Bridge SubstructureAbutments MAB1053 Bridge Engineering Faculty of Civil Engineering, UTM azlanfka/utm05/mab1053 1 Subtructures Substructures may be classified as ‘end supports’ or ‘intermediate supports’, according to their position along a bridge. End supports can be abutment walls with associated wing walls for closed side spans, and either skeleton abutments or bank seats for bridges with open side spans. Intermediate supports are the piers and columns in all bridges with more than one span. azlanfka/utm05/mab1053 2 Bridge Abutments Current practice is to make decks integral with the abutments. The objective is to avoid the use of joints over abutments and piers. Expansion joints are prone to leak and allow the ingress of corrosion agents into the bridge deck and substructure. In general all bridges are made continuous over intermediate supports and decks under 6m long with skews not exceeding 30° are made integral with their abutments. azlanfka/utm05/mab1053 3 Bridge Abutments Usually the narrow bridge is cheaper in the open abutment form and the wide bridge is cheaper in the solid abutment form. The exact transition point between the two types depends very much on the geometry and the site of the particular bridge. 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. However the wider bridges with solid abutments produce a tunneling effect and costs have to be considered in conjunction with the proper functioning of the structure where fast traffic is passing beneath. azlanfka/utm05/mab1053 4 Bridge Abutments Solid abutments for narrow bridges should only be adopted where the open abutment solution is not possible. In the case of wide bridges the open abutment solution is to be preferred, but there are many cases where economy must be the overriding consideration. azlanfka/utm05/mab1053 5 Open Abutments A bridge constructed at existing ground level to span across a road in cutting may need only nominal bank seats if good foundation strata are available at shallow depths. This may give rise to problems where negative reactions are likely to develop. azlanfka/utm05/mab1053 6 Open Abutments Spill-through or skeleton abutments are suitable where spread footings are needed at a level well below a bank seat. It is often advantageous to design a footing to offset the foundations in relation to the bearings, because the permanent horizontal loading shifts the reaction. 7 azlanfka/utm05/mab1053 Various Types of Open Abutments azlanfka/utm05/mab1053 8 Piled Foundation Where load-bearing strata are at considerable depth below the bank seat level, piled foundations have to be used. azlanfka/utm05/mab1053 9 Wall Abutments Mass Concrete Cantilever Mass concrete is economic for small heights, such as where headroom is less than that needed for vehicular traffic. Cantilever is simple to form but demanding high concentration of reinforcement in the stem as height increases 10 azlanfka/utm05/mab1053 Wall Abutments Counterfort Stub Counterfort Counterfort and Stub Counterfort abutments. Reduces weight of reinforcement compared with cantilever, but calls for more complex shuttering. azlanfka/utm05/mab1053 11 Hollow Abutment For high abutments on sloping ground, this construction offers advantages over heavy counterfort construction. azlanfka/utm05/mab1053 12 Other Types of Wall Abutments azlanfka/utm05/mab1053 13 Choice of Abutments Wall Abutments These are normally designed as a reinforced concrete cantilever fixed along the base slab. Strutted abutments may be used for square bridges up to 12m span, where advantage is taken of the propping action of the deck to relieve the foundation pressure under the toe of the footing. Backfilling to these walls is generally selected granular material and earth pressures are often assessed on the basis of an equivalent fluid density. Typical details : a) Wall height – from 5m to 9m b) Wall thickness – 0.7m to 1.1m azlanfka/utm05/mab1053 14 Choice of Abutments Skeleton Abutments This type of end support consists of transverse cill beam across one or more buried columns carrying the loads down to a base. It can be used where the road over a bridge is on embankment and a suitable foundation can be obtained near the previous existing ground level. Typical details : Columns spaced at 3.5m center and directly under deck bearings where possible to avoid large bending moments in the cill beam. Columns placed at ends of the cill beam since wing walls are cantilevered horizontally from each end. The rear face of a column is usually vertical and the front face battered at 1:6 since each column is designed to act as a vertical cantilever from the