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Design of continuity slabs and the Metro viaduct designs

020

Gajanan Chaudhari

Principal Bridge Engineer Middle East & India Atkins Abu Dhabi, UAE

Anand Panpate

Senior Bridge Engineer Middle East & India Atkins Abu Dhabi, UAE

Hassan Ibrahim

Structural Engineer Middle East & India Atkins Abu Dhabi, UAE

Abstract

The Emirate of Dubai is one of the major cities of UAE, which is expanding rapidly in all sectors, resulting in rapid growth of population and severe traffic congestion problems. The Dubai Roads and Transport Authority (RTA) has come up with "Dubai Metro" as a solution to the problem. Dubai Metro Project is owned by Dubai's RTA, and comprises two lines, the Red Line and the Green Line. The Red Line extends for 51.8km from Rashidiya to Jebel Ali; 47.4km of which is elevated and the remaining 4.7km in bored or cut and cover tunnel. The Red Line will serve 30 stations; four of which will be below ground including two interchange stations at Burjuman and Union Square. The Green Line extends for 16.8km from Dubai Health Care City to Dubai Airport Free Zone; 10km of which will be elevated and 6.8km in bored or cut and cover tunnel. The Green Line will serve 15 stations; six of which will be below ground. Atkins' team in Abu Dhabi was commissioned for the detailed design of the Green Line's special precast segmental decks with horizontal radius down to 200m and in particular to study the effect of the tight radii on the behaviour of the decks. This report presents the longitudinal and transverse analyses, including checking the effects of construction loading on the design, the computer models setup, analysis of the results and the detailed design carried out for the 200m radius decks with spans of 17m, 20m, 24m, 28m, 30.6m, 32m and 34.4m in accordance with BS 5400:Part 4. The report tries to shed some light on the 3D solid modelling of complex structural shapes, rail loading applied to bridge structures, segmental construction technology and its effect on the design.

1. Introduction

This report outlines some of the procedures employed in the analysis and design of the tight radius curved precast segmental spans of the Green Line viaduct. Analysis and design is based on a rigorous analysis using brick element solid modelling of the u-shaped deck section.

The deck is designed to satisfy all the specified code serviceability and ultimate limit state (SLS and ULS) criteria. The deck's cross-section geometry as seen in Figure 1 is an open thin-walled u-shaped section. For such an open web section, it would be difficult to analyse the section using a Finite Element (FE) shell element model. This is because of the transverse flexibility and three-dimensional behaviour of the section. If we take an example of a typical box girder as in Figure 2, an FE shell element model for this deck is easily set up by just extruding the centrelines of the slabs and the walls into the shown shell elements model. Unlike any typical box girder decks, the u-shaped deck is more complex to prepare its analysis model. The main girders on both sides of the u-trough have non-uniform cross section and the down stand cross section at supports differs from that at the mid41

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2. Deck analysis and design

The viaduct is designed for a special carriage defined by the owner as described in section 2.1, and the code applied for the design is BS5400: Part 4.

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Design of continuity slabs and the Metro viaduct designs

2.2 Solid element analysis:

For the purpose of studying the stresses in the deck due to its self-weight, super imposed dead load, and train load, we used LUSAS software. LUSAS is an FE analysis software specialized in solid element modelling, and has many features that eases the pre-processing (geometry input) and post processing (output extraction) operations. FIGURE 1. SEGMENT CROSS SECTION span. In addition, the top flange thickness is varying along the span. All this necessitated modelling of this deck using brick elements solid model. For ease of construction, the deck is made up of precast concrete segments, standardised in terms of their length. For most of the spans, the end segment, which incorporates a down stand, is 1.65m long, and the rest of the segments are approximately 4.0m long. For each segment, we used a naming convention of referring to its start or end by "Joint", a typical convention for segmental construction. When modelling in LUSAS, we used the element type (HX8M) for meshing. (HX8M) elements are 3-dimensional solid hexahedral elements comprising eight nodes each with three degrees of freedom. We used HX8M elements because it was more efficient for the computer run time, and its results are suitable for the design purposes (an element with 20 nodes could be used but it would take more computer run time, and its results are more useful for research purpose). Analysis using solid element modelling of the deck in LUSAS ensured that the stress distribution on the deck cross-section at different locations is reflecting the actual behaviour of the deck and that the shear lag effect near supports and effect of the lateral buckling of main girders at mid-span are accounted in the 3-D analysis. When modelling on LUSAS, we faced a problem that it does not have the ability to model prestressing tendons in brick elements. The software requires that prestressing be applied to frame elements only. That means for modelling the prestressing force using LUSAS, we would have to model the curved tendons in 3-D space as dummy frame elements with nominal properties; and then try linking those dummy frame elements with each node at the intersection between the solid element and the tendon; which is a very lengthy procedure and prone to error. To study the prestressing force effect, the prestressing force can be applied to a line beam model, and the stresses calculated at regular intervals along the deck due to this applied force (while studying serviceability limit states, prestressing effect is studied in linear elastic range of both steel and concrete). By using RM2006, we adopted this approach in the deck analysis and used the prestressing effect from RM analysis into the design. To study the moving load effect on the deck, train wheel locations were determined using the influence lines extracted from the RM model.

