a more economical facade design and lower facade reaction eccentricities, which is worth considering when engaging various contractors.

Movements of both the facade and the structure after installation need to be accommodated in the vertical and horizontal joints of the facade. Facade movement such as thermal, fabrication tolerances, and movements induced from vertical and horizontal building movements are all combined as appropriate to determine the opening and closing demand of the facade joints. The sidebar, “Movement Considerations” on page 23 notes where the various movements need to be considered in the design of the facade and its attachment to the structure. Adjustable anchors can adjust the unit attachment point to the nominal position while the movement joint between the units, known as the stack joint, accommodates movements after installation. The deflection of the structure due to the weight of the curtainwall can be accounted for in adjustment of the anchors or at the stack joint.

Early coordination of facade gravity and lateral anchors along with anticipated facade movements is critical between the facade engineer, engineer of record, and the architect to ensure coordination, movement coordination and joint sizes. Curtainwall extrusions are often custom on elaborate projects. Any change resulting in a modification of the design curtainwall movements can have significant fees resulting from changing extrusions and undesirable aesthetics due to larger curtainwall joints.

Facade design is constantly evolving as architects push the boundary of creativity. Custom curtainwall warrants individualized considerations with the information presented as key considerations. Ensuring the resilience of the primary structure and accessory structures demands close coordination between the structural engineer of record and the facade engineer of record. Doing so will facilitate the conveyance of digestible information without undue conservatism. ■

About the Author

Aaron J. Kostrzewa, PE, has served as the facade engineer of record for multiple projects and has designed curtainwall systems around the country. He serves as the managing member at Kosco Engineering Group and enjoys teaching structural concepts. Connect with him on LinkedIn.