REDESIGNING SKYSCRAPER FACADES USING FINITE ELEMENT AND WIND TUNNEL ANALYSIS TECHNIQUES

The building in the poster will soon be the new home of Cladtech USA, Ltd., the Algor customer which also designed the granite and glass exterior of this skyscraper.

Finite element analysis (FEA) on a personal computer is being used by an architectural engineering consulting firm in Chicago to optimize the design of skyscrapers' stone and glass facades, leading to greater strength and safety while reducing the amount of material used. The same firm - Cladtech USA, Ltd. - has also completed the industry's first integration of wind tunnel analysis results with FEA results to iteratively design and verify building frames, according to President L. Russell Buczkowski. This is especially useful for buildings in areas like Southern California, where there are stringent seismic code requirements. The consultants have also developed the first FEA model for evaluating the reaction of glass panels to both static pressure loading and dynamic loading.

Cladtech's experience with FEA has not only optimized design, but has shown that the classical methods for testing thick stone are not applicable to thinner pieces. "Therefore," points out Buczkowski, "the only way to design safely is to use finite element methods and spend a good deal of time performing small-scale stone testing to determine breakage modes, failure modes, and flexure."

Cladtech USA specializes in architectural engineering design. A sister company, Cladtech Associates, operates in London, United Kingdom. The Chicago company focuses their work on a building's curtain wall, which is the outside padding of a high-rise structure, a nearly independent system that bears neither gravity load nor floor load from the building itself. Curtain walls are generally fabricated from extruded aluminum; the vertical members, or mullions, are connected to the building at local anchors of force tabs; horizontal members, which frame windows and panels of stone or metal, are attached to the mullions.

Cladtech USA uses Algor FEA software to perform computerized analyses on IBM AT compatible computers with high-resolution VGA color monitors, as well as newly purchased 80386 computers. According to Buczkowski, who has been working with FEA since the days of Nastran, SAP IV and SAP 80 in the early 70's, "We evaluated many FEA programs and concluded that Algor is probably the most flexible system on the market on a price-performance basis. Nothing comes close to it for our type of work, which is heavily beam-oriented. Without Algor, we wouldn't be able to do as many projects as we're doing, and we certainly wouldn't be able to do the complex analysis that Algor can do." For these reasons, Cladtech's sister company in London will now also be using Algor's FEA System for design analysis.

Algor's compatibility with Cladtech's CAD system is a tremendous time-saver, Buczkowski points out. Cladtech's designing is done on AutoCAD 9.0, but for design analysis, they use Algor's SuperDraw II, a comprehensive graphics drawing program geared towards finite element model creation. As Buczkowski notes, "There's a constant process of transferring our drawings from AutoCAD to Algor. With its built-in mesh generator, SuperDraw II saves a lot of time." Algor's beam design editor, BEdit, enables Cladtech to edit the SuperDraw II file, inputting nodal boundary conditions, beam end releases, material properties, area properties, etc. "BEdit also saves us time," he adds, "by allowing us to work from AutoCAD-generated drawings. We don't have to spend time entering the nodal points and element properties since that information can come along as block attributes in the AutoCAD files."

Cladtech President Russ Buczkowski.

Why Use FEA for Curtain Walls?

The majority of modern stone high-rises have towers clad with thin layers (less than four to five inches) of granite, limestone or marble. Buczkowski says it has only been during the last decade that thin stone has come into use in true curtain wall-supported systems, instead of conventional masonry support using a block back-up wall or a steel truss system. "We have come up with some alarming analyses indicating that the sort of testing that has been classically done for thick stone just does not apply to thinner pieces," noted Buczkowski. "Furthermore, while safety factors of three or four are required for stone design in buildings, we can go back and look at some of the older projects and find they have safety factors barely over two. This just underlines the need for using finite element methods in the design stage."

A key part of designing a curtain wall is developing attachments to the structure that are able to withstand the dynamics of the structure under wind loading: torsional response and longitudinal displacement. Most of the stone panels today are supported by kerfs (channels cut into the edge of the stone) at the top and bottom, which allow the extruded-aluminum members of the curtain wall frame to support both the wind load and dead load. Because of the geometry of the kerf and the thin plate behavior of the stone, there can be very large stress concentrations. "Our major concern is that if the stone panel's deflection is high enough, its edge will rotate, reducing the kerf connection, possibly causing the panel to slide out," he says. Therefore, kerf design and the flexural rigidity of the supporting members become very critical.