2.1 Loads considered in analysis

For purpose of analysis and design of the decks, the following loads were evaluated from BS5400: Part 2 a) Deck self weight (SW) b) Superimposed dead load (SDL) -Trackform plinths running rails and fixings ­ cable trays and cables c) Vertical train load (as in Figure 3). d) Rail vehicle impact factor (determined from a separate 3D solid element model on ABAQUS software) e) Braking and traction forces f) Centrifugal forces g) Nosing forces h) Lurching forces i) Derailment loading j) Gantry, transporter and construction loading (defined by the contractor) k) Wind loading LUSAS analysis was done for the load cases a, b, c and d. For other load cases, we developed separate spreadsheets to calculate their values and effects on the overall design of the deck.

STRUCTURES

2.3 Line beam analysis

For the purpose of studying the prestressing effect on the deck and obtaining the influence lines, we used RM2006 software. RM2006 is a very powerful tool for assessing the prestressing effects, and for obtaining short and long-term losses in the prestressing due to creep and shrinkage of concrete, steel relaxation and elastic shortening of concrete.

FIGURE 2. LUSAS MODEL, FE SHELL ELEMENTS MODEL FOR A TYPICAL BOX GIRDER 42

Design of continuity slabs and the Metro viaduct designs

020

the viaduct piers, the Rear leg and the Auxiliary leg; both are supported on the adjacent deck's girders. The launching steel gantry is a heavy structure of about 515 tonnes; which during its launching forward, one of its legs (the Auxiliary leg) carrying about 260 tonnes is directly supported on a segment joint. During construction, tendons are not grouted. This means that when determining the ULS flexure capacity of section, during construction the prestressing tendons shall be considered as "external" tendons, thus reducing flexure ULS capacity of the deck by about 10 to 30%.

FIGURE 3. TRAIN CONFIGURATION AS SPECIFIED BY THE CLIENT From RM influence line analysis, we obtained the train locations that would give the maximum straining actions at segments' joints. As the span/width ratio was less than two for some spans, we made a grillage model based on the recommendations by EC Hambly "Bridge Deck Behaviour". The main difficulty of applying prestressing to segmental bridges' decks is the standardisation of tendon locations at each joint; these points must be respected while determining the cable profile, as this is a construction requirement. The fixed locations for tendons do not give any flexibility to the design, and care should be taken to make sure that the profile adopted in design is applicable and matching the real profile executed on site.

3. Design

Longitudinal design of the deck was done in accordance with BS 5400 code provisions for prestressed concrete sections; and the transverse design was carried out to the code provisions for reinforced concrete sections.

3.1 Longitudinal Design

For longitudinal design, SLS check of deck is done by extracting the LUSAS stress' output for the load cases: Self Weight, superimposed dead load and Train load. A combination is made from these effects with the effects from other load cases; applying the appropriate load factors. Then these stresses were combined with the stresses from RM analysis and three cases were checked. Case 1: Self Weight + Prestressing effect: Immediately after jacking, so that it includes immediate losses in prestressing force: wedge draw-in, friction losses, steel relaxation and concrete elastic shortening. Case 2: Service Loads + Prestressing + Short term Creep and shrinkage:

2.4 Construction load analysis

For the deck construction, a steel launching gantry and a segment transporter are used. The steel launching gantry used for construction has three main supports: The Front leg; which is supported on

STRUCTURES

FIGURE 4. LUSAS MODEL, SHOWING STRESS DISTRIBUTION DUE TO SELF WEIGHT OF DECK (34.4M SPAN WITH RADIUS 250M) 43

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Design of continuity slabs and the Metro viaduct designs

An additional "During Construction" case, where the stresses on the deck needs to be checked for the gantry load, and also where the stressing sequence of cables needs to be checked and accounted for in the end block design. Depending on the deck configuration (whether a straight or curved deck), the stresses on the left and right girders were studied to ensure that the final stresses are within the allowable stresses of 1MPa in tension (approved as departure from standard by RTA), 20MPa in compression. ULS flexure capacity of the section was computed considering tendons unbonded to concrete during the construction stage. Ultimate shear stresses on the deck were checked as the envelope for maximum shear stress due to the loads (SW + SDL + Train load) and the resulting stress checked against the code limits of (5.3/5.8 MPa for concrete grades 50/60 MPa respectively). The ULS shear capacity of section, capacity of shear keys and epoxy glue between concrete segments were evaluated according to AASHTO/ LRFD provisions for shear friction design.

