Prof Mahboob Ali Ch STEEL BRIDGES Bridges are

Prof. Mahboob Ali Ch. STEEL BRIDGES Bridges are the structures that allow movement of highway and railway traffic over natural or artificial gaps in the topology of the area such as canals, rivers, gap between hills and difference of level in crossing roads etc.

Prof. Mahboob Ali Ch. • • Selection of type of bridge mainly depends on: local conditions, availability and cost of materials, volume of traffic, site requirements, geographical conditions, aesthetics and expected economic return, etc.

Prof. Mahboob Ali Ch. • • The design of bridges is further influenced by: the required clearances, erection possibilities, foundation choices and hydraulic characteristics of the stream, if one is involved. For example, a longer span may become economical in case the piers are very expensive to construct.

Prof. Mahboob Ali Ch. There are many structural differences between a building and a bridge, some of these are: 1. Bridges are designed for heavy and concentrated moving loads whereas buildings are usually designed for static distributed loads. 2. The impact of moving loads is quite considerable as compared with residential and official buildings.

Prof. Mahboob Ali Ch. 3. Fatigue may become a problem and hence may reduce the strength due to large number of loading cycles. 4. Greater part of the structure is exposed to atmosphere. 5. The controlling design specifications for bridges are provided by organizations different from those dealing with the building design. For example, AASHTO Specification may be employed for bridges in place of AISC Specification for steel buildings.

Prof. Mahboob Ali Ch. Steel bridges are classified depending on their use into the following categories: 1. Foot or pedestrian bridge used to carry pedestrian traffic, bicycles or small hand driven carts. 2. Highway bridges. 3. Railway bridges. 4. Combined highway and railway bridges. 5. A bridge that enables one form of land communication over the other (called Over. Bridge).

Prof. Mahboob Ali Ch. Deck of Bridge • A deck is the actual carriageway of the bridge. • It consists of concrete or orthotropic slab and wearing surface. • Stringers and floor beams are also present for larger decks in addition to the slab (Figure 9. 1). • In case of longer bridges, the deck is supported over the longitudinal main members.

Prof. Mahboob Ali Ch. Stringer Floor Beam Figure 9. 1. Typical Steel Deck Supporting Elements.

Prof. Mahboob Ali Ch. • Depending upon the position of the longitudinal supporting elements with respect to the deck, the bridges may be deck type or through type. • A Deck Bridge is a bridge built at or near the top level of the main supporting members of the superstructure, which hang below the deck and are not visible from the bridge. • In case of Through Bridge, the carriageway is supported at the bottom of the main supporting members that are visible while traveling on the bridge.

Prof. Mahboob Ali Ch. TYPES OF STEEL BRIDGES Truss Through Bridges are the bridges where the deck is supported on the lower chord of the truss. • The upper chord of the two longitudinal side trusses is braced. • The traffic moves through the two trusses and the top transverse bracing (shown in Figure 9. 2).

Prof. Mahboob Ali Ch. Top Lateral Truss Sway Frame Truss Stringer Floor Beam Figure 9. 2. Typical Truss Through Bridge.

Prof. Mahboob Ali Ch.

Prof. Mahboob Ali Ch. • A Fixed Bridge permanently remains in one position. • One or more parts of a Movable Bridge are made movable to allow the vessels to pass through the stream underneath in case sufficient clearance is not available. The bridges are made movable in the horizontal and the vertical planes.

Prof. Mahboob Ali Ch. Figure 9. 10. Line Diagram of a Vertically Movable Bridge.

Prof. Mahboob Ali Ch. • The main supporting elements of a steel bridge may be rolled beams, plate girders, trusses and beams with suspension cables. • Rolled beams in non-composite bridges may be used for spans up to 28 m. • Composite rolled beams may be used for spans from 15 to 38 m. • Plate girders may be used for spans from 25 to 45 m. • Box girders are economical for spans from 45 to 75 m.