Plot of various material/component interactions that occur throughout the curtain wall.

Wind Tunnel Tests

Prior to the construction of large buildings, 1:400-scale models are generally evaluated in wind tunnels. A model of a 20-story building, for example, would be roughly 7?inches tall. The model is a rigid shell constructed in a true scale representing the geometry of the building. Pressure taps are drilled into the surface and connected by tubing to scanning pressure transducers. Instantaneous pressure response - which occurs as a result of simulated full-scale wind conditions surrounding the model - is then evaluated.

The wind forces are based on similitude scaling laws. Basically, the structure of the aerodynamic flow is related to the geometric size of the building, but the wind pressure itself is not. To extrapolate up to full scale loads, Cladtech simply multiplies by the prototypical wind pressure. Once the wind loads for the curtain wall are determined, they are used as performance criteria for the engineering design.

For buildings in areas like Southern California, where there are high seismic requirements, Cladtech will build a finite element model of the curtain wall frame and connect the model to the structure. Since seismic requirements are related to wind-induced displacements and forces, Cladtech will use the wind tunnel force balance results to dynamically load the connections to the curtain wall frame to evaluate various reactions, including induced stresses, deformations and displacements.

Creating a Curtain Wall Design

Once Cladtech has determined prototypical loading from either the wind tunnel or coding information, they work with the architectural designer, who produces a rendering or schematic-level design concept of the building. "As soon as we have shapes that will 'work' aesthetically," explains Buczkowski, "we go into a detailed analysis, building a fairly highly defined FEA model, using beam elements predominantly. Then, having obtained the response characteristics of the typical curtain wall module under wind load and seismic conditions, we go back and 'tune up' our supporting members to optimize the shapes on a basis of weight or tooling costs so we arrive at the most economical approach. Finally, once we have the Supersap output for the composite system, we look in detail at local stress concentration on individual elements to verify safety factors."

FEA of a Curtain Wall

A typical curtain wall FEA model will be two full floors high by a column-to-column bay wide, or about 25' x 20 - 30'. For example, a 25' x 25' model will often have glass vision lengths about six feet wide. Below the glass is a stone panel measuring roughly 5' x 5', supported on mullions that are five feet on center, spaced horizontally. The mullions are tied back to the structure at each floor, every 12 feet. This glass/stone/mullion construction is a unit that occurs all over the facade of the building. "Conceptually, what we do is slide this module around to various locations on the building, look at its response, determine the critical conditions and design accordingly," says Buczkowski. In analysis, Cladtech examines the stress distribution, moment reaction and displacement response of the module to various wind loads on the building.

Stone is an anomaly in terms of engineering design practice. Most of the stone, particularly granite, is not homogeneous and is quite anisotropic. There is not as wide a body of code requirements or industry-supported mechanical properties to fall back on as there are with other materials. When modeling stone, Cladtech creates many elements using 2000 to 4000 nodes with a high mesh density around the kerfs. Under full wind load, a typical 5' x 5' granite panel that is three centimeters thick may deflect approximately 1/8 inch at the center, leading to large stress concentrations in the kerfs. Hence, the need for the finer mesh in these critically studied areas. The elements Cladtech uses are mostly brick; complex three-dimensional shapes are modeled by breaking them down in many small bricks.

Principal bending stress maximums in the stone panels occur in diagonals near the corners, except under high loads; in this case, a plate will be highly distorted, whereupon it does not behave as much like a plate as a membrane.

In modeling the glass, Cladtech applies elements according to where they anticipate large clamping stresses, which arise because the glass is literally clamped into glazing rabbets (which retain the glass) with neoprene gaskets. The principal stress magnifies as the plate bends in diaphragm action. Glass measuring 5' x 5' and ?inch thick will deflect approximately 1 - 2?inches. Buczkowski points out that it is crucial to look at stress development near the edge and principal plane bending stresses near the center. The location and magnitude of these stresses varies depending upon the aspect ratio of the type of glass being considered.

Jon Weir, a designer at Cladtech, inspects an AutoCAD display of a piece of a curtain wall. A three-story high portion such as this is built life-size then tested.

