This paper describes the scheme outline and contractual set-up for the viaduct design and discusses the design and construction of the viaduct substructure. In particular, the design methodologies used for the piled foundations, single reinforced concrete columns and overstressed concrete pier heads are discussed as well as the design of electorates bearings used extensively for most of the viaduct spans. Seismic loading governed the design of many of the foundations and the seismic analysis and design methodology adopted are discussed, together with specific reinforcement detailing requirements.
Rail- structure interaction analysis and design are also covered. Other critical design issues resolved include, fatigue performance of cranked reinforcement and the treatment; of the onerous construction loading from overhead gantries used to erect the precepts deck segments. In July 2005, the Government of Dubbed Road and Transport Authority (ART) awarded a design and build contract to the Dubbed Rapid Link (DOUR) consortium for the construction of the first and second stages of the Dubbed Metro Red and Green Lines. The DOUR consortium comprises the Japanese companies Mediumistic Heavy Industries,
Mediumistic Corporation, Bobstays Corporation and Jaime Corporation together with Yap Merrier of Turkey. Construction of the infrastructure and stations was the responsibility of a Joint venture between Jaime, Bobstays and Yap Markers Pant-Turkey-Metro Joint venture or JHTML). The JHTML appointed Atkins as their designer in 2006. This project organization is illustrated in Figure 1. Client Government of Dubbed Roads & Transport Authority Engineer Stray Parsons The Dubbed Metro will be a drivels, fully automated metro network and will be the longest fully automated rail system in the world.
Completion of the first section of the Red Line is planned for September 2009, followed in 2010 with the first section of Green Line. Contractor Mediumistic Corporation / Mediumistic Heavy Industries Jaime – Bobstays – Yap Merrier Designer A further Blue Line (along Emirates Road) and Purple Line (an airport express route) are planned for subsequent years. The metro route map is illustrated in Figure 2, with a more detailed Red Line route map shown in Figure 3. STRUCTURES Project Organization viaducts – Substructure Figure 2 Proposed Route Map Figure 3 Red Line Route Map 2 The El . Billion, 52 kilometer long Red Line connecting Arachnids to Jibe All port comprises 42 kilometers of elevated viaduct with 22 foreground stations, 5. 5 kilometers of tunnels with 4 underground stations, 2. 5 kilometers of at-grade section and 2 depots. The IEEE million, 24 kilometer long Green Line runs around the city centre connecting Festival City to the airport free zone and comprises 16 kilometers of elevated viaduct with 12 foreground stations and 7 kilometers of tunnels with 8 underground stations (of which two are shared with the Red Line).
This paper discusses the design and construction of the viaduct substructure. A further appear covers the design ND construction of the viaduct superstructure. Figure 4 Conceptual viaduct form Figure 6 Prototype deck segment and finish 3 Figure 5 Typical section of I-I-trough deck and pier head 2. Viaduct form 3. Piled foundation details The proposed form of the viaduct was architecturally-led in appearance. Figure 4 gives an artist’s impression of the proposed viaduct at the conceptual design stage and much of this form has been retained in the final detailed design.
Dubbed lies directly in the Arabian Desert and much of the geology comprises fine sand overlying sandstone and mudstone. The fine, upper sand layers consist mostly f crushed shell and coral and are a combination of mobile dune sands and Kasbah deposits. These overlie clarinetist, Aeolian deposits (sands, weakly cemented sands and weak sandstone) and ecclesiastical. Beneath these, between around 20 meters and 40 meters below existing ground level, are Jurassic conglomerates, mudstone and siltstone. The viaduct superstructures were typically formed from U-shaped cross sections as illustrated in Figures 5 and 6.
The post-tensioned precepts segmental deck segments were cast using either long line or short line moulds. The following superstructure forms, all of post-tensioned segmental construction, comprise the majority of the scheme: The categorical design parameters were derived by with the Engineer’s representatives locally. Results of the extensive ground investigations and their interpretation were collated into several ground reports for typical sections along the full metro route. The categorical design parameters in the soil and rock layers were derived from SPOT-N values and unconfined compressive strength (USC) respectively.
