The constructor must understand the response of structural components to complex load states imposed during construction. Although they are primarily responsible for the behavior of the completed structure, designers must also make allowances for construction loads in some structures. For example, wall panels in tilt up construction must be reinforced to resist the loads imposed when the panel is being lifted in to place. "In-the-wet" construction of marine projects imposes a wide variety of loads on the structural components. In a typical application, large, hollow, precast concrete segments are constructed in a dry basin next a body of water. The basin in flooded to float the completed segment in order to tow it to the construction site. Cavities in the segments are flooded to position the structure on the foundation caissons. Once secured, the cavities are dewatered and filled with tremie concrete. Figure 1 shows a segment of the Braddock Dam project under construction in Leetsdale, Pa. This segment was floated out in late July 2001 for outfitting and was scheduled to be installed on the Monongahela River near Braddock, Pa. in September 2001 (Armistead, 2001). The structural design of the precast segment must account for hydraulic pressures during the floating, ballasting and filling operations. The thin concrete shell elements will experience significant flexural loads when one side is exposed to a higher level of pressure.

Although in-the-wet construction has become common in many marine applications such as offshore oil and gas platforms and bridge piers, each project presents new design challenges. A complete structural design must account for all construction loads. This can be accomplished by "over-designing" so that the structure is adequate for all possible load conditions. However, a more efficient design would be realized by accounting for actual construction conditions in the design. Detailed checking of all loads is tedious and time consuming using conventional methods.

Methods to automate much of the design and analysis process will help designers to develop more economical approaches. Design by Analysis (DBA) software is being developed to speed the learning process necessary to implement innovative techniques. In the DBA environment, engineers will be able to quickly develop preliminary models of a structure, analyze the structure under all significant loads, modify the structure based on those results and receive rapid feedback indicating the effect of those changes. The final analysis model should then be translatable into a solid model of the structure that can be used throughout the design and construction.

One objective of Design by Analysis is to significantly increase the efficiency of the designer by automating much of the modeling and design process. Custom software, tailored to a specific application, can reduce training and modeling time. The required analysis can be performed using commercially available, general-purpose finite element software, but substantial experience is required to use those programs efficiently. A program to design and analyze a specific type of structure can be developed with a small set of essential modeling and analysis options to minimize training time. With some assumptions about typical structural configurations, much of the modeling process can be automated such that large parts of the structural model are generated with minimal, usually "parametric," input.

Software to perform Design by Analysis is being developed using Microsoft Visual Basic 6.0. This powerful development environment is well suited for the researcher with minimal formal computer training. Later versions of MS Visual Basic allow the programmer to create an executable version that is typically an order of magnitude faster than the interpretive code when performing intensive calculations typical of finite element software.

The graphical interface is written using "primitive" graphics commands in order to simplify later implementation using Windows Application Program Interface (API) functions or more efficient graphics languages. All graphics are drawn in a Picture Box using PSet, Line and Print commands. The Polygon function from Windows API is also used.

The graphical user interface (GUI) for Design by Analysis is designed to create three-dimensional models. The objective of the GUI development is to provide a limited set of essential drawing options in order to insure that any designer, with minimal training, can create working models. Options were selected to allow the designer to model typical navigation structures with adequate detail for Design by Analysis. Other drawing options can be added as requirements are identified.

Model generation technique development focused on structures whose cross-sections normal to the Z-axis have fairly consistent shapes. The geometry for these structures is generated by first drawing a typical two-dimensional cross section of the structure using simple drawing commands such as Line, Polyline, and Copy. Each quadrilateral region is then designated for modeling with either shell elements or superelements. These regions are extruded along the Z-axis to create a solid model, and the cross section geometry can then be modified as required along the length of the structure. The skeleton for finite element meshing is also created in this process. Figure 2 shows the 2-D sketch prior to extrusion for a spillway segment.

Nodes and elements are generated automatically based on user input indicating the desired level of refinement. The user then defines material properties and applies boundary conditions using various options provided in the GUI.

