Civil Engineering and Construction: Millau Viaduct - Case Study Example

Table of Contents Table of Contents 1.0 Millau Viaduct 3 1 Introduction 3 2 Aesthetics 4 2 Fulfilments of Function 4 2.2 Bridge Proportions 4 1.2.3 Structure Order 4 1.2.4 Design Refinements 5 1.2.5 Environmental Integration 5 1.2.6 Surface Texture 5 1.2.7 Character 6 1.2.8 Complexity in Variety 6 1.3.0 Loadings 6 1.3.1 Dead Loads 7 1.3.2 Super Imposed Dead Loads 7 1.3.3 Live Traffic Loads 7 1.4.0 Formwork 8 2.0 Student Project 9 2.1 Introduction 9 2.2 Piers 9 2.3 The Deck 9 2.5 Pylons 9 2.6 The Cables 10 2.7 Materials 10 3.0 Static Longitudinal Scheme and Winds Studies 10 3.1 Static Longitudinal Scheme 10 3.2 Wind Studies 11 Reference List 13 1.0 Millau Viaduct 1.1 Introduction The Millau Viaduct provides a missing link in the A75 auto route connecting Paris to Barcelona. Before construction of the viaducts, traffic descended the Tarn Valley. This caused a bottleneck in the town of Millau particularly during summer months of July and August. Millau Bridge is a multi-span cable stayed bridge that passes over the Tarn Valley but at the lowest points between the two plateaus. This made it become the tallest bridge piers standing at 242m with structure rising to 343m at the top of the pylon. It is also the world’s longest multi-span cable stayed bridge of a total length of 2460m. The bridge has a slight gradient of 3% from North to South and a slight curve of approximately 20000m radius. The pylons are made of steel and the piers of post tensioned concrete (Saxton, 2007). 1.2 Aesthetics The aesthetic value of the structure is categorized in ten different areas. 1.2.1 Fulfilments of Function The concrete piers are huge and portray the magnitude of construction and the task of building across the Tarn Valley. This shows how the bridge is supported and the purpose of the strength, and rigidity of the piers. The bridge shows the load path as well as its members. 1.2.2 Bridge Proportions A perpendicular look at the bridge shows that the pylons and abutments have identical width at the deck. The abutments and pylons split and taper out and meet at the deck. A parallel look along the structure reveals a different case with the concrete abutments being wider than the steel pylons. The tapering out of the piers creates good proportion with the outside of piers remaining constant resulting to an increase in the distance between the two halves with the rise. The joint between the piers and the deck is well proportioned particularly with the wind barrier giving increase depth to the deck. 1.2.3 Structure Order Despite being the tallest bridge in the world, the bridge looks simple. Repetition of the pylons across the bridge appears easy by sight at the constant height at which the piers split. The span between the piers is equal just as the effective spans between cables. A good continuity is reflected in the way the piers and pylons flow as one upon reaching the deck. 1.2.4 Design Refinements The aspect ratio of rectangles between the ground, piers, and deck must be kept constant when crossing a valley. This is not the case for the bridge since it would need differing spans between the piers resulting to different number of cables supporting the deck between each pier. Such a scenario would not make the bridge look good. It would also mean that the pylons differ in height for constant spacing of cable anchorage at the deck. A longitudinal look along the bridge gives an appearance of piers of equal thickness to pylons. The piers have hexagonal shape to produce the effect of leading face of hexagon (Saxton, 2007). 1.2.5 Environmental Integration Cable stayed bridge spanning across the entire valley should be integrated with the environment. Careful considerations were made to make the bridge look as natural as possible. Morning mist and low lying clouds hide the concrete piers giving the impression that the deck delicately floats on them. This natural feature was taken into consideration in the design phase of the structure to produce this effect. 1.2.6 Surface Texture The piers for the bridge are roughly finished with the deck and pylons having a smooth finish. This adds aesthetic value to the structure especially on a sunny day. The pylons sparkle and glisten making the bridge more noticeable. The rough texture reveals the material used whereas the concrete left untouched and the steel give glossy white finish. The rough appearance of the concrete gives the piers an organic appearance and sense of rigidity and strength to assure users. The steel deck, cables, and pylons have smooth texture giving a floating appearance in the midst of the clouds. 