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Bs5400 DesignUploaded by Mahesh PokhrelRelated InterestsDeep FoundationBearing (Mechanical)Prestressed ConcreteStructural LoadConcreteRating and Stats0.0 (1)Document ActionsDownloadShare or Embed DocumentEmbedView MoreCopyright: Attribution Non-Commercial (BY-NC)List price: $0.00Download as DOCX, PDF, TXT or read online from ScribdFlag for inappropriate contentClause 6.2
α2 = 0.0137[bL(40-L)+3.65(L-20)] α2 = 0.0137[3.65(40-34.0)+3.65(34.0-20)] = 1.0 Note: For loaded lengths less than 20m the load is proportioned to a standard lane width of 3.65m, i.e. 0.274bL = bL/3.65. For a metre width of deck : W = (31.6 x 1.0)/3.65 = 8.66 kN/m KEL = (120 x 1.0)/3.65 = 32.88 kN
γfL = 1.50 (Ultimate limit state - combination 1) Design HA loading for a metre width of deck : W = 1.5 x 8.66 = 12.99 kN/m KEL = 1.5 x 32.88 = 49.32 kN Maximum mid span Bending Moment with KEL at mid span = Mult Mult = (12.99 x 342)/8 + (49.32 x 34)/4 Mult = 1877 + 419 = 2300 kNm Note: Use of γf3 BS 5400 Pt.3 & Pt.5 - γf3 is used with the design strength so Mult = 2300 kNm. BS 5400 Pt.4 - γf3 is used with the load effect so Mult = 1.1 x 2300 = 2530 kNm.
Span Deck Type Up to 20m Insitu reinforced concrete. If the bridge is to cross a road that is on a curve. Prestressed pre-tensioned box beams with insitu topping. Headroom requirements have to be maintained below the deck. 30m to 40m Prestressed pre-tensioned SY beams with insitu slab. etc) Rivers and streams (liability to flood) Existing property and rights of way Access to site for construction traffic
Selection of Bridge Type The following table is intended to be a rough guide to the useful span ranges of various types of deck.3. Existing services (Gas. Prestressed post-tensioned beams with insitu slab. Prestressed post-tensioned beams with insitu slab. Prestressed pre-tensioned inverted T beams with insitu fill. 16m to 30m
Insitu reinforced concrete voided slab. Electricity. Steel beams with insitu slab.2(1) quotes clearances from roadway surfacing to the underside of the deck to avoid impact damage.preliminary design stage. It is important to determine the condition of the bridge site by carrying out a comprehensive site investigation. Prestressed pre-tensioned Y and U beams with insitu slab. The preliminary design usually settles the appearance of the bridge. 30m to 250m Box girder bridges .As the span increases the construction tends to go from 'all concrete' to 'steel box / concrete deck' to 'all steel'. iv.for spans up to 50m they are generally less economic than plate girders. Water. Prestressed pre-tensioned box beams with insitu topping. then the width of the opening may have to be increased to provide an adequate site line for vehicles on the curved road. 150m to 350m
. The Eurocode Standard (EN 19911-7 clause 4. the sizes of individual members are finalised at detailed design stage. EN 1997-2: 'Ground investigation and testing' covers the requirements for the Soil Survey. Truss bridges . iii. The economic implications of raising or lowering any approach embankments should then be considered. Insitu prestressed post-tensioned concrete voided slab. Constraints The construction depth available should be evaluated. Steel beams with insitu slab. Insitu prestressed post-tensioned concrete. Other topics which need to be considered are: i. By lowering the embankments the cost of the earthworks may be reduced. the minimum standards for UK Highway bridges are given in TD 27 of the Design Manual for Roads and Bridges. ii. but the resulting reduction in the construction depth may cause the deck to be more expensive.
useful when headroom is restricted or access is difficult. Increases sensitivity to differential settlement. Reduces number of expansion joints. Suspension bridges.
Costing and Final Selection The preliminary design process will produce several apparently viable schemes. iii. Factory made units i. and will be true if the structure is not particularly unusual
Beam and Slab Solid slab bridge decks are most useful for small.Cable stayed bridges. iii. v. Voided slab and beam and slab bridges are used for larger. The shortest structure is not always the cheapest. The structure should be considered as a whole.