I-span (single span) decks – Simply-supported I-I-section decks constructed by the span-by-span method from an overhead gantry -span (twin span) continuous decks – I-I-section decks constructed by the span- by-span method from an overhead gantry and made continuous over internal supports by subsequent in situ concrete stitching of adjacent decks The vast majority of the viaduct spans are supported on single circular reinforced concrete columns with flared pier heads to support the decks, although a few portal structures are used in specific locations.
Most single columns are supported on 2. Mm and 2. Mm diameter bored monopoles for speed of construction and to minimize the footprint required for excavations in the congested urban environment. Figure 7 shows a typical monopole detail. -span continuous decks – Comprising a combination of I-I-section and box-section decks, erected by crane using the balanced cantilever method Station spans – 3- or 4-span continuous I-I-section decks constructed by the span-by-span method from an overhead gantry, subsequently made continuous over internal supports by stitching adjacent spans together Single track decks – Simply-supported I-I-section decks constructed by the span- overhead gantry (similar to I-span decks).
These decks are used at depots and bifurcations of the main lines t the largest stations The viaduct substructures generally comprise reinforced concrete piers, with flared pier heads to support the for the single spans, twin spans and station spans were constructed using thin precepts reinforced concrete shells which were unfilled with in situ concrete and overstressed in stages once erected on site. The pier heads for the single track and 3-span continuous internal piers are of in situ reinforced concrete construction. All piers and abutments are founded on large diameter bored piles.
Figure 7 Typical monopole arrangement for simply-supported spans and finish 4 Figure 8 Construction of typical pier on a monopole foundation Figure 9 Typical pier-pile connection details The use of large diameter piles suited the interface with the circular piers. Typically diameters of 1 . Mm or 2. Mm were used for the piers supported on 2. Mm diameter piles, and 2. Mm for piers supported on 2. Mm diameter piles. The connection detail between the pile and pier was constructed like that for a pile cap; the pile was broken down and pier starter bars introduced, making allowance for piling tolerances.
Figure 8 shows a typical monopole foundation under construction with the column tarter bars in place and Figure 9 illustrates the alternative cranked reinforcement connection details that were adopted depending on the relative sizes of pier and pile and the percentage reinforcement content. Conservative calculation method previously approved. Pile shaft resistances were calculated using the following relationships depending on soil type and pile construction 1. Spot-N + 6 kappa (after Decorate) for soils xx(USC design value)O. (after Ghana and Einstein) for rock, with k = 0. 35 for polymer modified water supported pile shafts and k = 0. 25 for Benton supported pile shafts as minimum values. . Design methodology for LULLS and SSL The piles needed to be large enough to resist the significant moments that are generated from lateral seismic loading (see Sections 6 and 7) and from out-balance forces from the deck due to horizontal alignment curvature, wind loading, eccentric train loads and other effects.
These moments increase down the length of the pile towards a peak at the effective point of fixity and reinforcement was provided and curtailed to suit the specific force and moment envelopes generated from a range of load cases for each foundation. Critical to the use of monopole foundations was the ululation of pile length. For foundations with only a single pile, these needed to be suitably conservative as there is clearly no potential for load distribution between adjacent piles as is possible within a pile group.
The original design basis proposed an allowable pile working load (Callow) as the sum of the shaft resistance (Sq) divided by a factor of safety of 2 and the end bearing resistance (CB) divided by a factor of safety of 3. The final resistance calculation method, as agreed with the Engineer, uses Sq / 3 and ignored end bearing due to the potential problems that could arise if a monopole ere to bear into a local void in the weak sandstone or mudstone layers.
Initial pile testing indicated that twice the calculated ultimate shaft resistance and 50% of the calculated ultimate end bearing resistance would give good agreement to the test results and thus potentially a more refined Callow = Sq + CB / 2. 5 might have been adopted, but the design was completed using the more The usual BBS 5400 load combinations from 1 to 5 were assessed to determine critical design load effects. In addition, a sixth load combination was added to cover seismic loading (see Section 6).