1.2.7 Character The design of the bridge is fairly unique from the norm that each pylon has stays anchoring the pylons to the ground on each side and support the deck on the other. For Millau, the cables on each side of the pylons support the deck. The complicated design and construction of the bridge produced a plane curvature on a radius of 20km. This adds character to the structure. The curvature enables motorists see how the bridge works as they drive along it. 1.2.8 Complexity in Variety The aspect of complexity has been put at a minimum in the bridge. There is no structural confusion and it is clear to denote the role of each component at 342m interval and 204m at the end spans. Additional support is provided by cables attached to steel pylons situated above concrete piers. The cables are fixed at equal distance over certain area of the pylon not at the top or equally spaced in the entire height of the pylons (Saxton, 2007). The curve is measured to perfection with apparent complexity since none of the cables seem to cross therefore avoiding unnecessary confusion. 1.3.0 Loadings The first study of the bridge was done according to French standards with the final design of the structure complying with the French standards as specified in the contract document. Eurocode 3 was applied in the temporary steel support and steel deck design to check for instability. This implies that the loadings used in the actual design process are likely to differ with the ones discussed. The discussion focuses on loadings according to BS 5400. Despite the basic loads applied to all bridges, the geometry and design for the structure leads to other loads and effects that must be considered. The constant curvature of the bridge introduces horizontal centrifugal loading and single plane of cables need consideration to establish torsion effects (Saxton, 2007). 1.3.1 Dead Loads In the structure, the dead load is the steel deck, cornice, and windscreen. The fixings for the cables and the pylons are also considered as dead loads. 1.3.2 Super Imposed Dead Loads The black top surfacing, concrete, steel crash barriers, handrails and all drainage are super imposed loads. They are permanent but can be removed. They were added after the dead load for the structure had been completed (Saxton, 2007). 1.3.3 Live Traffic Loads The bridge has two lanes of traffic and narrow hard shoulder located on each side. The carriage way is 23.3 m wide including steel crash barriers on the outside. This makes the carriage way for the structure to be 19-22.8 m in width. Two types of loading, HA and HB are placed at the adverse locations. The construction phase of the bridge ensured that the pylons were wheeled out into location on crawlers with the weight of the convoy stipulated at 8MN. This represented a load of 4MN per crawler. The BS5400 stipulates that the total loading per HB vehicle is 1.8MN, spread over 4axles with each wheel comprised of 112.5kN per wheel. HA and HB loading act vertically in the form of UDL’s, Knife Edge Loads (KEL) and point loads. Centrifugal loading is also generated by the curvature of the bridge. This is calculated by the following equation; Since the curvature of the bridge is on a radius of 20,000m, the horizontal force is 1.49kN. This gives a very low figure that could not be considered in the design. Bracking from trucks cause horizontal loading on the deck exerting a force of 8kN/m assumed acting along one notational lane. An additional point load of 200kN and are all associated with HA and HB loading. Accidental loading causes point loads of 250kN. This acts in any direction in any of the notational lanes and also applies associated HA loading (Saxton, 2007). 1.4.0 Formwork The piers were poured in 4m lifts with the use of climbing form system called automatic rail climbing system (ACS). The formwork shutter and work platforms were integral. Hydraulic Rams pushed the shutter and carriage up the rail. The Rails ran through shoes fixed to concrete as the Rams lifted the rail up when the carriage fixed in its position. This form of formwork ensured pure repetition of procedure with each climb hence simplifying high-rise concrete construction leading to tremendous speed of turn round. 