4D JOURNAL. inaugural issue of LARSA s quarterly journal. L-Tips: Tools tor rapidly modeling post-tensioning tendons page 3

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1 LARSA, Inc. s January D JOURNAL The 27.5m wide superstructure comprises a classical Pi-section RC deck with two edge girders connected by regularly spaced transverse post-tensioned girders. The main girders are also post-tensioned in the main span region. The RC pylons are hollow box H-shaped shafts and feature draped legs of a variable cross section that are connected by postthe inaugural issue of LARSA s quarterly journal The Phu My Bridge A case study by Tony Gee and Partners of staged construction and dynamic analysis of the cablestayed bridge in Vietnam. The People s Committee of Ho Chi Minh City on behalf of the Socialist Republic of Vietnam commissioned the construction of a bridge crossing the Sai Gon river. The investment owner of the project, the Phu My Bridge Corporation (PMC), commissioned the UK based consulting firm Tony Gee and Partners LLP (TGP) to undertake an independent Technical Design Investigation Consultancy. The main contractor for the project is the BBBH consortium which is a joint venture between Bilfinger Berger and Baulderstone Hornibrook. BBBH are also responsible for the design of the bridge through their design consultants. The scope of TGP s services under the contract spread over a broad range of checks and overviews but particularly included detailed numerical and analytical checks on the bridge structure using high end engineering software. The Cable-stayed Structure The all-concrete cable stayed structure comprises a threespan arrangement with spans of 162.5m + 380m m. continued on page 4 ALSO INSIDE... Welcome Message from LARSA s president Ali Karakaplan page 2 News in Brief page 2 L-Tips: Tools tor rapidly modeling post-tensioning tendons page 3 Meet The 4D Team: Ali Koch page 6 What s Ahead: Composite section construction activities page 7 Conference Schedule page 8

2 2 4D Journal January 2008 Welcome Message We would like to welcome you to our first issue of the 4D JOUR- NAL. This quarterly publication will share and exchange information among LARSA 4D users, and we hope to shape the publication around suggestions and contributions from you. The JOURNAL will update you on the features we are developing and interesting ongoing projects using LARSA 4D. In the issues later this year, you can expect to read about new capabilities for design. LARSA 4D is growing from a 4D analysis package useful for any type of complex structure, such as cablesupported structures, to an analysis and design tool with a special focus for bridge engineers. Bridges represent the most challenging structures due to their exposure to the environment, loading conditions, seismic response, and construction sequence. Engineering technology has made tremendous advances in the past decade, and we hope to keep LARSA 4D ahead of the curve. Engineers require the ability to predict the performance level of structures with a prescribed degree of confidence. Shortcuts are no longer an option for the types of bridges now being built, and the margin for error is narrowing. Now 20 years in the business of rigorous analysis, we will continue the development of analytic tools designed with our clients needs in mind. It has only been through developing new capabilities with clients, one-on-one, that has allowed us to produce cutting-edge software. Material time effects, tendons, momentcurvature analysis, and steel I girder design, among other components of LARSA 4D, were developed in this way. We greatly value our close relationships with our clients and enjoy working together on extending the software for whatever new needs arise. Best wishes for the new year, Ali D. Karakaplan, Eng.Sc.D. President, LARSA, Inc. News in Brief Company and software updates of note: Our new design module performs AASHTO LRFD 2006/7 code check on curved and skewed steel I, plate, and box girder bridges. A wizardlike interface can be used to generate bridge models or perform code checking on existing LARSA models. This module complements the analysis capabilities of the program in our aim to be a complete analysis & design package. Moment curvature analysis can now be performed on concrete sections from Tools > Moment Curvature Analysis. The moment-curvature curves computed can be used for plastic analysis using hysteretic members. A new iterative shape finding tool has been added to the program under the Tools menu. This tool finds the initial state of the structure so that after loading, including staged construction activities and time effects, the deformed geometry reaches a certain state. It has been used successfully to find the initial prestress on the cables of a cable-stayed bridge and the main suspender in a suspension bridge so that the deformed structural geometry matches the intended architectural form. Our relationship with Bentley Systems has reached an end. In 2004, Research Engineers subsequently incorporated into Bentley in 2005 partnered with us and became a reseller of LARSA products. The reseller agreement was mutually terminated this month. About the 4D JOURNAL LARSA 4D is analysis and design software for bridges, buildings, and other structures, developed by LARSA, Inc. in New York, USA. This journal publishes quarterly and is sent as a courtesy to our clients and corporate friends. We welcome feedback and suggestions for future stories to info@larsa4d.com. LARSA, Inc. 105 Maxess Road Melville New York info@larsa4d.com