4. A span to depth ratio of 20 will give a starting point for estimating construction depths. but the deck costs will increase. By increasing the length of the structure the embankment. iv. Specials tend to be expensive. Alternative designs for piled foundations should be investigated. In circular voided
. Reduces the need for soffit shuttering or scaffolding. including appraisal of piers. The procedure from this point is to: i. Reduces site work which is weather dependent. ii. single or multi-span bridges. Substructure i. ii. piling can increase the cost of a structure by up to 20%. Special permission needed to transport units of more than 29m long on the highway. Obtain prices for the schemes. retaining wall and abutment costs may be reduced. abutments and foundations. single or multi-span bridges and are easily adaptable for high skew. Continuity over supports i.they need not be up to date but should reflect any differential variations.
The final selection will be based on cost and aesthetics. Apply unit price rates . 350m to ?
3. iii. Preliminary Design Considerations 1. This method of costing assumes that the scheme with the minimum volume will be the cheapest. Reduces maximum bending moments and hence construction depth or the material used. Estimate the major quantities. ii. Length of structure i. 2.
5. Dependent on delivery dates by specialist manufactures.
. Skew decks develop twisting moments in the slab which become more significant with higher skew angles. it is sometimes convenient to split the deck into two halves longitudinally along the centre line. My and Mxy where Mxy represents the twisting moment in the slab. this is then continued to the footing. However these directions vary over the slab and two directions have to be chosen in which the reinforcing bars should lie. Extensive tests on various steel arrangements have shown the best positions as follows
Foundation types depend primarily on the depth and safe bearing pressures of the bearing stratum.
Design Considerations The design of foundations comprise of the following stages : i. and the maximum area of void should be less than 49% of the deck sectional area. My and Mxy. From the site investigation report decide upon which stratum to impose the structure load and its safe bearing pressure. ii. one for each pier and abutment. the most economical way of reinforcing the slab would be to place the reinforcing steel in the direction of the principal moments. Bridge foundations generally fall into two categories: i. Computer analysis will produce values for Mx. whereas multi-span continuous decks 10 mm is usually considered as a maximum. Strip footings. Piled foundations. For skews greater than 25° then a grillage or finite element method of analysis will be required. However.79. Generally in the case of simply supported bridge decks differential settlements of about 20 to 25 mm can be tolerated. also restrictions placed on differential settlement due to the type of bridge deck. Wood and Armer have developed equations for the moment of resistance to be provided in two predetermined directions in order to resist the applied moments Mx. Due to the influence of this twisting moment. piers being piled with abutments on strip footings).decks the ratio of [depth of void] / [depth of slab] should be less than 0.
It is possible to have a combination of both (i.e. Analysis of Deck For decks with skew less than 25° a simple unit strip method of analysis is generally satisfactory.
The working load of an individual pile is based on providing an adequate factor of safety against the soil under the toe failing in shear and the adhesion
. Cover to reinforcement should never be less than values given in BS 5400: Part 4: Table 13. Bored and Cast In-Situ Piles. Driven type piles can. formed by driving a hollow steel tube with a closed end and filling the tube with concrete. The art of selecting the right sort of pile lies in rejecting all those types which are obviously unsuited to the particular set of circumstances and then choosing from those which remain. depending on the strata. none of them however give the most economic and satisfactory solution under all conditions. Bored piles are generally end bearing and are often of large diameter. the one which produces the most economical solution. Preformed Driven Cast In-Situ Piles.8 to 1. In most all cases the rejection of conventional pad or strip foundations arises because the computed settlement is more than the structure can safely withstand and hence the main purpose of the piled foundation will be to reduce this settlement. the ideal solution is where piles support the load wholly in end bearing on hard rock where the settlement will be negligible.ii. c. the remoulding effect of driven piles may well increase the settlement of the soil under its own dead weight and thus increase the settlement of the foundation itself. Choice of pile type depends largely on the strata which they pass through. It follows that piles wholly embedded in the same soil that would under-lie a conventional foundation has very little effect in reducing settlement. d.0 m but must be capable of withstanding moments and shears produced by piers or abutments. However. therefore. that if more compressible strata exists within reasonable distance of the surface. be either end bearing or friction piles. The critical shearing stress may be assumed to occur on a plane at a distance equal to the effective depth of the base from the face of the column. no shear failure in the soil and no excessive settlement.
Aspects of design of piled foundations which influence choice of pile type All foundations must satisfy two criteria. Ensure that the factor of safety against shear failure in the soil is not reached and settlement is within the allowable limits. it is very desirable that a high proportion of the foundation load should be carried by this more stable strata. With soft normally consolidated alluvial clays. but the second presents more of a problem. sometimes a combination of both. To increase their bearing capacity the bottom can be under-reamed to produce a greater bearing area. preformed piles of concrete or steel driven by blows of a power hammer or jacked into the ground. The thickness of the footings is generally about 0. A specialist form of pile consisting of stone aggregate consolidated by water or air using the 'Vibroflotation' technique is suitable in some granular soils.