continuous based slab and horizontal loads have a large effect. azlanfka/utm05/mab1053 15 Choice of Abutments Bank Seats If the road over a bridge is at or near to existing ground level, then a bank seat may be sited at ground level after either a s a simple base or carried on piles. A bank seat carried on piles driven through fill is usually preferable to a skeleton abutment carried on piles at a lower level. The height of a bank seat is often only 2-4 metres so that it is possible to employ mass concrete wall sections. Where the foundation level is above the level of a nearby open surface, a slip circle analysis should be made to check the stability of the bank slope. azlanfka/utm05/mab1053 16 Choice of Abutments Wing Walls These walls are included at all end supports in order to contain the immediate areas of back-fill. There are two basic types to be considered and the choice is normally made on purely structural or economic reasons. Horizontal cantilevered wall – this type is very economic since it requires a minimum amount of material and saves on excavation for additional footings. Vertical cantilever free-standing wall – this type is similar to a normal retaining wall except that horizontal cantilever extensions are often used. They are suitable beyond the lengths and skew angles at which horizontal cantilevered walls become unpractical. The main disadvantage is the large height of these walls and the amount of buried structure which causes the cost to become excessive. azlanfka/utm05/mab1053 17 Wing Walls azlanfka/utm05/mab1053 18 Wing Walls azlanfka/utm05/mab1053 19 Wing Walls azlanfka/utm05/mab1053 20 Modes of Failure The stability of an abutment should be checked for several modes of failure : Sliding failure Overturning Foundation yield Slip Circle Structural failure azlanfka/utm05/mab1053 21 Abutments – Modes of Failure Sliding Failure azlanfka/utm05/mab1053 22 Abutments – Modes of Failure Sliding Failure Resisted by friction in granular soils or adhesion in cohesive soils, aided by the passive resistance of the soil in front of the toe. If public utilities are to install services in front of the wall, the location or depth of the trenches may invalidate the passive resistance. Sliding resistance can be increased by incorporating a heel below the foundations. Factor of safety = 2.0 considering passive resistance. JKR use f.o.s = 1.5 not considering passive resistance. azlanfka/utm05/mab1053 23 Abutments – Modes of Failure Foundation Yield azlanfka/utm05/mab1053 Overturning 24 Abutments – Modes of Failure Foundation yield (bearing failure) – produces similar effect to overturning Overturning – In practice overturning is usually associated with some yielding of the foundation, since this produces very high pressures under the front of the footing. azlanfka/utm05/mab1053 25 Abutments – Modes of Failure Slip Circle Structural Failure azlanfka/utm05/mab1053 26 Abutments – Modes of Failure Slip Circle – Only a problem in cohesive soils. Structural failure – Failure can occur in the stem of the footing if an inadequate section is provided (design fault). Factor of safety for reinforcement is provided in code. Substructure : nominal f.o.s. = 1.0 (piles). Use partial safety factors for material. azlanfka/utm05/mab1053 27 Basic Components of Abutment azlanfka/utm05/mab1053 28 Forces on an Abutment azlanfka/utm05/mab1053 29 Forces on an Abutment Dead load due to the superstructure. Proper dead load include self-weight of beams and deck. Superimposed dead load include premix, surfacing, services and railings etc. Live load on the superstructure. BS 5400 – HA UDL and HD KEL BS 5400 – HB (45 units) abnormal vehicle load JKR Standard – special vehicle (SV) azlanfka/utm05/mab1053 30 Forces on an Abutment Self-weight of the abutment – Components of the abutment include main body, wing walls and approach slab. Traction force – Horizontal forces due to braking and acceleration of vehicles. BS 5400 specifies maximum traction force. JKR puts a maximum value of 253 kN. azlanfka/utm05/mab1053 31 Forces on an Abutment Temperature variations – Expansion and contraction due to temperature variation will induce force in the substructure. Substantial movements occur in steel bridges. The temperature induced movements or deflections give rise to forces which will be transferred to the abutments. Creep and shrinkage – These are time dependent properties of concrete. For both creep and shrinkage, it is assumed (JKR practice) that about 50% occurs after 3 months and about ¾ has taken place after 6 months. 