3 3 L-Tips Each issue of the 4D Journal will feature a short guide to an aspect of LARSA 4D that you may not already be familiar with. How to place tendons rapidly using the Model Data Explorer and the Tendon Editor Tendons in LARSA 4D are used to model internal and external pre- and post-tensioned strands within beam sections. The program computes the resulting forces on the beam elements along 3D tendon paths based on the curvature and other properties of the tendon specified by the user, taking into account long-term time effects such as steel relaxation. Because crosssections frequently contain several to a dozen tendon ducts, the ability to model tendons rapidly in a LARSA 4D model can be very important. While tendon data can be entered entirely through the Geometry > Tendons spreadsheet, three tools speed up the process of entering tendon geometry: drawing tendons from a member chain, the Tendon Editor, and the Model Data Explorer s tendon right-click menu. Drawing Tendons One way to create a tendon quickly is to define it based on selected members. You will need to have selected just the members through which the tendon passes, so Unselect All. You could use the standard graphical selection tools to click on each of the members that the tendon passes through to select them i.e. the portion of the girder the tendon goes through, if this is a bridge model. However, selecting members one by one is a tedious process and is in most cases unnecessary. Instead, first go to the graphics window and turn on the Pointer Tool (the arrow-like tool in the toolbar). Then click any member that the tendon passes through with the right mouse button. A menu of useful commands to apply to the particular member clicked is shown, such as Select, Rotate Principle Axis (90 degrees), and Reverse Direction. See Figure 1. Click on the Select Chain command. LARSA will select that member and then look at which members it is connected to at either end and select all of those up and down the model so long as the members don t make any sharp turns. Figure 1. Unselect All, and then right-click a member. The Select Chain command will select it and all of the members up and down from that member that form a connected chain. This command can be used to select entire girders, tall columns made of multiple member elements, etc. very quickly. With the so-called chain of members selected, go to Draw > From Selected Members > Tendon. See Figure 2. A tendon record is created and set up to pass through the selected chain of members. The Tendon Editor is also brought up for the new tendon. The Tendon Editor The Tendon Editor is used to enter the geometry of the tendon: the profile of the tendon through the members it passes through. The Tendon Editor is an alternative to the (older) standard tendon path spreadsheet. The path spreadsheet opens with a 3D view of the tendon. The Tendon Editor, on the other hand, shows horizontal and vertical profile diagrams and a crosssection diagram. In a curved bridge, these profile diagrams factor out the curvature so that it is easier to see the entire profile at once, compared to the 3D view, and the cross-section shows all of the tendons passing through a member at any location. The paths of tendons are controlled by a series of geometry points: shown as filled circles in the Tendon Editor profile images. To change the tendon s profile, you can click the geometry points and set their locations by offset distances from a member s reference axis (which is usually the centroid, but can be positioned elsewhere in the Section Composer) or from the extreme edges of the member where section Stress Recovery Points are usually set. Curvature options at geometry points can also be set in the editor. Figure 2. This command creates a tendon passing through the selected members. Tendon material, jacking, and other information must still be set in the Geometry > Tendons spreadsheet. After entering or modifying the proposed jacking force, always remember to right-click the tendon in the spreadsheet and choose Compute Jacking Force so that LARSA will ensure that the force along the tendon does not continued on page 6