Strip Footings The overall size of strip footings is determined by considering the effects of vertical and rotational loads. piled foundations also have to meet this criteria. and crack control calculation must be carried out to ensure the crack width is less than 0. It follows. There are well established methods for ensuring that the first criteria is met. formed by boring a hole and filling it with concrete. Driven Cast In-Situ Piles. and the problems of calculating the load carrying capacity and settlement require a different approach to that for bored piles. Cover to reinforcement will need to be increased to comply with BS 8500 requirements. possibly comparing the suitability of several types.
a. simultaneously withdrawing the tube. formed by driving a hollow steel tube with a closed end and filling the tube with concrete. Design the foundation to transfer and distribute the loads from the structure to the ground. iii. to c. are known as displacement piles. Driven Piles. The combination of these two must neither exceed the safe bearing capacity of the stratum or produce uplift. because this very often influences the choice.
Select the type of foundation. additional safety precautions are required with larger diameter piles. b.
Piled Foundations The type of piles generally used for bridge foundations are : a. Concurrently with the choice of pile type must go the choice of the strata which will carry the main loads from the structure.25mm (Table 1).
By means of test piles. then the horizontal movements will impose moments at the base. corresponding to a structural pin joint. Through soil parameters i. Whereas the pier has to be designed to resist major derailments. By means of dynamic formulae i. their magnitude will depend on the pier flexibility. A similar arrangement is often required by the rail authorities to prevent minor derailments striking the pier.e. It is the proportions and form of the bridge as a whole which are vitally important rather than the size of an individual element viewed in isolation. The objective is to avoid the use of joints over abutments and piers.e. ii. Sometimes special requirements are imposed by rail or river authorities if piers are positioned within their jurisdiction.
. The ultimate bearing capacity is usually modified to compensate for the driving effect of the pile. A slender bridge deck will usually look best when supported by slender piers without the need for a downstand crosshead beam. iii.
Wherever possible slender piers should be used so that there is sufficient flexibility to allow temperature.between the shaft and the soil surrounding it passing its ultimate value and the whole pile sinking further into the ground.
The overall configuration of the bridge will determine the combination of loads and movements that have to be designed for. Piling contractors 'know how'. Also if the pier should be completely demolished by a train derailment then the deck should not collapse. Hiley formulae which equates the energy required to drive the pile with its ultimate bearing capacity. ii. iii. or intermediate joints in the deck.
Current practice is to make decks integral with the abutments. For example if the pier has a bearing at its top. In general all bridges are made continuous over intermediate supports and decks under 60 metres long with skews not exceeding 30° are made integral with their abutments. There are basically four methods for assessing this effect : i. iv. Expansion joints are prone to leak and allow the ingress of de-icing salts into the bridge deck and substructure. iv. In the case of river authorities a 'cut water' may be required to assist the river flow.
Different Pier Shapes Design Considerations Loads transmitted by the bridge deck onto the pier are : i. Vertical loads from self weight of deck Vertical loads from live loading conditions Horizontal loads from temperature. creep movements etc and wind Rotations due to deflection of the bridge deck. shrinkage and creep effects to be transmitted to the abutments without the need for bearings at the piers. summing shaft friction and bearing capacity. or independent fenders to protect the pier from impact from boats or floating debris.
In addition to the structure loads. However. sliding (PTFE). iv. but there are many cases where economy must be the overriding consideration. The exact transition point between the two types depends very much on the geometry and the site of the particular bridge.
Design Considerations Loads transmitted by the bridge deck onto the abutment are : i. creep movements etc and wind Horizontal loads from braking and skidding effects of vehicles. In the case of wide bridges the open abutment solution is to be preferred.
These loads are carried by the bearings which are seated on the abutment bearing platform. on top of the abutment at carriageway level. or rolling. However the wider bridges with solid abutments produce a tunnelling effect and costs have to be considered in conjunction with the proper functioning of the structure where fast traffic is passing beneath. This entails the abutment wall being designed as a propped cantilever. the full braking effect is to be taken. in either direction.