Prof. Mahboob Ali Ch. • Simply supported trusses are used for spans from 45 to 180 m. • Continuous trusses are preferred for spans ranging from 75 to 240 m. • For simple spans of 45 to 60 m, Warren or Pratt trusses are common. • For spans of 55 to 110 m, Curved Chord Pratt trusses (also called Camel Back or Parker trusses) are used. • For spans up to 180 m, Baltimore and Ktype trusses are preferred.

Prof. Mahboob Ali Ch. • Semi-Through Bridge is a type of truss bridge in which no bracing is provided above the traffic connecting top chords of the two opposite trusses. • If the roadway lies between the top and bottom chords and no room is left for the overhead bracing, the bridge is said to be a half-through or pony bridge.

Prof. Mahboob Ali Ch. • Slab Bridge is the one consisting of a simple one-way reinforced concrete slab without any beams or trusses. These are used to cover short openings in the road topology. • Beam and Slab Bridges consist of reinforced concrete slab supported over longitudinal steel beams (Figure 9. 3). These beams may either be present at the two edges only or more may be provided in the central width of the deck. This type may be used for spans up to 28 m.

Prof. Mahboob Ali Ch. Figure 9. 3. Cast-in-Place or Precast Concrete Slab Supported by Steel Beams.

Prof. Mahboob Ali Ch. If the steel beams in the above type are continuously connected to the slab by providing shear studs or the top flange of the beam is cast within the concrete, Composite Beam Bridge is obtained. This type may be economical up to a span of 38 m (Figure 9. 4).

Prof. Mahboob Ali Ch. Figure 9. 4. Composite Steel Beam Bridges.

Prof. Mahboob Ali Ch. Plate Girder Bridges are used for spans of 25 to 45 m. The deck in these bridges is supported on two longitudinal plate girders present on the sides of the roadway. If the plate girders are provided below the deck, the bridge is called plate girder deck bridge (Figure 9. 5).

Prof. Mahboob Ali Ch. Figure 9. 5. A Typical Plate Girder Deck Bridge.

Prof. Mahboob Ali Ch. In case of plate girder through bridge, the traffic moves on the deck supported at the lower flange or at a certain depth of the main member. Figure 9. 6. A Plate Girder Through Bridge.

Prof. Mahboob Ali Ch. In case of Orthotropic Deck Bridges, an orthotropic deck consisting of longitudinal folded steel plate resting on cross girders, provided at a spacing of 3 to 5 m. The cavities of the plate are filled with tar and gravel and topped by wearing surface (Figure 9. 7). Wearing Surface Folded Steel Plate Tar Plus Gravel Figure 9. 7. An Orthotropic Deck with Steel Folded Plate.

Prof. Mahboob Ali Ch. Box Girder Bridge is used for curved and longer span bridges (Figure 9. 8). These bridges decrease the total depth requirement and can resist torsion to a large extent. Hybrid Girder Bridges are those plate girder or box girder bridges where high strength steel is used for flanges and ordinary steel is employed for the web of the supporting elements.

Prof. Mahboob Ali Ch. Figure 9. 8. Cast-in-Place Concrete Slab over Closed Steel or Precast Concrete Boxes.

Prof. Mahboob Ali Ch. For spans in excess of 160 m, Suspension or Cable Stayed Bridges may become economical. Line diagrams of two types of these bridges are shown in Figure 9. 9. Suspension and Cable Stayed Bridges.

Prof. Mahboob Ali Ch. Prestressed Steel Bridges are those plate girder bridges in which the high bending moment sections are prestressed by high strength steel tendons in a direction opposite to the applied loading. Figure 9. 11. Example of a Prestressed Continuous Bridge Girder.