4 4 4D Journal January 2008 Phu My Cable Stayed Bridge, Vietnam Tony Gee and Partners LLP, U.K. continued from cover tensioned cross beams. The tie-down piers are of solid RC construction and are integrally connected to the deck. The 144 cable stays are orientated in a semi-harp configuration in two symmetrical cable planes. Modelling and Analysis In view of the high level of complexity associated with the analysis a global 3D model comprising representative beam and cable elements was developed. In order to economize on data preparation and to make the modelling process more efficient, a database was constructed in LARSA Section Composer for generating the section properties of the prismatic and non-prismatic members comprising the global model. This database was interfaced with the global model using the facilities provided within the software. Additionally, the model also featured the construction and de-construction of a variety of temporary works that were installed and sequentially removed at various stages during the construction of the cable stayed structure. Use was made of linear and non-linear springs in order to simulate such scenarios in conjunction with members representing the temporary works. Appropriate 6x6 stiffness matrices were also developed to model the soil-structure interaction between the large diameter RC piled foundations under the main pylons and the abutments. TGP principally used LARSA 4D for a broad range of linear as well as non-linear static and dynamic analyses for the cable stayed structure including: staged construction with time-dependent creep and shrinkage effects global and differential temperature effects extraction of eigenmodes and eigenfrequencies dynamic seismic response spectrum analysis ship impact analysis cable replacement and nonlinear push-over cablesnap analysis Key aspects of some of the analyses undertaken by TGP are briefly outlined below. Staged-construction Analysis (SCA) with Time-dependent Effects A major component of the analytical work involved the staged construction analysis of the cable stayed structure incorporating time dependent effects such as creep, shrinkage and relaxation to CEB-FIP 90. Key aspects of the SCA are outlined below. The general sequence of construction input in the SCA was based upon the construction program provided by the contractor. The emphasis of this analysis was to closely model the evolving geometry, force distribution, stiffness and material character of the structure with reference to the applied loading as well as time. Relevant stages of superstructure construction also accounted for the installation, sequential movement as well as release of the erection gantry. A major input in this analysis comprised the application of pre-determined forces applied to various cable stays during the process of stay installation. These forces were applied and managed at appropriate stages using the facilities provided within the software. In the instance of Phu My cable stayed bridge, LARSA 4D provided us with a versatile analytical tool for investigating the structure under a variety of static and dynamic load effects. We particularly found useful the range of options it offered with regard to staged construction and dynamic seismic analyses. Sam Khan, Principal Engineer, Tony Gee and Partners

5 Temporary supports, members and falsework installed and removed during the construction were also modelled. These included temporary bearings, wind buffeting cables and propping supports. Some temporary members were assigned unidirectional stiffness such as tension-only and compression-only. The Figure 1. The cable stayed structure during various stages of construction as modelled in LARSA 4D. participation in the three principal directions. Carrying out a multimodal dynamic response spectrum analysis using site-specific parameters and combining the extracted forces in accordance with the codal provisions. post-tensioning tendons installed in various members at different stages were also modelled. The SCA yielded the build-up of stresses in the members including those locked-in during construction. In the postclosure phase, further stages were investigated for time dependent effects up to 125 years into the life of the structure. Dynamic Seismic Analysis The dynamic seismic response of the structure was investigated by carrying out a multimodal eigenvalue analysis which was followed by a Response Spectrum Analysis to AASHTO LRFD. The dynamic seismic analysis focussed on the following key aspects. Appropriate representation of the material and structural character of the structure as well as the mass distribution. Extraction of the first 100 eigenmodes and eigenfrequencies to yield an appropriate cumulative mass Nonlinear Cable-snap Pushover Analysis As opposed to the cable change scenario, which was considered as a controlled event, the cable-snap scenario was modelled as a nonlinear pushover scenario incorporating the strand-by-strand fracture of a critical cable stay. The basic impact dynamic force considered in the analysis was derived from the Post-tensioning Institute Guide Specification for Stay Cable Design Testing and Installation, Ship Impact Analysis The ship impact analysis was undertaken as a pseudodynamic analysis in which the dynamic force resulting from ship impact was applied in accordance with the provisions of AASHTO Guide Specification and Commentary for Vessel Collision Design of Highway Bridges. Further analysis in this regard was also carried out using proprietary software. As an additional consideration the ship impact scenario was initially investigated under a transient dynamic force. This course however was not pursued in detail because of the lack of project-specific information. (Text and figures courtesy of Tony Gee and Partners.) Figure 2. Selected output from the multi-modal eigenvalue analysis carried out in LARSA 4D.

