CHAPTER 17 - SPECIALIST CONSTRUCTION SYSTEMS

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1 CHAPTER 17 - SPECIALIST CONSTRUCTION SYSTEMS WJ Martin 17.1 SCOPE Specialist construction systems refer to the construction systems falling outside of the normal reinforced and prestressed bridges that make up the bulk of bridges in South Africa. These structures generally fall outside of the experience of the average South African resident engineer. Specialist construction systems include: Bridges constructed using the Incremental Launch Method (ILM); Balanced Cantilever bridges (BC); Precast Segmental construction (PSC); Jacked bridges; Cable stayed bridges and suspension bridges; Concrete arch bridges; Steel bridges and composite steel bridges; Bridges constructed from timber, brick, stone or composite materials Although the other chapters in the Construction Monitoring Manual cover many aspects which are applicable to the above specialist construction systems, they do not cover all of the requirements on the construction site. This chapter deals very briefly with the specialist requirements but, as very few bridges of this nature are constructed in South Africa, this chapter cannot be considered as comprehensive. FOR THESE SPECIALIST CONSTRUCTION SYSTEMS, SPECIALIST LITERATURE SHOULD BE CONSULTED VOIDED SLAB DECKS Although voided slab decks are relatively common, problems do occur with the void formers. Because of this, it is discussed in more detail below. These comments also apply to any other formwork which may have resultant concrete pressures which are not horizontal Void formers In order to understand the need to have void formers that are strong and properly tied down, it is necessary understand the buoyant or hydrostatic properties of wet concrete. Wet concrete is about 2.5 times denser than water; therefore it is 2.5 times more buoyant than water. If you imagine the forces on a void former in water, then the forces of the same void former in concrete will be 2.5 times greater. When a void former is immersed in wet concrete, the hydrostatic forces trying to crush the void former are very large. Adequate stiffening inside the void former is required. Originally this used to be timber cross-bracing but recently polystyrene disks at frequent centres are used. The closures at the ends of void formers also need to withstand this hydrostatic pressure and be watertight. It is not adequate to have a few pop-rivets around the circumference there need to be quite a REHABILITATION AND REPAIR OF STRUCTURES 17-1

2 few closely spaced pop-rivets. If two void formers are butted up against each other, with a small gap, the hydrostatic pressure will be the same as if there was a large gap. Once again, the ends need to be adequately fixed. While concrete is being cast, the voids will want to float on top of the concrete. They must be tied down with sufficient ties to withstand this buoyancy. If concrete is cast higher on one side of a void than the other, as is inclined to happen, the void will want to move sideways as well. The ties must be able to resist this sideways movement. In summary, the site monitoring staff need to ensure that the contractor has considered the following: There is sufficient bracing inside the void formers; The ends of the void formers are firmly attached to the void formers; The ties holding the void formers down can resist the buoyancy forces; and The ties holding the void formers down will restrain lateral movement INCREMENTAL LAUNCH METHOD (ILM) ILM bridges have become more common recently. This section does not give guidance on construction monitoring of ILM bridges. It only highlights some major construction aspects that need to be monitored. When monitoring the construction of and ILM bridge, specialist literature must be referred to. Some of the major construction aspects include: Construction tolerances these are more critical than in normal construction. Special attention and accurate surveying is required to ensure millimetre accuracy. Temporary works: This is highly specialized Launching nose, temporary bearings, launching jacks, braking system, etc. Pier deflections: This must be limited to avoid damage to the piers and excessive movement when bearing friction is overcome. Launching nose and connection: This is a critical piece of equipment and needs to be properly fabricated and installed. If its stiffness varies from the design launching nose, it needs to be checked. Emergency stop equipment: When something goes wrong such as a bearing pad installed incorrectly, the launching must be stopped immediately to avoid damage or injury. Construction loads and superimposed dead loads (parapets, etc): do these comply with the design requirements? Cumulative deck length: allowance needs to be made for shortening due to prestressing, creep and shrinkage. Stability during construction: This is significant during the first few launchings. Lateral and longitudinal fixity between launches is also a consideration. Launching downhill will require braking and restraining devices. Launching bearing pads must resist rotation. Installation of the permanent bearings needs consideration. Concrete strength before prestressing and launching is an issue. REHABILITATION AND REPAIR OF STRUCTURES 17-2