Prof. Mahboob Ali Ch. ADVANTAGES OF STEEL BEAM BRIDGES 1. Steel is a high quality, homogeneous and isotropic material that is perfectly elastic up to its yield point. 2. It has equal and high strengths in tension and compression. 3. The material remains uncracked and exhibits appreciable ductility. 4. Lesser construction time, compared with reinforced and prestressed concrete bridges, reduces the overall cost. 5. The basic skeleton of steel bridges may very easily be erected over various gaps in natural surface.

Prof. Mahboob Ali Ch. 6. The design, erection and fabrication procedures for steel bridges are very well established. 7. Due to lesser self-weight of these bridges, the foundation cost is also reduced. 8. For their lesser depths, the steel bridges are preferred where underneath clearance is important. 9. Repair, rehabilitation and upgradation of steel bridges are usually easier than concrete bridges.

Prof. Mahboob Ali Ch. General Terms • Stringers: These are longitudinal bridge deck beams spanning between the transverse floor beams and placed parallel to the roadway (refer to Figure 9. 1). • Floor Beams: Floor beams are the main girders of the bridge deck spanning between trusses or plate girders and running perpendicular to the roadway (refer to Figure 9. 1).

Prof. Mahboob Ali Ch. • Core width is defined as the width of the monolithic deck without the overhangs. • Equivalent Strip is defined as an isolated predefined width of the deck slab in the longitudinal or transverse direction This when designed individually represents the full design of the deck and the same design is used throughout. • Footprint is the specified wheel contact area over the roadway.

Prof. Mahboob Ali Ch. • Force Effect is defined as a deformation, stress or stress resultant caused by the applied loads, imposed deformations or volumetric changes. • Lever Rule means the statical summation of moments about any point to calculate the reaction at some other point. • Skew Angle is defined as the angle between the centerline of a bridge support and a line normal to the roadway centerline. • Two closely spaced and interconnected axles of equal weight are together called a Tandem.

Prof. Mahboob Ali Ch. Concrete Slab Haunch: A layer of concrete is usually projected below the slab surface to surround top flange of the steel beam, called concrete haunch. This haunch provides lateral support to the top flange of the steel beam and thus prevents lateral buckling of the beam. Haunch Figure 9. 12. A Concrete Haunch.

Prof. Mahboob Ali Ch. Diaphragm: This is a single steel member or a frame used to connect the longitudinal steel beams of a bridge, provided at the required interval. Part of a typical cross frame is shown in Figure 9. 13. o o o Figure 9. 13. Cross Frame.

Prof. Mahboob Ali Ch. Following requirements must be satisfied for the bridge diaphragms: • Rolled beams and plate girders used for bridges must be reinforced in perpendicular direction by cross frames or diaphragms at each support and at spacing not exceeding 7. 5 m. • For rolled beams, the minimum depth of diaphragms is equal to one-third the beam depth with a preferable value equal to half the beam depth. For deeper plate girders, the diaphragms may consist of cross frames.

Prof. Mahboob Ali Ch. End diaphragms must be proportioned to transmit all of the lateral forces to the bearings. The horizontal wind loads are transferred to the diaphragms by the lateral wind bracings. Force in the transverse diaphragms, FD = 1. 14 W SD where W = wind loading along the exterior flange (N/m) SD = diaphragm spacing (m)

Prof. Mahboob Ali Ch. Assuming the cross frame at the interior pier of a multiple span bridge with wind load equal to 2000 N/m 2, spacing of piers equal to 30 m and total height of girder, deck and side guard equal to 3 m, the force in the transverse cross frame should be as follows: FD = 1. 14 2000 3 30 / 1000 = 205. 2 k. N

Prof. Mahboob Ali Ch. This force may be equally divided into the four angles of the cross frame and area of the angle section may be calculated for the force while the required minimum radius of gyration of the angle is checked for the length. The section may be selected accordingly for the two requirements.

Prof. Mahboob Ali Ch. Design Lane • The design lane has a width equal to the lesser of 3600 mm or width of the traffic lane. • Roadway widths from 6000 to 7200 mm shall have two design lanes, each equal to one-half the roadway width. • The number of design lanes is taken as the integer part of the result when the clear roadway width in mm between curbs is divided by 3600.