6 6 4D Journal January 2008 L-Tips: Modeling Tendons continued from page 3 exceed the peak allowable stress. Model Data Explorer for Tendons After a tendon has been created, either through the geometry spreadsheets or the Draw menu, they are listed in the Model Data Explorer in the Tendons section. This list has two functions. First, the tendon selected in this list by clicking on it is drawn thicker in the graphics window to distinguish it from other tendons. This is a good way to pick out a tendon from a crowd. Second, you can click tendons with the right mouse button in this list for a menu of useful commands to apply to the tendon. The commands are: Rename: Asks for a new name for the tendon. Edit: Opens the Tendon Editor for the tendon. Path Spreadsheet: Opens the older standard path spreadsheet and 3D view of the tendon. Duplicate: Creates an exact copy of the tendon. Translate in Y/Z: Offsets the tendon in local member Y or Z directions by some distance. Mirror Y/Z Offsets: Inverts the sign of the offset values on either the Y or Z axis, effectively mirroring the tendon across the local member Y/Z axis. Generate Equivalent Loads: Creates a load case with equivalent member loads that represent the effect of stressing the tendon. This is an alternative to using tendon activities in the Staged Construction Analysis. Figure 3. The Tendon Editor shows Y/Z profile and cross-section diagrams, and can be used to edit geometry control points and curvature. When a span contains multiple similar tendons, it is easier to define the second, third, etc. by duplicating and modifying the first, rather than starting from scratch for each tendon. Use the Duplicate, Rename, Translate, and Mirror commands. After creating the first tendon using the spreadsheet or Tendon Editor, use the Duplicate command in the Model Data Explorer to create a second tendon, and then use the Rename command to identify it. Then use Translate and Mirror to move the second tendon into its actual position. Repeat the process of Duplicate followed by Rename, Translate and Mirror to create the remaining tendons. Meet the 4D Team Each issue we will introduce you to a different member of the LARSA 4D team. Ali Koch, M.S., director of research, development and support, holds a master s degree in computer science and has been with LARSA, Inc. since He is currently a PhD student in the City University of New York. Koch was most recently responsible for the development of the steel I-girder and box girder AASHTO LRFD 2006 code check module and moment curvature analysis. He is known to be the fastest support handler of the west, able to type separate s with each hand, drink coffee, and juggle textbooks, all at the same time. Between support issues and development, he finds time to travel. He takes short trips to Europe, one-day trips to the Caribbean, and last summer explored China in a 15 day trip with his girlfriend. Koch grew up in Istanbul, Turkey. Because Koch and LARSA president Ali Karakaplan share the same first name, Koch goes by his last name in the office.

