contact | contents | bibliography | illustration credits | ⇦ chapter 7 |
8. OPENING REMARKS ON NONSTRUCTURAL FAILURE
I have written previously about nonstructural building failure. My paper, "Designing Building Failures" examines the "relationship between building envelope failure and attitudes towards design," with a concluding section that "examines the implications for pedagogy and practice."1 "A Probabilistic Approach to Nonstructural Failure" takes a closer look at one of the conclusions suggested in the first paper: that only a risk-based approach to the design of nonstructural building elements—analogous to limit-state design methods in structural engineering—can create conditions in which building design becomes rational and nonstructural failure is thereby reduced.2 In the second of the two papers cited, I outline two characteristics of buildings that can increase or reduce the risk of nonstructural failure: a greater degree of peculiarity or complexity can increase the risk, while certain types of redundancy reduce the risk.
By nonstructural building failure, I mean problems with the actual constructed elements of a building that include things like water and thermal control issues; sloppy, dysfunctional, or dangerous details; maintenance issues; and blotched or cracked finishes. I have excluded a discussion of structural failure since the design of structural systems has been largely removed from the purview of architects and is not only strictly regulated by building codes which reference design manuals promulgated by the major consensus-based structural materials organizations, but is also largely in the hands of professional engineers who are inclined by training and temperament to follow best practices embedded in those codes.
Whereas the probability of structural failure (i.e., the actual collapse of buildings or structural components like beams or columns) is made explicit within the design methods enforced by building codes and, in fact, forms the very basis of structural design, the design of nonstructural parts of buildings typically has no underlying probabilistic basis. In other words, when architects create drawings and specifications for buildings, they often have no basis for determining the probability of nonstructural failure. Where a clear pattern of architectural failure emerges, building codes may or may not be modified, depending on the severity of the problem. Even in those cases, however, the recommended "fixes" do not approach the problem from an explicitly probabilistic standpoint, so that it is still not possible to assess the reliability of one system in comparison with another, or to assume that an equivalent level of risk resides in all systems sanctioned by the codes.
A probabilistic basis for architectural failure is beginning to be acknowledged in theory but is still difficult to implement in practice. Nevertheless, it is still possible to draw some important conclusions about the nature of such failure.
Peculiarity
The most important conclusion derives from the fact that, for unusual architectural designs, the interaction of materials, systems, geometries, environmental conditions, installation methods, and so on, is rarely systematically tested or theoretically grasped. Conventional construction details and methods, on the other hand, have at least a track record of generally successful (or unsuccessful) application. While the lack of a consistent measure of reliability applies to such conventional systems as well, there is at least an informal understanding of how such systems perform over time. For this reason alone, one can state that architectural failure will generally increase as the peculiarity of the architecture (i.e., the deviation of the design from well-established norms) increases.
This conclusion requires a disclaimer: it presupposes an ordinary level of attention given to all aspects of building design and construction. In other words, it is assumed that little or no original research (i.e., research following protocols such as those sanctioned by ASTM) is undertaken to establish the behavior of unusual design elements or their interactions; and that little or no additional time is spent in order to properly identify and document all special building conditions resulting from unusual geometries or materials. Of course, if one has the budget, the time, and the expertise, it is certainly possible to reduce the probability of failure when designing unusual or complex buildings. However, doing so requires not only a commitment to research, but also sufficient time and money to conduct the research, produce the necessarily complex and complete construction documents consistent with the research results, and hire contractors willing and able to carry out such a project.
Clearly, the parameter "peculiarity" has not been rigorously defined, but it is worth noting the following characteristics of peculiarity in architectural construction:
- Within a given length, area, or volume, the number of building elements is unusually large, or unusually small; what constitutes an unusual density of such elements is simply a comparison to what is usual. In general, increasing the number of building elements increases the probability of failure since it is typically at the intersection or interface of such elements that failure occurs (and increasing the number of elements increases the quantity of such intersections). However, there are instances where reducing the number of elements actually increases the probability of failure. For example, a smaller number of uninsulated facade panels means that thermal movement of the panels, relative to an insulated structural frame, is concentrated over fewer joints, so that joint movement is greater. Greater joint movement can increase the likelihood of certain types of sealant failure, for example.
- The number of different types of building elements is unusually large.