FIGURE 5. LUSAS MODEL, SHOWING STRESS DISTRIBUTION DUE TO SELF WEIGHT OF DECK (SLICES AT JOINTS LOCATIONS)

3.2 Transverse design

For transverse design, the segment is considered as a reinforced concrete section, and code checks are made accordingly. For this design, local distribution of wheel load, together with Self Weight and SIDL results, were extracted from LUSAS. A useful tool in LUSAS to extract results called "slices". By using "slices" command, we extracted the stresses distribution in the transverse direction of the deck under the train loads. In addition, from these slices, the software would integrate these stresses, and extract an output of Flexure, Shear and Torsion straining actions, to be used later in the design. We used SAM software to evaluate the ultimate flexure capacity of the deck's transverse cross section, and to check the SLS crack width criteria of (0.2mm). For transverse design, a special case of bearing replacement was also checked. In this particular loading case hogging moments near girders had to be checked, and top reinforcement at edges were introduced to cater for this particular loading case.

FIGURE 6. RM MODEL, SHOWING PRESTRESSING TENDONS Representing working loads of the structure plus short term creep and shrinkage losses in prestressing for a period of 180 days. Case 3: Service Loads + Prestressing + Long term Creep and shrinkage: Representing working loads of the structure plus long term creep and shrinkage losses in prestressing for a long term period or the structure life time of 16500 days. The stresses were checked for the points on the deck cross section shown in Figure 10.

STRUCTURES

FIGURE 7. SCHEMATIC FOR GANTRY CONFIGURATION 44

Design of continuity slabs and the Metro viaduct designs

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4. Deck construction

A special area is established for the assembly of reinforcement, called rebar jigs, as seen in Figure 12, and then reinforcement cages are moved into the casting cells for concrete casting. Segments are made in a purpose built casting yard, where a specially designed steel formwork known as longitudinal mould or casting cell is used to cast the precast segments, as seen in Figure 13. The casting cell consists of side shutters, bulkheads, a soffit form and an internal shutter, as seen in Figure 14. The side shutter forms the external shape of the u-trough; the inner shutter forms the inner shape of the u-trough, while the bulkheads form the shear keys and tendons' duct locations. The bulkheads vary in terms of shear keys shapes and sizes and ducts' locations according to the cast segment's location along the deck.

FIGURE 8. GANTRY'S FRONT SUPPORT RESTING ON A PIER, ERECTED DECK IN POSITION AFTER ITS PRESTRESSING IS COMPLETED

FIGURE 9. GANTRY'S REAR AND AUXILIARY SUPPORTS RESTING ON DECK'S GIRDERS

FIGURE 12. ASSEMBLY AREA FOR SEGMENTS' REINFORCEMENT, REINFORCEMENT CAGE IS READY TO BE MOVED TO CASTING MOULD FIGURE 10. CONTROL CHECKPOINTS FOR SLS STRESS CHECK

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FIGURE 11. SHEAR KEYS 45

FIGURE 13. THE STEEL FORMWORK USED FOR SEGMENTS CASTING, SHOWING THE TENDONS' FIXED POINTS

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Design of continuity slabs and the Metro viaduct designs

Inner shutter is adjustable for the down-stand segments, as the girder web thickness is thicker than that for a midspan segment. For some decks, the top flange thickness at mid span increases compared to that at near ends. This also will need to have an adjustable inner shutter. For this type of construction, the gantry used will have a height over deck more than twice the deck height. First all segments will be lifted by cables hanging from the gantry and adjusted on two levels, to ease the application of epoxy glue to the segments' interfaces. The epoxy glue used in this project is a tropical grade to give a setting time of about 2 hours. The workers should apply this glue manually, so a suitable gap between segments is required. After applying the epoxy glue, the segments are matched together, and temporary prestressing using steel brackets (Figure 18) is applied to hold the segments until finishing threading of prestressing tendons, and application of permanent prestressing force.

FIGURE 16. ONE SEGMENT AFTER CONCRETE HARDENING WHILE STILL ON THE SOFFIT SHUTTER, READY TO BE MOVED TO STORAGE YARD

FIGURE 17. STORAGE YARD

FIGURE 14. INNER SHUTTER AND BULKHEAD FOR A MID-SPAN SEGMENT

FIGURE 18. STEEL BRACKETS FOR APPLYING TEMPORARY PRESTRESS

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FIGURE 15. STEEL CAGE OF ONE SEGMENT AND THE DUCTS IN POSITION, BULKHEADS IN POSITION, WHILE INNER SHUTTER BEING SLIDE INTO THE DECK

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