7 7 What s Ahead Composite Section Construction Activities The What s Ahead section highlights some of the new program capabilities we have in the works. When the components of a single cross-section are assembled at different times, or when parts of a cross-section of different materials are behaving independently with respect to timedependent material effects, composite section construction must be used to maintain the continuity of behavior of a single cross-section. A composite member is a single continuous line element made up of shapes of different materials or of shapes constructed or cast at different times. Coming to LARSA in 2008 are new capabilities for composite members. Expect the program capabilities described below to be released later this year in the coming major update to LARSA software. They are currently in a testing phase but can be previewed upon request. Composite members are defined in In the Section Composer, each shape that makes up a cross-section can be assigned a different material, and will be able to be constructed at different times. the LARSA Section Composer. While members made of multiple materials can already be modeled using the Section Composer, they cannot undergo time-dependent material effects and the shapes that make up composite members must all be constructed in the model at the same time. Composite Member Element The new composite construction capabilities coming in 2008 allow each shape that makes up a complete crosssection to be cast at different points during Staged Construction Analysis. Further, each shape may undergo creep, shrinkage, the time effect on elastic modulus, and thermal loads independently. These effects are computed separately for each shape and then transformed and applied to the member s overall centroid automatically. For creep, the stress and strain history of each shape is maintained independently. The casting of shapes at different times and the differential effect of the time-effect on elastic modulus on parts of a section with multiple materials causes the centroid of members to move over time. This is also automatically accounted for. A common use of composite construction will be to model a girder made up of a steel or concrete tub and a concrete plate cover which are cast at different times, but when it is desirable to consider the tub and cover a single cross-section. This might be because they act together continuously along the length of the girder. While a more detailed finite-element model may treat the tub and cover as independent elements, such a model could fail to capture the continuity of the connection between the tub and cover along the length of the girder. In a finite-element model of the girder, the continuity must be captured by rigidly connecting the girder to the deck at regular intervals along the bridge. One might use Member End Offsets or rigid elements (members with artificially high stiffness) to do this. But creating connections at small intervals is a highly labor-intensive task. Composite members instead model the continuity but allow for some degree of independence of their parts. Composite Section States Composite section states determine which shapes are active at any given time. A shape can be included in a state with weight and stiffness, weight only as a simple method to model concrete before it sets, or stiffness only if the weight of a composite part has been modeled independently of automatic self-weight computation. Weight-only is an alternative to using time-dependent material properties and setting the casting day. The composite state of a member can be changed as an activity during Staged Construction Analysis. Loading, including self-weight, and timedependent material effects like creep continued on page 8

8 4D Journal January 2008 What s Ahead: Composite Construction continued from page 7 are applied on a shape-by-shape basis to whichever shapes are active in a member s current composite state. Forces are transformed as needed from a shape s centroid to the member s overall centroid. Composite construction affects member stresses as well. Stresses will reflect the possibly different strains in each shape that makes up a member, as well as the different modulus of elasticity of each shape, and the differential changes in modulus of elasticity over time for each shape. New Composite Construction Activities Composite section construction is an addition to the Staged Construction Analysis. Two new activity types become available: Cast Concrete: In a time-dependent staged construction analysis, this activity sets the casting day for concrete shapes within composite members. Different casting days can be set for different shapes of the same cross-section, and the differential effects of the time effect on elastic modulus on each shape within each member element will computed based on the casting days set with this activity. Composite Activity: This activity changes the cross-section make-up of a group of member elements in the model by adding or removing shapes to members cross-sections during the course of a Staged Construction Analysis. This activity is used to model the addition of a slab of concrete above a tub shape, for instance. This activity changes the cross-sectional properties of the members, which are automatically calculated by the Section Composer. The centroid of a member may move as a result of this activity, and internal forces are automatically transformed to the new centroid location as needed. When composite shapes are removed, the internal forces in the removed shape are applied automatically onto the remaining structure. New Nonlinear Thermal Gradient Loads Along with new construction activities, a new nonlinear thermal gradient load will be available. This is a thermal load applied at a single cross-section, with an arbitrary thermal profile along the member s local y or z axis in the cross-section. The effects of a nonlinear thermal load require the careful modeling of member cross-sections, as the effect is determined by integrating the section geometry along the cross-section and applying the resulting forces and moments at the member centroid. New construction activities will be added to the Construction Stages Explorer (left). The cast concrete activity (left below) sets casting days of concrete members and sub-shapes for the time effect on elastic modulus. Composite activities (far bottom) add or remove sub-shapes into and from member elements, such as for modeling the pouring of a concrete deck above a pre-cast box. Conference Schedule Come see us at these conferences that we plan to attend in 2008: Accelerated Bridge Construction Technologies Baltimore, Maryland, USA; March North American Steel Construction Conf. Nashville, Tennessee, USA; April 2-5 Structures Congress Vancouver, B.C., Canada; April th International Bridge Conference Pittsburgh, Pennsylvania, USA; June 2-4 PCI Annual Convention & Nat l Bridge Conf. Orlando, Florida, USA; October 5-8 ASBI 20th Anniversary Symposium San Francisco, Calif., USA; November 17-19