Prof. Mahboob Ali Ch. If the design lanes are more than one, reduction factor of Table 9. 1 is applied on the live load force effect called Multiple Presence Factor denoted by m.

Prof. Mahboob Ali Ch. Table 9. 1. Multiple Presence Factors. Number of Loaded Lanes Multiple Presence Factor m 1 1. 20 2 1. 00 3 0. 85 >3 0. 65

Prof. Mahboob Ali Ch. Design Vehicular Live Load • H 20 means a highway truck with two axles and weighing 20 tons. • HS 20 means a highway truck similar to H 20 truck but having a semi-trailer with one additional axle. • H 15 and HS 15 are defined in a similar way. • The new specification uses HL 93 (highway loading of 1993).

Prof. Mahboob Ali Ch. • In case of HL 93 loading, the vehicular live load on the bridge roadway consists of a combination of design truck (or design tandem) and the design lane load. • The loads shall occupy a width of 3000 mm transversely within a design lane • All design lanes must be loaded simultaneously by the truck or tandem and the lane loads.

Prof. Mahboob Ali Ch. Design Truck (HL-93) • A standard truck consists of front axle of 35 k. N, rear truck axle of 145 k. N at 4. 3 m spacing from the front axle and trailer axle of 145 k. N having a variable spacing of 4. 3 to 9. 0 m from the truck rear axle. • The spacing of trailer axle producing the maximum force effect must be used.

Prof. Mahboob Ali Ch. 35 k. N 4. 3 m 145 k. N 4. 3 to 9. 0 m 145 k. N Fig. 9. 14 a. Longitudinal View of HL-93 Design Truck Showing Axle Loads.

Prof. Mahboob Ali Ch. 1. 8 m Design Lane 3. 6 m 0. 6 m in general 0. 3 m for deck overhang Fig. 9. 14 b. Back View of Truck Showing Transverse Clearances. Figure 9. 14. AASHTO Standard Truck Loading.

Prof. Mahboob Ali Ch. • Dynamic load allowance of 33 % is to be applied on these loads. • The design truck or tandem shall be placed transversely at 300 mm from the face of curb or railing for the design of bridge overhang and 600 mm from edge of the design lane for the design of all other components.

Prof. Mahboob Ali Ch. • For both negative moment between points of dead load contraflexure and reaction at interior piers, 90 % of the effect of two design trucks spaced 15 m between the front axle of one truck and trailer axle of the other may be considered. • The distance between the two 145 k. N axles of both the trucks must be taken equal to 4. 3 m. • A simultaneous action of 90 % of design lane must also be included.

Prof. Mahboob Ali Ch. Design Tandem (HL-93) • The design tandem shall consist of a pair of 110 k. N axles at a longitudinal spacing of 1200 mm with the transverse center-to-center spacing of the wheels being 1800 mm. • Dynamic load allowance of 33 % is to be applied on these loads. • For negative moment and reaction at the interior supports, pair of tandem may be considered at a spacing of 8 to 12 m.

Prof. Mahboob Ali Ch. Design Lane Load (HL-93) • The design lane load shall be 9. 3 k. N/m along the length, having a width of 3000 mm. • The load intensity becomes 3100 N/m 2. • Dynamic load allowance is not to be applied on lane loading.

Prof. Mahboob Ali Ch. Pedestrian Loads • A pedestrian load of 3600 N/m 2 is used on all sidewalks simultaneously with the vehicular design live load. • Separate bridges for pedestrian and bicycle traffic should be designed for a live load of 4100 N/m 2. • The dynamic load allowance is not considered for these loads.

Prof. Mahboob Ali Ch. Pakistan Code Of Practice Loading For Highway Bridges (1967) The highway loading according to the Pakistan Code of Practice for Highway Bridges consists of Class A, Class B and Class AA loadings.