. In highway bridge bearings movements are accommodated by the basic mechanisms of internal deformation (elastomeric). In most cases the open abutment solution has a better appearance and is less intrusive on the general flow of the ground contours and for these reasons is to be preferred. It is the cost of the wing walls when related to the deck costs which swings the balance of cost in favour of the solid abutment solution for wider bridges. horizontal pressures exerted by the fill material against the abutment walls is to be considered. ii. Solid abutments for narrow bridges should only be adopted where the open abutment solution is not possible. Also a vertical loading from the weight of the fill acts on the footing. For certain short single span structures it is possible to use the bridge deck to prop the two abutments apart.Solid Side Span with Full Height Abutments Usually the narrow bridge is cheaper in the open abutment form and the wide bridge is cheaper in the solid abutment form. iii. A large variety of bearings have evolved using various combinations of these mechanisms. The horizontal loads may be reduced by depending on the coefficient of friction of the bearings at the movement joint in the structure. Vehicle loads at the rear of the abutments are considered by applying a surcharge load on the rear of the wall.
Bridge bearings are devices for transferring loads and movements from the deck to the substructure and foundations. Vertical loads from self weight of deck Vertical loads from live loading conditions Horizontal loads from temperature.
transverse and vertical directions. The keys resist movement. but vertical loads only can generally be resisted. This allows the deck to expand and contract freely. but also resists loads in the longitudinal.Multiple Roller Bearing Design Considerations The functions of each bearing type are : a.
b. A 'Fixed' Bearing does not allow translational movement. and loads in a direction perpendicular to the keyway.
c. Longitudinal or transverse loads can be accommodated by providing mechanical keys. This bearing does not accommodate rotational movement in the longitudinal or transverse directions and only resists loads in the vertical direction. Bearings are arranged to allow the deck to expand and contract.
The designer has to assess the maximum and minimum loads that the deck will exert on the bearing together with the anticipated movements (translation and rotation). Bearing manufacturers will supply a suitable bearing to meet the designers requirements. polytetrafluoroethylene (PTFE). but retain the deck in its correct position on the substructure. Roller Large longitudinal movements can be accommodated by these bearings. Expansion joints are prone to leak and allow the ingress of de-icing salts into the bridge deck and substructure. Elastomeric The elastomeric bearing allows the deck to translate and rotate. Loads are developed. The objective is to avoid the use of joints over abutments and piers.
Current practice is to make decks integral with the abutments. Plane Sliding Sliding bearings usually consist of a low friction polymer. ' Sliding' Bearings are provided for vertical support to the deck only. sliding against a metal plate. or the deck and substructure have been designed to incorporate deck joints then the following guidance is given in BD 33/94 for the range of movements that can be accommodated by the various joint types:
. 'Sliding Guided' Bearings are provided to restrain the deck in all translational directions except in a radial direction from the fixed bearing. In general all bridges are made continuous over intermediate supports and decks under 60 metres long with skews not exceeding 30° are made integral with their abutments. Where it is intended not to use road salts. and movement is accommodated by distorting the elastomeric pad.
Buried joint under continuous surfacing.
6. Nosing with preformed compression seal.
. Elastomeric in metal runners. 5 * 3 5 * 3 7. Asphaltic Plug joint.
5. Nosing joint with poured sealant.3
2.LONGITUDINAL MOVEMENT Min (mm) Max (mm) MAXIMUM ACCEPTABLE VERTICAL MOVEMENT BETWEEN TWO SIDES OF JOINT (mm) 1. Cantilever comb or tooth joint.
5 20 1. Reinforced Elastomeric.
Thermal Movements BS 5400 Part 2 Chapter 5. The nominal effective temperature used in the calculations will also have to be specified to enable the correct adjustments to be made on site when the joints are set.The minimum of the range is given to indicate when the type of joint may not be economical. together with a list of suppliers can be obtained from theBridge Joint Association
. The width of joint between the end of the deck and the abutment is set during construction of the bridge. Having determined the range of movement at the joint then the type of joint can be specified. usually when the concrete curtain wall is cast. Hence if a maximum effective temperature of 40°C is calculated from BS 5400 Part 2 then a joint width will have to be provided at the end of the deck to allow for an expansion caused by a temperature increase of (40-2)=38°C.4 specifies maximum and minimum effective bridge temperatures which have to be accommodated in the bridge structure. The maximum expansion of the deck is therefore determined from the minimum effective temperature at which the curtain wall is allowed to to be cast. The maximum contraction of the deck is determined in a similar manner. Joint Manufacturers An overview of the various types of bridge joints. usually 2°C. but using a nominal effective temperature at which the joint is set. * Maximum value varies according to manufacturer or type.
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