Structure and Facade Coordination

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The concrete superstructure has topped out on your latest project, but there’s a problem. The window wall facade can’t be installed because the panels don’t fit between the slabs. The contractor blames your design, saying the slab edges are deflecting more than the panels can accommodate. You’re the structural engineer—you didn’t design the facade. Could this really be your fault?

Facade systems are among the most complex building components, in part because they require close coordination among several parties. When facade problems occur, structural engineers can be drawn into disputes. In most facade litigation cases, the structural engineer is not the prime defendant. Design errors are generally attributable to the architect and facade designer (often under a contractor’s delegated design). Construction defects are generally attributable to the contractors. However, the structural engineer has an important role to play in achieving a successful facade design and installation. When those responsibilities aren’t met or when a structural engineer’s decisions directly contribute to facade installation and performance issues, the engineer can be sued. For cases involving construction delays or modifications to the facade or structure, damage claims can exceed the engineer’s design fees by orders of magnitude.

Most facade litigation claims stem from poor coordination between the design professionals and contractors. Structural engineers should understand their coordination responsibilities to minimize their facade claims risk. Standard practice is not directly defined in building codes, but guidance can be found in industry standards, such as:

Broadly speaking, the structural engineer is responsible for:

In the structural engineer’s contract, scope related to facade coordination should be clearly defined, along with explicit callouts for excluded tasks such as review of facade submittals beyond imposed loads and connection tolerance capability.

Common Errors

Facade litigation claims against design professionals have a few recurring themes in terms of coordination, design, and installation.

This straightforward design error occurs when slab edge or spandrel beam details designed by the structural engineer are not sufficiently stiff to limit deflection or not sufficiently strong to provide adequate support for the facade. Inadequate stiffness is far more common than insufficient strength.

Frequently, deflections are an issue at unreinforced masonry facades, where the spandrel beams and lintels must typically be designed for a dead + live load deflection of L/600 (TMS 402, Building Code Requirements for Masonry Structures, Section 5.2.1.4.1)—much more restrictive than the deflection requirement for typical steel framing members. Deflection requirements for spandrels and lintels should always be confirmed with the architect and facade designer. Failure to design to the deflection requirements can result in cracking, visible deflections, and other serviceability problems.

For heavy facades (e.g., brick, stone, and concrete), the structural engineer must also pay close attention to torsional rotations of the spandrel beams. If the facade is highly eccentric to spandrel beams of a steel-framed building, a small amount of spandrel rotation can result in significant deformations at the facade support locations (Fig. 1). While facade panels anchored at two successive floors during installation may help to restrain outward tilt at the building’s lower stories, the problem can resurface at the top floor/parapet, where the facade cannot provide rotational restraint. Check spandrel rotation and, where required, design kickers or full-depth shear connections to minimize torsional effects.

Inadequate strength can be an issue when communications are insufficient. Examples include changes in facade support strategies, loading locations, weights, and other forces that occur after the structure is designed. To avoid that communication gap, diagrams on structural drawings clearly showing typical facade loads and support arrangements used for framing design can provide a basis for discussions.

Facade installation problems can arise if structural and facade movements are incompatible. Most structural engineers know to present anticipated slab edge live load deflections and interstory drifts on drawings for use by the facade designer. In practice, these movements are rarely the source of facade failures. One reason may be the reality that most buildings never experience design seismic and wind events or full design live load. While these movements must be properly communicated to the facade designers, other types of movement—such as thermal, moisture expansion, and creep behaviors—are more commonly overlooked and more likely to result in performance issues.

Masonry facades can prove particularly tricky for designers who are not aware of one simple fact: in contrast with concrete shrinkage, brick masonry expands after installation, as it absorbs moisture from the atmosphere. Consider a cavity wall facade with a concrete masonry unit (CMU) bearing wall as backup for a clay brick exterior wythe. For this type of construction, the CMU structure and brick exterior move in opposite directions. The brick undergoes moisture and thermal expansion, while the CMU experiences drying shrinkage as well as axial shortening as loads are added. Failure to detail the brick facade with sufficient joints of suitable width to accommodate the total relative movement can result in cracking and spalling of the masonry (Fig. 2).

While the architect is typically responsible for this detailing, developing appropriate details without input from the structural engineer is difficult. Brick and CMU movements are determined from shrinkage and expansion coefficients that come from Section 4.2 of TMS 402. Further detailing recommendations are available in Tech Note 18A, Accommodating Expansion of Brickwork, published by the Brick Industry Association (BIA). Structural engineers should be familiar with these recommendations and should confirm that the facade designers have the structural movement information needed.

Building lateral displacements under gravity loads can also adversely impact facade installation. Consider a high-rise concrete building with an offset elevator core. Experienced structural engineers would anticipate that during construction, the building will begin to lean away from the core due to larger axial deformations and creep of columns under gravity loads compared to the less-stressed core. In recent facade litigation, the structural engineer of record correctly calculated during the design phase that a 50-story building would lean by approximately six inches. The lean was accounted for in the structural model, so the structural engineer did not see a need to communicate the magnitude to the remainder of the design team. Unfortunately, the aluminum facade panels were detailed with much less adjustability than required to accommodate the building movements. The panels could not be made to fit around the corners of the leaning superstructure and needed to be refabricated. For tall concrete structures, best practice is to discuss building lean magnitudes early in the design process. In this case, costly facade panel refabrication could have been avoided by structural compensation measures during construction, such as column cambering, by facade panels with more adjustability, or a combination of measures.