Prof. Mahboob Ali Ch. Table 21. 2. Load of Trucks/Tanks. Standard Truck/Train Weight of Truck/Tank W (k. N) Class A 275 Class B 165 Military Tank 700

Prof. Mahboob Ali Ch. Table 21. 3. Ground Contact Dimensions. Axle Load Longitudinal Tire Contact Length BL Transverse Tires Contact Width BT (k. N) (mm) 110. 4 255 510 66. 4, 69. 0 205 380 27. 6 150 205 41. 5 150 305 16. 6 125 180

Prof. Mahboob Ali Ch. Table 21. 4. Design Transverse Spacing Between Trucks (J). Clear Road Width Rw (m) Distance - J For Most Critical Design Condition (mm) 5. 0 or less 0 5. 0 to 5. 5 800 (Rw - 5) 5. 5 to 7. 3 400 + 450 (Rw - 5. 5) Above 7. 3 1210

Prof. Mahboob Ali Ch. 1. 2 0. 40 W EACH 19. 8 1. 0 3. 2 0. 10 W EACH 1. 2 0. 40 W EACH 4. 3 3. 0 0. 25 W 19. 8 1. 0 0. 10 W EACH 0. 25 W AXLE LOADS AND DISTANCES FOR CLASS A & B TRUCKS (All distances are in meters) J BL WHEEL IN LONGITUDINAL VIEW 150 + BT/2 1830 J + BT BT TRANSVERSE POSITION OF TRUCKS (All distances are in mm) Fig. 21. 25. Class A and B Loadings.

Prof. Mahboob Ali Ch. 3. 66 m 7. 3 m 91. 0 m MIN. LONGITUDINAL TANK VIEW

Prof. Mahboob Ali Ch. W = 700 k. N W/2 840 mm W/2 1220 mm 840 mm PLAN VIEW OF TANK Fig. 21. 26. Class AA (Tank) Loading. 3660 mm

Prof. Mahboob Ali Ch. W = 250 k. N W/3 2/3 W 5. 62 m FRONT AXLE 0. 43 W 4. 85 m REAR AXLE 0. 43 W 4. 14 m TRAILER AXLES Fig. 21. 27. Axle Loads For Mercedes Benz Truck.

Prof. Mahboob Ali Ch. EMPIRICAL DESIGN OF CONCRETE DECK SLAB • This design method is only applicable for concrete deck slabs supported by longitudinal beams. • The concept of design is based on internal arching developed by a complex internal inplane or membrane forces.

Prof. Mahboob Ali Ch. Conditions For Empirical Slab Design • The supporting beams are of steel or concrete, made composite with the deck. • The design depth of the slab should not be lesser than effective length divided by 18 but should not be more than effective length divided by 6. For monolithic slabs, effective length is the face-toface distance of the beams or walls. For slabs supported on steel or concrete girders, the effective length is taken as the distance between the flange tips plus the flange overhang from the web and not any fillet.

Prof. Mahboob Ali Ch. • The depth should be uniform except any local thickening. • The effective length as defined above should not exceed 4100 mm and the slab depth should not be less than 175 mm. • The core depth of the slab must not be lesser than 100 mm. The core depth of the slab is defined as the distance from the top edge of the top-most bars to the bottom edge of the bottom-most bars.

Prof. Mahboob Ali Ch. • The specified 28 -day strength of the deck concrete is not less than 28. 0 MPa. • There should be an overhang beyond the centerline of the exterior girder equal to a minimum of 5 times the depth of slab or 3 times depth of slab with vertical barrier. • A minimum of two shear connectors shall be placed at a spacing of 600 mm C/C between the steel girders and the deck slab. For concrete girders, stirrups extending into the slab satisfy this requirement.

Prof. Mahboob Ali Ch. Empirical Slab Reinforcement • Isotropic bottom layer steel should have a minimum area of 0. 570 mm 2/mm width of slab. • Isotropic top layer steel should have a minimum area of 0. 380 mm 2/mm width of slab. • The outermost layer of steel along the depth of the slab is to be placed in the direction of the effective length.