3 Construction Tolerance For ILM construction, the accuracy of the sliding surfaces and alignment plays a primary role in creating a stress-free construction site. Many problems during launching can be traced back poor accuracy or poor alignment. Incorrect deck or segment lengths: If there is an error here, especially if the shortening due to prestressing, shrinkage and creep has not been taken into account, then the deck ends up being too short and the bearing diaphragms or bearing attachments are in the incorrect position. Allow for shortening due to prestressing, creep and shrinkage; and Check the length of the bridge between launches, starting from the front of the deck. Levels on sliding surfaces in casting yard not to specification: If the levels are out, this causes additional friction and makes it difficult to feed the launching bearing pads in correctly. Horizontal alignment is not to specification: If the horizontal alignment between segments is out, this causes part of the deck or all of the deck to curve inwards or outwards during launching. This leads to problems with feeding in the side guide sliding pads. This occurs with curved decks as it is much easier to align straight decks. It also leads to problems where at some of the piers, the deck can be out transversely by more than 70 mm, affecting the positioning of the temporary launching bearings and the permanent bearings. Vertical alignment is not to specification: This can change the bearing reactions during launching significantly. It may overload some bearings, causing cracking in the deck and damage of the sliding surfaces. It will also increase the horizontal loads which can damage piers. Casting yard foundations too elastic: This affects the vertical alignment as discussed above. Before a segment is cast, the sliding surface levels are checked in the casting yard and found to be within specification. However, when the segment is cast, the foundations deflect downwards and sometimes tilts, causing the vertical alignment to now be out of specification. Launching bearing pad thickness: When the bearing pads arrive on site, their thicknesses should be checked against the specifications. Furthermore, they should be sorted in terms of thickness to the nearest 0.2 mm so that on each pier, all of the pads used are of similar thickness. This will reduce friction and bearing overload and make it easier to feed the pads in. REHABILITATION AND REPAIR OF STRUCTURES 17-3

17.3.2 Launching nose The contractor needs to produce calculations showing that the launching nose is strong enough to withstand the launching forces including any additional forces due to

4 Launching nose The contractor needs to produce calculations showing that the launching nose is strong enough to withstand the launching forces including any additional forces due to lack-of-fit caused by the construction tolerances. The prestressing in the launching nose connection should be correctly stressed. Too little stress will lead to the connection opening up and increasing the load on the next pier. The prestressing force in the launching nose connection should be checked from time to time between launches. During launching when the bending force in the launching nose is approaching its maximum, the connection should be checked for the nose lifting off the deck Monitoring pier deflections If pier deflections are within specification, this means that the friction generated during launching falls within design limits. When pier deflections exceed specification, this means that something has gone wrong. It is therefore important that pier deflections be monitored continuously during launching. Photo 17.1: Zambezi River Bridge constructed using ILM REHABILITATION AND REPAIR OF STRUCTURES 17-4

17.4 BALANCED CANTILEVER CONSTRUCTION (BC) Very few balanced cantilever bridges have been constructed in South Africa. The Mfolozi River Bridge on the north coast of KZN is an example. Photo 17.

5 17.4 BALANCED CANTILEVER CONSTRUCTION (BC) Very few balanced cantilever bridges have been constructed in South Africa. The Mfolozi River Bridge on the north coast of KZN is an example. Photo 17.2: An abnormal load crossing the Mfolozi River Bridge Some issues needing specialist consideration include: During construction, the deck levels change continuously. Very careful calculation is required to ensure that the bridge reaches its required vertical alignment. Creep and shrinkage affects the vertical alignment and needs to be continually monitored on site. Constant correction is needed to ensured correct vertical alignment. The contractor s construction sequence and weight of temporary works will affect the alignment if these are different to the design assumptions. Stability during construction is an issue. Until the cantilevers are joined up, they are vulnerable, especially to wind loading Vertical alignment during construction As the deck is constructed, each load added or moved causes the deck to deflect. Creep and shrinkage increases the deflection. Once the joints between the cantilevers are closed and the deck made continuous, the deck continues to deflect due to creep and shrinkage. The objective is to get the vertical alignment correct for some stage after construction, usually when the bridge is opened to traffic or at some time after that. REHABILITATION AND REPAIR OF STRUCTURES 17-5

17.5 PRECAST SEGMENT CONSTRUCTION (PSC) One of the first PSC bridges in South Africa was the Buffalo River Bridge in East London. The Gautrain Viaducts were also constructed using this method.

6 17.5 PRECAST SEGMENT CONSTRUCTION (PSC) One of the first PSC bridges in South Africa was the Buffalo River Bridge in East London. The Gautrain Viaducts were also constructed using this method. Some issues needing specialist consideration include: Segments need to be match-cast and glued together. Temporary prestressing is required during the gluing process. Operation of the launching gantry is important to prevent overloading of the segments during prestressing. Prestressing across the construction joints need attention to ensure durability. Photo 17.3: Buffalo River Bridge at East London: One of the precast segments can be seen in the foreground REHABILITATION AND REPAIR OF STRUCTURES 17-6

7 17.6 JACKED BRIDGES It is becoming more common to construct jacked bridges under railway line embankments these days. Photo 17.4: Bridge on the N17 about to be jacked under the rail embankment Some issues that need specialist consideration during bridge jacking include: Railway tracks need to be supported so that rail traffic can continue during construction. Jacking forces need to overcome very high friction between the sides of the structure and the retained earth. These forces need to be resisted in some way. The foundations need to be constructed first through the embankment, usually by pipe-jacking. These form the sliding surface during construction. Stability of the front excavation face needs careful consideration REHABILITATION AND REPAIR OF STRUCTURES 17-7

VIADUCTS BY PUSHING: INCREMENTALLY LAUNCHED BRIDGES

Istanbul Bridge Conference August 11-13, 2014 Istanbul, Turkey VIADUCTS BY PUSHING: INCREMENTALLY LAUNCHED BRIDGES J. E. Erdoğan 1 and Ö. Özkul 2 ABSTRACT Despite its widespread use around the world, constructing