2.0 Student Project 2.1 Introduction This project entails a 1:10 scale three span version of Millau Viaduct that links two raised plateaus separated by 3.4m deep and 18m wide dry river valley that is flat bottomed with retained vertical sides. Its construction involves two tapered concrete piers on the foundations and concrete bank-seats on the edge of each plateau. The two 10 by 1.5 m deck steel structures are assembled and launched to meet at the centre of the structure. The deck is made of two steelwork with cables, four steel bearing plates and scaffold hand railing (Saxton, 2007). 2.2 Piers 2.3 The Deck A trapezoidal profiled metal box girder with an upper orthotropic decking made of sheet metals on the greater part of the main spans would serve best. Resistance to fatigue is achieved by a 14mm thick for the whole length of the structure. The thickness is increased towards the pylons. Trapezoidal stiffeners 7mm thick provide longitudinal stiffening of the upper orthotropic decking. The bottom side box of the girders require sloping base plates of 12mm steel on the span and 14-16mm around the pylons. 2.5 Pylons The pylons are set on the deck longitudinally and transversely. Longitudinal setting ensures continuity between the metal sheets of the webs that are central to box girder and the walls of the pylon legs. Traversal setting provides rigidity by the frame that covers the bearings on each pier shaft. The legs of the pylon 38m high are made of stiffened metal box girders. The pylons are surmounted by a 49m high mast onto which the cables are anchored (Saxton, 2007). 2.6 The Cables The cables are anchored along the axis of central reservation at regular intervals following the curvature of the structure. They support each span and arranged in a single plane in a half-fan pattern. The cables can be made of T 15 strands of class 1860MPa, supergalvanised, sheathed and waxed. The cables can be protected by an aerodynamic sheath made of non-injected PEHD. This protects the cables from UV light and combat vibration from combined effects of wind and rain (Saxton, 2007). 2.7 Materials Steel of grade S355 and S460 are preferred for the decks and pylons. The piers are made of concrete of class B60 (Saxton, 2007). 3.0 Static Longitudinal Scheme and Winds Studies 3.1 Static Longitudinal Scheme Just like the Millau Viaduct, flexibility of adjoining spans ensures that restraint is not provided. The head of the pylon is defected towards the span that is loaded. This implies that the pylons and the piers contribute to resistance of the bridge to longitudinal bending. When the deck is fixed to the piers and pylons, rigidity of the structure is increased whereas the vertical movement is reduced in the loaded span as forces are transmitted to adjacent spans are also reduced. In the design phase, the dimension of the deck in relation to resistance and deformality is linked to flexibility of the piers and the pylons. This implies that with flexible pies and pylons, it is necessary that the design of the deck should be rigid and thick. On the other hand, when the piers and pylons are rigid, the deck should have reduced inertia and less thickness in design. Millau Viaduct’s case considered the scale of effects due to wind. This led to the adoption of reduced thickness with rigid piers and pylons (Saxton, 2007). For this particular case, flexible piers and pylons with a rigid and thick deck is the best. This is so because fixing of deck to piers that are inflexible poses challenges in relation to temperature variations and variations arising from creep and shrinkage for a concrete deck. 3.2 Wind Studies Since the structure is high above the valley, stresses generated by wind are critical and need consideration so that the dimensions of the structure can be developed. Complete studies in the wind tunnel are required. The wind tunnel studies will enable understanding characteristics of wind at site, determination of the wind model, aerodynamic behaviour of various elements of the structure exposed to the wind such as the piers, deck, pylons, and temporary piers, establishment of aerodynamic admittances in bending and drag from aeroelastic trial on the model of the structure during construction phase, determination of torsional admittances of deck from trial on cross-sectional model, and calculation of stresses and movements in structure. The wind tunnel will also assist determine the safety coefficients from the calculations based on extreme conditions during construction and in service (Saxton, J. L. (2007). Reference List Saxton, J. L. (2007). Report on the Millau Viaduct, Proceedings of Bridge Engineering 2 Conference 2007, 27th April 2007, University of Bath, Bath; U.K. Read More