Prof. Mahboob Ali Ch. • Maximum spacing of steel should be 450 mm. • Minimum yield strength of steel is to be 400 MPa (Grade 400 steel). • The minimum depth of the concrete slab to be used as a deck should not be less than 175 mm. (NHA suggests a minimum value of 220 mm). The wearing surface is separately provided.

Prof. Mahboob Ali Ch. DISTRIBUTION OF LIVE LOAD • The truck loads on the bridge deck are moving at different locations along the width and length of the slab. • When these loads occupy certain critical positions, maximum forces occur in the members. • For approximate design of the deck, usually onedimensional analysis is carried out considering only the girder. • In such cases, it becomes very important to find the effect of loads along the lateral direction of the member.

Prof. Mahboob Ali Ch. • The position and contribution of various loads along the lateral direction to analyze 1 -D members for an actual 2 -D structure and to find the design forces is called distribution of the live loads. • Estimating contribution of the transversely placed loads (with respect to direction of traffic movement) over the centerline of a particular member, spanning along the length of the bridge, is called lateral distribution of loads. • During the lateral distribution, the combined action of all the structural members in resisting the applied loads is considered.

Prof. Mahboob Ali Ch. • Thus, by the lateral distribution, equivalent loads are obtained at the members having the transverse load effect included in them. • These equivalent loads are then placed along the length of the member according to the criteria of maximum forces in case of moving loads. • Maximum force effects are then obtained from this longitudinal distribution of the loads.

Prof. Mahboob Ali Ch. • However, for beams or slab strips placed transverse to the traffic direction, longitudinal distribution of loads is to be performed first to get the equivalent loads. • These equivalent loads are then placed transversely at suitable locations to get extreme forces. • This method of performing manual 1 -D analysis is called Approximate Method of Analysis.

Prof. Mahboob Ali Ch. Conditions For Approximate Method • Spacing of beams, S, should be between 1. 1 and 4. 9 m. • Thickness of deck slab, ts, should be between 110 and 300 mm. • Length of beam should be between 6. 0 and 73. 0 m. • Number of longitudinal beams in the cross-section, Nb, should be greater than or equal to 4.

Prof. Mahboob Ali Ch. • The deck cross-section should be one of the standard types given in the AASHTO Specification. • The width of deck should be constant. • Multiple presence factor is not to be applied when the using the given expressions. However, it is always to be considered if the lever rule is used to the find the force effects. • If beam spacing exceeds 4. 9 m, the live load on each beam shall be the reaction of the loaded lanes based on the lever rule.

Prof. Mahboob Ali Ch. • Beams should be parallel and should have approximately the same stiffness. • The roadway part of the overhang, de, does not exceed 910 mm. • The curvature in plan is less than the specified AASHTO limit. • The given expressions are only applicable to concrete deck on steel or concrete beams.

Prof. Mahboob Ali Ch. Notation Used S eg S = spacing of beams or webs (mm) L = span of beam (mm) Nb = number of beams, stringers or girders ts = depth of concrete slab (mm) n = modular ratio between beam and deck materials I = moment of inertia of beam (mm 4) = distance between the centers of the basic beam and deck (mm), considered zero for non-composite beams

Prof. Mahboob Ali Ch. A Kg = = Kg/L ts 3 = g de area of stringer, beam or girder , longitudinal stiffness parameter (moment of inertia of one beam modified to equivalent concrete section and transferred to a point at the center of the slab) a parameter proportional to the ratio of beam stiffness to total slab stiffness in transverse direction at the level of the slab centerline = distribution factor, and = distance between the center of exterior beam and the interior edge of curb or traffic barrier mm). It shall be taken positive if the exterior web is inside the curb and negative when it is outside the curb.

Prof. Mahboob Ali Ch. To be continued in the next file……