To create a mesh from the AutoCAD outline of the model, Cladtech imports the file into SuperDraw II then uses the Construct:Mesh option, indicating the number of mesh divisions to use along each side of a specified region. The mesh is automatically generated according to the conditions that have been stipulated. SuperDraw II also features a Construct:Clean command that will eliminate any duplicate lines or vertices that may have been created during the drawing or meshing processes. Throughout the definition process, SuperDraw II provides a graphic display of the geometry.

Once the mesh has been completed in SuperDraw II, Cladtech uses BEdit to apply loads to the model. These are mostly uniform wind loads (ranging from 30 to 100 pounds per square foot) and gravity loads. At this point, Cladtech can display applied loads as different colored arrows at the points of application on the model.

The actual FEA calculations are performed using one of Algor's new high-speed HyperSap processors, which maximize the computing power of the 80386 machines recently acquired by Cladtech. Comments Buczkowski, "These new processor modules run much, much faster and allow us to use single block models in certain cases." After running the analysis, Cladtech again uses BEdit, this time to examine the results. The Post: Both command automatically overlays the original and deformed models onscreen in two different colors for visual appraisal. The principal bending moments in the mullions occur as negative restraining moments at the intermediate anchor, due to the fact that Cladtech uses twin span beams. As a result, the highest induced bending stresses occur at the intermediate anchor. Stress levels that result from out-of-plane bending moments in typical framing members are 20 - 60,000 inch-pounds.

Cladtech uses the FEA output to reduce the section properties, minimizing the metal weight. "We'll reduce the wall thickness to the point where it's all light as possible," related Buczkowski. "Additionally, we will often develop asymmetrical extruded sections that focus the section properties in areas that are subject to high local stresses. One of the noticeable differences you would see in our FEA-designed curtain walls as opposed to those done totally manually is that our wall will be lighter and our sections generally will not have a constant wall thickness."

Further Evaluation

Cladtech's approach to determining the curtain wall's dynamic behavior is to use the results of the static analysis to determine the curtain wall's basic frequency and then dynamic analysis to find the natural frequencies.

With its modes of vibration identified, Cladtech can predict how the model will behave at each of its resonant frequencies and ascertain the source of the disturbance, its propagation path, and how it is radiated into the environment. After the analysis, Algor's POST, which creates a stress output file from the processed model, gives Cladtech a listing of the stresses and deflections. They also use Superview - Algor's latest menu-driven graphics display program - to evaluate the results of the analysis.

Plotted mock-up of three-story test specimen.

Glass is commonly a subject of dynamic analysis. While performance under static uniform load develops one stress regime, if the wind-induced stresses are the result of fluctuating pressures, whose fundamental frequency is near the fundamental frequency of the glass light (i.e., the pane of glass), there can be very large stress magnifications. Failures occur in the compression layer near the interface when there are anomalies in the glass and at the surface on the tension side.

Cladtech uses brick elements to model glass, normally defining enough elements to require use of 2000 to 4000 nodes. "Because we know the principal failure mode, we won't use a constant mesh density," says Buczkowski. "That is, we will develop the mesh in the compression layers to a higher density than in the amorphous region trapped between the two compression layers." The glass panels are typically exposed to wind forces of 1500 to 2000 psi, though they sometimes reach as high as 6000 to 8000 psi.

"If we see a glass section whose natural frequency response is in the range of 0.5 hertz to 1.5 hertz," he explains, "we will then look at the model dynamically and see if changes in the support system or clamping will take the natural frequency out of the frequency of the loading domain. If that doesn't work, generally, the only resort is to thicken the glass. However, using FEA techniques lets us accurately determine the optimal thickness."

"Not only does Cladtech benefit from the use of FEA," Buczkowski asserts, "but more importantly, our clients also profit. The curtain walls we design use the least amount of material necessary in designs that are proven to be safe using cost-effective PC-based testing methods. As a consequence of our use of the Algor FEA System, clients can be sure that Cladtech provides them with a state-of-the-art design."

Unless otherwise noted, the computer displays and plotter drawings accompanying this article are for components to be used in the construction of the 1999 Avenue of the Stars building in Century City, California, owned by Urban Investment and Development Company, a division of JMB Realty. The architect is Johnson, Fain & Pereira Associates; structural engineering is by A.C. Martin Company (NOTE: According to Russ Buczkowski, this engineering firm is planning to purchase and use Algor's FEA System after having seen it in action at Cladtech.)