Closely related to the issue of movement incompatibility is failure by the structural engineer to communicate slab edge deflections and tolerances. Contractors understand that facade attachment points may not end up exactly where they were specified to be. But input from the structural engineer is required for the contractor to understand exactly how much they will deviate. Structural tolerances should be communicated during the design development phase, or even earlier, so the facade can be detailed accordingly. The structural engineer should provide actual construction tolerance values instead of general references to the standards. For example, a sketch that shows that the slabs on a concrete building may be ¾ inches higher or lower than specified is much more helpful than a note broadly stating that the facade must accommodate tolerances from ACI 117 (Fig. 3). Specifying erection tolerances more stringent than those provided in industry standards (e.g., ACI 117 Specification for Tolerances for Concrete Construction and Materials, and AISC 303 Code of Standard Practice for Steel Buildings and Bridges) should be considered only for special circumstances, with the implications discussed ahead of time with the design team and contractors.

Similarly, slab edge deflections must be clearly communicated on the design documents. A facade designer will want to know three things:

Separate deflection and story drift values should be provided for atypical conditions, such as podium floors or longer spans; it is inefficient to design the entire facade to accommodate worst-case deformations that occur at only a few bays or stories. Buildings with window wall facade systems require additional attention to detail. Unlike curtainwalls, which typically have generous adjustability, window walls must fit between the slabs. Problems occur most frequently when window wall facades are used in conjunction with non-prestressed concrete slabs that have aggressive span-to-thickness ratios. Because window wall head tracks are often set straight and level, slabs with significant deflections can “pinch” the available clear dimension between slab (Fig. 4).

Deflection information should also be clear and easy to follow. The general notes excerpt in Figure 5 comes from a set of structural drawings. The facade designer interpreted the note to indicate that the total maximum slab edge deflection was 1.23 inches, inclusive of both the “Short Term LL” and “Short Term DL” values. The structural engineer later testified that the values were additive; the total maximum slab edge deflection was actually 1.23 inches + 0.28 inches + 0.77 inches = 2.28 inches (close to twice as much as what the facade designer had considered). It was also unclear what loads were included in the “Short Term DL” deflection. Clearer communication of the slab edge deflections could have prevented serious facade installation issues that led to a lawsuit.

While the structural engineer of record does not typically design the facade attachments, they should still have some understanding of the types and locations of the planned facade anchors. The structural drawings should include diagrams showing facade loads and locations, along with exclusion areas where facade anchorage is not permitted (e.g., as seismic deformation zones and areas with post-tensioning strands).

For concrete buildings, conflicts between rebar and facade anchors can cause serious construction delays. To avoid these headaches, the best practice is for the structural engineer to work with the architect and contractor to ensure the implementation of one of the following strategies:

While the general contractor is responsible for overseeing coordination between the concrete and facade subcontractor, the structural engineer should avoid making the coordination more difficult than necessary. In one facade litigation case, the structural engineer insisted, for no clear structural engineering reason, that edge rebar be located exactly 3 inches from the edge of the slab—directly in line with the facade anchors. Not surprisingly, the contractor blamed the structural engineer for the resulting anchor conflicts.

Involvement from the structural engineer is required throughout the design and construction process to help facilitate a smooth facade installation process. With that in mind, the following is a list of key items for the structural engineer to consider during each phase of the project:

Review the facade concept with the architect to identify potentially problematic conditions. For example, speak up if the design team and owner are pushing for unusually thin concrete slabs and long spans. Suggest that the project team consider details that can prevent facade installation conflicts, such as cast-in facade anchors instead of post-installed anchors. Communicate structural movements and tolerances to the design team so that the facade can be detailed accordingly from the get-go. For concrete structures with long-term creep, make sure the facade designer understands both the initial and long-term slab edge movements, and that the tolerances and deflections must be combined to get the total potential slab edge deviation.

Include section cuts that clearly show the structural slab edge details with the relationship to the facade. Clearly list the structural movements (slab edge deflections, inter-story drift, creep, etc.) and structural tolerances in the construction documents. Provide the actual +/- tolerance values, not just references to industry standards.

Review facade submittals, while including clear disclaimer language indicating you have not designed the facade and that your review is limited to reviewing loads imposed on the structure. Look for and take exception to potentially problematic facade detailing, such as inadequate facade adjustability and facade anchorage details that will potentially conflict with structural components.

Require the contractor to perform slab edge surveys before and after the shores are removed. Review the first few surveys to identify slab placement tolerance issues or slab deflections that exceed what would be predicted by analysis.

Every project is different, and the recommendations in this article are not always applicable. However, facade coordination is not something that the structural engineer can ignore. Being a proactive participant in the coordination process—and being aware of the common coordination pitfalls—can go a long way in preventing facade disputes and potentially substantial litigation costs for structural engineers. ■

About the Authors

Joseph Schuster, PE, SE, is a Principal at Thornton Tomasetti and leads the forensics practice in TT’s Newark office. His work focuses on facade and structure-related litigation support work, including expert testimony.

Matthew Kuba, PE, is a Principal at Thornton Tomasetti and is the Midwest Regional Facade Engineering practice leader. He specializes in facade analysis and investigation, including finite element modeling and thermal performance analysis of glass and steel specialty structures.