32 azlanfka/utm05/mab1053 Forces on an Abutment Earth pressures – The equivalent fluid concept (Rankine’s or Coulomb’s theory) is normally used for calculating the earth pressures on an abutment, but the selection of the appropriate intensity depends on the degree of restraint offered by the wall and the particular calculation being considered. In a situation where a wall can move by tilting or sliding and the backfill is a free draining granular material, active pressures are assumed. A common design approach is to use an equivalent fluid pressure of 5H kN/m2, where the active coefficient, Ka is normally 0.25. Modern compaction technique for placing the backfill material and the use of more rigid type of construction have caused many designers to estimate design pressures for the at-rest condition. The value of the earth pressure coefficient at-rest, Ko is often taken to be 1.5-2.0 times the active coefficient, Ka. azlanfka/utm05/mab1053 33 Forces on an Abutment Surcharge pressure – The effect of HA and HB loadings on the carriageway behind the abutment is arbitrarily treated as an additional surcharge loading. The nominal values suggested in BS 5400 for live load surcharge are 10kN/m2 for HA loading and 20kN/m2 for HB loading. The weight of granular material is assumed to be 19kN/m3. azlanfka/utm05/mab1053 34 Forces on an Abutment Wind loading – must be considered only for bridges with spans greater than 20m. A typical value for wind speed of 40 mph is assumed for 30m span. Seismic loading – There was only one case so far in 1960 of medium size disturbance. Long span bridges such as Penang Bridge include seismic loading consideration in the design. 35 azlanfka/utm05/mab1053 Forces on the Abutment azlanfka/utm05/mab1053 36 Abutment (Load Case 1) WA Self Weight during construction azlanfka/utm05/mab1053 37 Abutment (Load Case 2) DL + HA 1/3 PSHB Tr + Fstc + W Pa WA azlanfka/utm05/mab1053 38 Abutment (Load Case 3) DL + HB 1/3 PSHB Tr + Fstc + W Pa WA azlanfka/utm05/mab1053 39 Abutment (Load Case 4) DL Fstc WA azlanfka/utm05/mab1053 40 Design Standards for Abutments British Standards BS 5400: Part 2: Specification for Loads BS 5400: Part 4: Code of Practice for the Design of Concrete Bridges BS 8002: Code of Practice for Earth Retaining Structures BS 8006: Strengthened/Reinforced Soils and Other Fills azlanfka/utm05/mab1053 41 Design Standards for Abutments Design Manuals BD30: Backfilled Retaining Walls and Bridge Abutments BD37: Loads for Highway Bridges BA41: The Design and Appearance of Bridges BA42: The Design of Integral Bridges BD42: Design of Embedded Retaining Walls and Bridge Abutments BD57 and BA57: Design for Durability BD70: Strengthened/Reinforced Soils and Other Fills for Retaining Walls and Bridge Abutments azlanfka/utm05/mab1053 42 Basic Design Considerations Cantilever Wall Abutment azlanfka/utm05/mab1053 43 Cantilever Retaining Wall The CONCRETE CANTILEVER RETAINING WALL is constructed of reinforced concrete and it supports backfill soil by a cantilever action. The cantilevered stem portion is fixed at the bottom and is free at the top. The base slab serves as a fixed support and prevents against sliding and overturning. There is an option to install a key at the bottom of the base slab to ensure further safety against sliding. These walls provide prolonged durability and serviceability. They are widely used due to their ease in construction and cost-effectiveness. azlanfka/utm05/mab1053 44 Cantilever Retaining Wall azlanfka/utm05/mab1053 45 Analysis & Design of Cantilever Retaining Wall Stability Analysis Design of Concrete Members azlanfka/utm05/mab1053 46 Modes of Failure Overturning Sliding/Translation Bearing capacity Bending or shear failure of stem Bending or shear failure of heel Bending or shear failure of toe Bending or shear failure of key azlanfka/utm05/mab1053 47 Design Considerations The design of the wall must: Resist sliding along its base Resist overturning Not exceed the bearing capacity of the soil beneath the base Avoid excessive settlement. Built structurally strong to resist failure from the build up of internal stresses produced by external forces azlanfka/utm05/mab1053 48 Forces and Pressures on Retaining Walls The basic objective is to apply the conditions for static equilibrium, which are: 1. All the forces in the horizontal direction must add to zero. 2. All the forces in the vertical direction must add to zero. 3. The clockwise moments (or torques) must equal the counter-clockwise moments. azlanfka/utm05/mab1053 49 Forces on Cantilever Wall azlanfka/utm05/mab1053 50 Lateral Earth Pressures Lateral earth pressure is normally calculated based on Rankine or Coulomb’s theories. Lateral earth pressure is assumed distributed triangularly. The location of resultant is at 1/3 of height. If there is surcharge, lateral earth pressure from surcharge is distributed uniformly. The resultant is at ½ of height. The lateral earth pressure is calculated at the edge of heel. azlanfka/utm05/mab1053 51 Lateral Earth Pressures Ka.wH Pa = 1/2Ka.