The cost of constructing the granite exterior wall system of the 31-story 75 State Street office building in Boston ran over budget and the walls were not completed in the expected time frame. In the upcoming legal action, Desktop Engineering Int'l, Inc., a forensic engineering firm, will demonstrate that mockup test results were interpreted incorrectly. This resulted in unnecessary changes that caused cost overruns. Desktop Engineering has developed three-dimensional CAD drawings that will convey the concepts involved to the judge and lawyers associated with the case.

Intergraph's (Huntsville, Alabama) CAD package Engineering Modeling System (EMS) was utilized to construct three-dimensional computer models of the wall system. Intergraph's presentation package ModelView was used to produce the enhanced views of the EMS models. With the aid of computer modeling and a Sony RVP 6010Q multiscan rear projector system, it will be possible to: 1) recreate images of the mockup specimens that were used during the construction planning phase, 2) identify the wall system components and explain the related engineering terminology for parties involved in the litigation, 3) reproduce the various design changes and show that there had been unnecessary modifications, 4) demonstrate how test results had been interpreted incorrectly, and 5) show how the objective of the mockup testing had been changed.

Mockup testing involves constructing a good-sized section of the wall of the building and then measuring its performance under wind loading and water penetration. This type of testing is frequently done on buildings that use curtain wall construction to evaluate the behavior of integrated window/curtain wall units. The mockup is inherently different from the actual building in that the mockup is supported by a special structure rather than by the rest of the building.

In this case, the mockup was 2-1/2 stories high, 50 feet long and included one corner. Air pressure up to 60 psi was applied to the mockup to determine its deflection under loading and to check for air leakage through the curtain wall. Water was also applied to the outside of the building to check for water penetration. The building architects did not approve the results of the initial tests because, among other things, they thought deflection was excessive and because they felt some permanent damage occurred to the wall structure during the testing. The mockup design was changed and retested and it failed the test again. This cycle was repeated two more times before the architect approved the results.

The CAD presentation will demonstrate that the mockup actually should have passed the first test insofar as structural criteria were concerned. One important factor is the way in which the mockup's supports differ from the actual building. These differences are concentrated at the top and bottom of the mockup because of the absence of supporting structure. The middle of the mockup, on the other hand, is fairly representative of the actual building. In assessing the results of the test, it is logical to concentrate on this middle section. In evaluating the results of the tests, it was discovered that the middle section performed well while the excessive deflections were concentrated at the top and bottom. The CAD images make it possible to convey this concept to people who are not familiar with the technical concepts involved.

Another important issue is that the architects found that the wall did not return fully to its original position after deflecting under wind pressure. This sort of permanent deflection generally indicates damage in the structure under evaluation. However, Desktop Engineering consultants will be able to demonstrate that this permanent deflection was attributable not to the wall itself but rather to the fact that the test structure was not mounted with sufficient rigidity.

Although the changes demanded by the architect appeared to be minor, they had ripple effects throughout the building and ended up causing an increase in cost. It will be important to convey this complexity to the judge in order to demonstrate the fact that seemingly minor changes in the building design had an impact that rapidly escalated its cost. This will be a difficult task because the test mockup and the actual building itself consisted of many different components layered beneath the facade panels with complex geometry that are very difficult even for an engineer to visualize.

The exterior wall system consists of windows and granite stone pieces that are attached to steel frames. These steel frames, in turn, are attached to the building exterior steel columns and beams, and to the edge of the concrete floor slabs. Small mounting fingers hold the granite sheathing to the frames. There is a series of gravity supports that hold the weight of the frames and stone and other supports that prevent the panel from moving in and out under wind loading. The complexity of the design of the building was dictated not only by architectural concerns but also by the fact that it had to fit within a very tight city configuration.

The use of three dimensional CAD will greatly facilitate the presentation of this complex system by making it possible to peel away one layer at a time to show the one underneath. The role of CAD is vital in this scenario because it makes it possible to produce simplified graphics that communicate the basic concepts involved in the case. Another CAD model of the building will be constructed to demonstrate that changes in the aesthetic design of the wall system resulted in major disruptions to the shop drafting, manufacturing and erection of the exterior wall system.

Another important concern is a change in the manner in which the building was to be constructed. Originally, the walls were to be constructed using a special crane called a stone buggy. This type of crane rides on tires on the floors of the building and carries each panel with granite pieces into position so it can be attached from the inside. Just prior to beginning this phase of construction, the architect stated that the floors would not withstand the load of the cranes. This made it necessary to use a tower crane to position the wall sections. This delayed construction and substantially increased costs because at the same time that the walls were being built, the tower crane was being used to work on other parts of the building. Three-dimensional models have been developed to describe the engineering terms and design decisions that led to the change of the construction method. The models together with engineering analyses of the floor slabs will show very clearly why the decisions were in error.

The forensic engineering application benefits in two ways from the specialized use of CAD: it clarifies a complicated three dimensional system to enable determination of cause and effect in design decisions and modification, and it permits presentation of investigative findings in a simplified manner using advanced computer aided design techniques.