- Well-understood details are distorted/twisted/altered—or else simply invented without reference to any precedent—to accommodate unusual geometries, or to subvert conventional formal expectations. In particular, the right angle is eschewed in favor of bent, curved, or otherwise non-orthogonal geometries, and conventional expectations about "walls" and "roofs" are discarded in favor of more abstract characterizations.
- Materials are used in combinations, or in applications, that have not been well tested.
As a result of this peculiarity, the following outcomes become more likely:
- Structural movement in buildings with "peculiar" geometries is less well understood and less well modeled and predicted. Complex structural geometries make it more difficult to, first, coordinate the interaction of things like structural movement and cladding, and second, model the structure accurately. Even if a geometrically simple building is modeled inaccurately, the simplicity and uniformity of the model suggest that errors will at least correspond to behavioral tendencies of the actual structure, even if numerically out of scale.
- Junctions (intersections) of materials or systems deviate from well-established norms.
- Architectural drawings and specifications are less likely to address the full range of conditions present within the building, especially in their three-dimensional manifestations.
- Contractors are more likely to apply conventional knowledge to unconventional situations. Ironically, in the case of so-called green buildings where more environmentally benign, but less well-understood, materials are employed, the opposite situation may occur with the same result: contractors are more likely to apply unconventional knowledge to conventional situations (see the following bullet point).
- Untested material combinations are more likely to interact in unpredictable ways.
- Basic strategies for enclosure (continuity) are more likely to be violated: membranes become penetrated rather than continuous, or penetrated in ad hoc ways; surface complexities promote discontinuities in thermal/vapor/water/air membranes or materials.
Redundancy
The benefit of redundancy, examined from a probabilistic standpoint, is a relatively unexplored and potentially fruitful area of research. For example, providing two roof membranes instead of one doesn't merely cut the risk of failure in half, but—assuming that the failure of each membrane is independent of failure in the other—rather decreases the risk of failure by an order of magnitude. Of course, it is crucial that any strategy employing redundancy take into account the specific mode of failure: adding an extra (redundant) layer of paint over an improperly prepared substrate confers no particular advantage since the utility of the redundant layer depends on the integrity of the layer below. In other words, the conditional probability of failure of the redundant layer, given failure of the layer below (and therefore failure of the system as a whole), is 1.0, conferring no advantage. At the other extreme, the conditional probability of system failure for the two membranes discussed earlier—if each membrane is assumed, for example, to have a failure probability of 0.1—would be 0.1 × 0.1 = 0.01, a significant improvement.
Conventional practices, such as the provision of roof overhangs, can be reevaluated in this light. For a given exterior wall surface area, if the probability of failure due to water intrusion through an unintended hole in the wall is, say, 0.05, and if the probability that wind-driven rain will reach that wall surface is 0.07 when an overhang is in place (both values are entirely hypothetical), then the conditional probability of failure with an overhang is 0.05 × 0.07 = 0.0035, a dramatic reduction in risk compared with the hypothetical failure probability of 0.05 without the overhang.
The failure mode interaction described above—involving a combination of two or more failure modes where the redundant combination actually decreases the probability of failure—can explain the benefits of redundancy from a probabilistic standpoint. Having two barriers instead of one doesn't just double the safety (cut the probability of failure in half), but rather can be shown to be much more significant.
Roof overhangs could also reduce the probability of icicles forming on an exterior wall. In this case, the formation of icicles requires two things: on the one hand, a portion of the wall needs to be warm enough to melt wind-driven snow while a lower portion of the wall needs to be cold enough to freeze the melted water, causing icicles to form. On the other hand, wind-driven snow must be able to reach the wall surface. Now compare the use of overhangs on Frank Lloyd Wright's Robie House with the lack of overhangs on Milstein Hall. While leaking roofs are not unknown within Frank Lloyd Wright's oeuvre, the likelihood of icicles forming on the brick walls of the Robie House is dramatically reduced by the use of roof overhangs (Figure 8.1 top). On the exterior facade of Milstein Hall, on the other hand, icicles can form through the same process associated with classic ice damming. Snow melts on floor-to-ceiling glass panels, or perhaps on stone cladding panels—where radiant heat originating in the concrete floor slab (near the top of the stone panels) works its way through various insulation layers via thermal bridges—and then freezes (at the bottom of the stone panels) where the stone is colder. Such icicles, especially if they become bigger, pose a threat to pedestrians circulating directly under this cantilevered corner of Milstein Hall (fig. 8.1 bottom).