γH2 H/2 H/3 Ka.w Due to surcharge Ka.γH Due to backfill soil 52 azlanfka/utm05/mab1053 Pressure Coefficients The Rankine active earth pressure coefficient Ka for the specific condition of a horizontal backfill surface is calculated as follows: Ka = (1 – sin(φ)) / (1 + sin(φ)) φ is the angle of internal friction of soil backfill. The equation is modified if the backfill surface is sloped. azlanfka/utm05/mab1053 53 Stability Analysis 1. Check factor of safety against overturning. 2. Check soil bearing pressure. 3. Check factor of safety against sliding. azlanfka/utm05/mab1053 54 Overturning The rotating point for overturning is normally assumed at bottom of toe. The height of soil used to calculate lateral earth pressure should be from top of earth to the bottom of footing. Elements that resist overturning are weight of stem, weight of footing, weight of soil above footing. If there is a surcharge, the weight of surcharge can also be considered. The factor of safety against overturning is resisting moment divided by overturning moment. Acceptable factor of safety is between 1.5 to 2. azlanfka/utm05/mab1053 55 Factor of Safety for Overturning Overturning moment is calculated from : The resisting moment is calculated as : Where γ is unit weight of soil, Ka is active pressure coefficient, and H is the height from top of earth to bottom of footing, q is surcharge. where Ws,Wf,We,Wk,Wq are weight of stem, footing, earth, key and surcharge, Xs,Xf,Xe,Xk,Xq are distances from the center of stem, footing, earth, key, and surcharge to the rotation point at toe. 56 azlanfka/utm05/mab1053 Factor of Safety for Overturning The factor of safety against overturning is determined from : FoS = Resisting Moment = MR Overturning Moment Mo FoS should be > 1.5 azlanfka/utm05/mab1053 57 Bearing Pressure The centre of the total weight from the edge of toe is Where W is total weight of retaining wall including stem, footing, earth and surcharge. The eccentricity, e = B/2-X, where B is width of base footing. azlanfka/utm05/mab1053 58 Checking for Bearing Pressure Σ W e Eccentricity, e = B/2 –X B/2 X Either, e ≤ B/6 or e > B/6 azlanfka/utm05/mab1053 59 Bearing Pressure If e ≤ B/6, the maximum and minimum footing pressure is calculated as: Where, Qmax, Qmin are maximum and minimum footing pressure, B is the width of footing. 60 azlanfka/utm05/mab1053 Bearing Pressure If e > B/6, Qmin is zero, Qmax should be less than allowable soil bearing capacity of footing soil. azlanfka/utm05/mab1053 61 Sliding The driving force that causes retaining wall to slide is the lateral earth pressure from soil and surcharge. The forces that resist sliding are passive pressure at toe, the friction at the base of the footing; and the passive pressure against the key if used. The factor of safety against sliding is the total resisting force divided by total driving force. Acceptable factor of safety is between 1.5 to 2. azlanfka/utm05/mab1053 62 Factor of Safety for Sliding The driving force for sliding is calculated as The friction resisting force at the base of footing is calculated as where µ is friction coefficient between concrete and soil. µ is often taken as tan(2/3 φ). φ is internal friction of the soil. 63 azlanfka/utm05/mab1053 Factor of Safety for Sliding The passive resistance (if any) at the toe of retaining wall is calculated as Where Kp is passive earth pressure coefficient, h is the height from top of soil to bottom of footing at toe. If a key is used to help resist sliding, h is the height from top of soil to the bottom of the key. azlanfka/utm05/mab1053 64 Factor of Safety for Sliding The factor of safety is calculated as Resisting Force, ΣF > Sliding Force, μΣW azlanfka/utm05/mab1053 65 Forces on the Abutment azlanfka/utm05/mab1053 66 Design of RC Members 1. 2. 3. 4. 5. 6. 7. Check thickness of stem for shear stress. Design stem reinforcement for bending. Check thickness of heel for shear stress. Design heel reinforcement. Check shear stress for toe when the toe is long. Design toe reinforcement for bending. Check shear stress in key when key is deep and narrow. 8. Design key reinforcement for bending. azlanfka/utm05/mab1053 67 Design of Stem azlanfka/utm05/mab1053 68 Design of Heel eu ≤ B/6 eu > B/6 azlanfka/utm05/mab1053 69 Design of Toe azlanfka/utm05/mab1053 70