After analysis the user can plot stress, strain and deflected shape results at any cross section in the model. An option to plot shear, moment and thrust diagrams in a selected region of the model is also available. Figure 3 shows Y-direction stress contours in the spillway segment model that clearly indicate flexural stresses in the walls when the adjacent cells are being filled with water. Figure 4 is the Shear, Moment and Thrust diagrams for the right-most vertical element shown in Fig. 3.

Linear elastic finite element methods are used to analyze the structure. Two principle objectives of the Design by Analysis research is to generate a model that represents the actual 3D geometry of the structure and to be able to quickly modify the design and evaluate the effect of the changes on the structural response. These are accomplished by creating a global model consisting of a relatively small number of high order elements. Standard shell elements are used where appropriate, but details, such as regions where multiple shells are joined, must be modeled using solid elements. These regions are condensed to superelements in order to reduce the number of degrees of freedom and make the nodes compatible with 5-DOF shell nodes. The condensation process is computationally intensive, but, after the initial model formulation and analysis, this process is only repeated for regions that are modified. This greatly reduces reanalysis time.

A typical finite element model of a shell structure consists of shell elements that connect at lines or curves. Although these models provide good results for flexural stresses throughout most of the element, the model at the joint is oversimplified. Typical design considerations at the joints involving reinforcement details and increased thickness cannot be easily modeled with shell elements. These joint regions are modeled using eight-node, hexahedral elements. In order to be compatible with the shell elements these joint regions are connecting, they must be condensed to superelements with 5-DOF shell nodes. This is accomplished using standard static condensation methods (e.g. Weaver and Johnston, 1984) in which all nodes at the interface between a joint region and a shell are mapped into the mid-plane node assuming that through-the-thickness deflections are linear. Figure 5 shows regions designated for meshing with solid elements and subsequent superelement generation.

An additional objective of Design by Analysis is to automatically determine the required concrete reinforcement and perform other design checks such as shear capacity. At this time a shell region can be highlighted for Shear/Moment/Thrust plots at a cross section. Top and bottom reinforcement ratios can be calculated for the worst-case condition in the cross section. The program then proposes a reinforcement configuration to provide the required reinforcement, and the finite elements are modified to reflect the specified steel. Figure 6 indicates that recommended reinforcing is #7 @ 12" and #9 @ 12" on the two faces of the shell analyzed in Figure 4.

Once the preliminary design is completed, it should be checked for all load cases to be encountered throughout the construction and service life of the structure. It would be difficult to define all of the load cases using general-purpose software, but it can be largely automated using software developed specifically for this application. The construction sequence is input by indicating the order in which cells are flooded and later filled with tremie concrete.

The dam segment is initially constructed on dry land. It must be supported by formwork during the initial fabrication, and the structure must support its own weight once the formwork is removed. As the construction basin is flooded, water pressure will generate flexural stresses primarily in the bottom panels. Some concentrated loads will be caused by towboats as the segment is delivered for installation. Significant differential pressures will be applied to the vertical stiffeners as the individual cells are flooded and again during the placement of the tremie concrete. Changing the structural design or the construction sequence can reduce excessive stresses identified during the analysis. The tremie concrete will cure over time and transition from being a hydrostatic pressure to an integral part of the finished structure modeled with solid elements.

Finite-element-based Design by Analysis methods are being developed to provide an automated tool to determine the structural response during all phases of "in-the-wet" construction of marine structures. These methods allow the designer to create a more complete structural model in a fraction of the time currently required for a more complete analysis. The results will provide additional confidence in these construction procedures and allow designers to easily study various design options leading to further improvements in design and construction.

Armistead, T.F. (2001) First Segment Floated to Site, Engineering News Record, August 6, 2001.

Weaver, W., Jr. and P.R. Johnston (1984) Finite Elements for Structural Analysis, Prentice-Hall, Englewood Cliffs, New Jersey.