Other specific load cases noninsured included temporary loading from gantries (Section 5) and vehicular collision. The design of the viaducts was based on BBS 5400:Part 44 and associated British Standards, with additional International Standards used to supplement the scope in such areas as seismic loading and detailing, and rail dynamic factors. The American Concrete Institute technical design standard IAC 358. 1 R-925 was used to determine the dynamic factors to be applied to the vertical train loading for deck longitudinal design for the continuous spans.
For the simply supported spans, the dynamic factors were derived from bespoke dynamic analyses for the respective span lengths. For transverse design, the recommended dynamic factors of BBS 5400:Part 26 for RL loading of 1. 2 to 1. 4 were verified again using a finite element dynamic analysis. In accordance with the IAC code, the dynamic impact factors were not applied to the design of viaduct foundations, but were included in the pier head and bearing design. The maximum operating speed of the trains is intended to be 90 kip; the maximum design speed was taken as 100 kip. . Design during construction – gantry loading Typically, class CO/40 was used for the pier and pile cap concrete. Class CO/40 concrete was also used for the piles, but to account for possible weakening during the placement (under Benton or polymer modified suspension fluid) the cube strength design value used was reduced by 10 Amp. All reinforcement used in the substructure design was high yield, type 2 deformed bars with a yield stress of 460 Amp. The aggressive ground conditions meant that durability considerations were paramount.
As a result, additional waterproofing was applied to the top 5 meters of all piles to improve resistance to chloride attack and pile cover to reinforcement of 120 mm was used to improve assistance to sulfate attack. Generally crack widths (under combination 1 loads) reinforcement was also a critical design consideration in some locations (see Section 8). The majority of the simply supported decks and 2-span continuous decks were constructed by overhead gantries (illustrated in Figures 10 and 11).
The temporary loading from the various gantries used on the scheme was defined by the temporary works subcontractor, FRR (a consortium comprising VS., Freestones and Arizona De Cheer), appointed to undertake the deck construction. These loading regimes were continually developed throughout he design programmer, as various configurations of gantry were developed to cater for the many permutations of span configurations and access restrictions on site. The gantry loads included the effects of the most severe loading configuration carrying deck precepts elements and also the unloaded case, when the gantry was potentially subject to stronger winds.
In some locations, the gantries were also required to travel over previously constructed (by the balanced cantilever method) 3-span continuous decks and the temporary effects of these conditions needed to be designed for. Precepts deck segments were mostly levered to their required location at ground level, but in some locations, where access was more difficult, some segments were delivered over the previously constructed deck using special transporters. The additional load effects from these cases on permanent works also needed to be considered in the detailed design.
The construction programmer called for the initial design of over 1200 unique foundations in the first 9 months, to take the viaduct construction off the critical path. This was achieved through automation of the bulk of the design process and the use of conservative simplifying assumptions in the early stages of design. As the team got ahead of the programmer, the conservatism was removed from the process and more refined calculation methods introduced into the automated procedures to optimism the designs for the foundations yet to be constructed.
For the substructure, the temporary construction load cases were generally not governing for the pier and pile designs as they were typically less onerous than the seismic design effects. The design of the pier heads however, was discussed in Section 9 below. The design programmer capitalized on the locations of the UK-based structures team and the Dubbed-based alignment team, which handled setting-out and local issues such as utility diversions.
Advantage was taken of the staggered weekends between Dubbed and the I-J; the alignment of a given section was distributed at the start of the I-J week and the appropriate design data added and sent back to Dubbed at the end of the I-J week. Coupled with the automation process developed, this allowed a peak output of 100 bespoke designs per week to be achieved. Figure 11 Use of overhead gantries to construct typical viaduct sections Figure 10 typical viaduct sections 6 6.
Seismic design methodology and detailing The onset of plasticity controls the magnitude of forces that can be transmitted to the rest of the structure, which is particularly beneficial in the event of an extreme earthquake whose magnitude exceeds the design value. However, over-strength of the hinge zone in bending could lead to greater forces being attracted than expected. For such cases, it is important that the piers have adequate shear strength to enable a ductile response to develop, rather than permit a brittle shear failure to occur.