Figure 8.1. While leaking roofs are not unknown within Frank Lloyd Wright's oeuvre, the likelihood of water issues on exterior walls or windows of his Robie House is dramatically reduced by the use of roof overhangs (top); the exterior facade of Milstein Hall, on the other hand, forms icicles as snow striking the surface melts and then freezes (bottom).
Of course, the problem with icicles on the facade of Milstein Hall should have been addressed by decreasing the U-value of glazing or, as discussed later in this section, by eliminating thermal bridges through the stone panels. Both strategies not only reduce the probability of icicle formation, but also reduce gratuitous energy consumption. The point is that buildings are constructed in a probabilistic environment where the risk of nonstructural failure is reduced by employing redundant strategies. In this example, even if an unexpected thermal bridge creates the conditions for icicle formation, an overhang could prevent wind-driven snow from reaching the wall surface in the first place.
Complacency
Aside from causes originating in the complexity or peculiarity of buildings (or their lack of redundant details), buildings also experience nonstructural failure because of designers' "complacency." I use this term to includes things like sloppy detailing and inattention to functional considerations. Some of this is related to the peculiarity or complexity of their buildings since such buildings require a great deal more attention to detailing. This means that a great deal more time, money, and expertise needs to be devoted to such detailing; it is dangerous to assume that the complexity will be somehow dealt with "in the field."
Architects do not necessarily need to sacrifice the expressive qualities of their designs in order to reduce the risk of nonstructural failure. But an architectural design strategy that starts off with heroic intentions and then attempts to "make it work" by superimposing some rational elements will be more likely to experience nonstructural failure than a design strategy that starts off on a rational basis and then "adds" expressive elements that leave the rational basis intact.
Nonstructural failure in Milstein Hall
Milstein Hall is a classic example of a peculiar and complex building for which only routine attention was given to nonstructural detailing and performance. Contract documents were produced, and contracts for construction were signed, without having established a clear and comprehensive understanding of critical construction details. Even from casual observation, without having official access to records or correspondence, several instances of this phenomenon can be seen, including rainwater infiltration through building enclosure elements, extensive cracking of concrete slabs, blotching of concrete wall finishes due apparently to VOC-compliant form-release agents, staining of concrete floor finishes apparently due to premature contact with plywood protection boards, and cracked exterior lighting fixtures. Given the secrecy surrounding the actual construction process—the ongoing crises, panicky phone calls, hastily-called meetings, negotiated remedies, and the change orders that invariably accompany such complex projects, are not made public—it is likely that those defects and failures immediately visible in Milstein Hall represent only a small fraction of actual nonstructural failure incidents.
Yet is it fair to classify Milstein Hall as a "peculiar" building? Unlike building designs that obviously deviate from traditional constructional geometric norms (e.g., those manifesting things like "splines, nurbs, and subdivs"3), Milstein Hall is, at least in part, designed with a regular orthogonal grid of columns, rigid frames, girders, and beams, and is clad with an expensive, but otherwise conventional, glass and stone veneer curtain wall. It is true that the lower-level geometry is far more complex, consisting of a reinforced concrete doubly curved "dome" and inclined glazing. However, even the "conventional" orthogonal steel framework is itself highly unusual (peculiar) in terms of its large cantilevers, hybrid trusses, and moment-connections for lateral-force resistance. As a result of both the peculiarity of the design and the lack of adequate attention given to its detailing, numerous sites of actual or potential nonstructural failure can be identified. These are described in the chapters that follow.
Notes
1 Ochshorn, "Designing Building Failures."
2 Ochshorn, "A Probabilistic Approach to Nonstructural Failure." The discussion of peculiarity and redundancy that follows is largely derived from this paper.
3 "Instead of the classical and modern reliance on ideal (hermetic, rigid) geometrical figures — straight lines, rectangles, as well as cubes, cylinders, pyramids, and (semi-) spheres — the new primitives of parametricism are animate (dynamic, adaptive, interactive) geometrical entities — splines, nurbs, and subdivs — as fundamental geometrical building blocks for dynamical systems like 'hair', 'cloth', 'blobs', and 'metaballs' etc. that react to 'attractors' and that can be made to resonate with each other via scripts." Schumacher, "The Parametricist Epoch."
contact | contents | bibliography | illustration credits | ⇦ chapter 7 |