CHAPTER 2 INVESTIGATION FOR BRIDGE 2.1 ELEMENTS OF A BRIDGE PROJECT

School of Civil Eng., IoT., Hawassa University CHAPTER 2 INVESTIGATION FOR BRIDGE 2.1 ELEMENTS OF A BRIDGE PROJECT In general a bridge project can be considered to have three major stages. They are, 1. Investigation stage 2. Design stage & 3. Construction stage Unlike the building structure constructions, bridge projects require an intensive investigation based on the feasibility, requirement or necessity, population benefited, economic development expected, topography, hydraulic data and soil characteristics prior to the approval and design stages. After all such investigations being over, the design stage commences. The design stage, consists of mainly three elements; hydraulic design, geometric design and structural design. Hydraulic design accounts for calculation of flood discharge, scour action near the bridge supporting structures, characteristics of river channel to fix the level of the bridge, clear water way of the bridge and thus the bridge spans. Foundation depth based on hydraulic characteristics is also a point to be considered. In geometric design, vertical and horizontal alignment and curvatures required are to be established. Traffic flow characteristics, projected traffic over one or two decades are to be considered. Thus the geometric design concerns more with transportation engineering point of view. Structural design involves the selection of component types and providing an economical solution for the purpose intended based on strength and serviceability point of view. At the end of design stage estimations, drawings and approvals are vital roles to be performed. At the construction stage, one cannot start the construction of bridge all of a sudden without certain preparatory works. Apart from primary construction surveys, river training works, coffer dam construction, approaches for machinery and equipments, storage and security for materials are important elements of bridge project under construction stage. Material and manpower management are also vital tasks for construction managers at this stage. There are design specific and bridge type specific construction technologies that could be adopted at this stage (like slip form, cantilever form techniques etc.). 2.3 DESIGN OBJECTIVES The general objective of bridge design is to provide economic, viable and safe solution to cross an obstacle such as river, valley and other traffic flow, by means of proper selection of site, material, type, technology and design. Specific objectives can be listed as follows: 1. to provide economic, strong and durable design of bridge 2. to provide the shortest structure across the obstacle 3. to forecast and decide the expected traffic flow in the future decades to come and to finalize the structural dimensions 4. to study the hydraulic data and fix economic spans for the bridge superstructure 5. to include applicable load combinations to design the structural components with the help of appropriate design code DESIGN WORKING LIFE Concrete, stone and steel bridges shall be designed for 100 years working life. Concrete and Steel culverts with an opening or diameter less than 2.0 m and all timber bridges shall be designed for 50 years working life. __________________________________________________________________________ Fundamentals of bridge design – CEng 5121 – Chapter 1 Instructor: M. K. Chandrasekar 1 School of Civil Eng., IoT., Hawassa University 2.4 DESIGN PHILOSOPHY AND SPECIFICATIONS Design philosophies evolved for the design of bridge structures in the world based on how safely a bridge be constructed. Economy played a secondary role only as some of the bridges collapsed due to the nature’s effects and soil’s conditions and shown the importance of safety. The design philosophies reflected the engineers’ confidence while correlating analysis results to make the final design. Due to the development of new materials and their ways of usage with other materials as composites and advancements in realistic analysis techniques, the design philosophies also got changes over time. The general concept of structural design is that the resistance offered by the structure or structural components shall be greater than the demands put on them by the applied loads, i.e. Resistance offered by the components should be greater than Effects of loads applied. There are uncertainty in measuring the exact values of resistance offered by the components and as well as the effects of loads. The earlier formed philosophy of design by name the allowable stress design or working stress design did not consider the uncertainty associated with the loads. Also, the material uncertainty was not adjudged based on specific criteria but fixed based on safety aspect only, that too expecting a high range of safety. Currently the widely used design philosophy for bridge design is Load and Resistance Factor Design (LRFD) philosophy, which is based on Limit State theory. In the equation said above, i.e. Resistance offered by the components > Effects of loads applied, the variability should be accounted in both sides of inequality. In LRFD method, the resistance side is multiplied by a statistically based resistance factor φ, whose value is usually less than one, and the load side is multiplied by a statistically based load factor γ, whose value is usually greater than one. If the nominal resistance is given by Rn, the safety criterion is φRn ≥ effect of Σ (γi.Qi), where Qi refers to different load effects in a combination and γi refers to corresponding load factor. The Ethiopian Roads Authority (ERA) has produced bridge design manuals to assist the designers to follow certain design procedures using LRFD method. These Standards deal with small and medium sized structures and shall be used for all structures within the ERA throughout the country. These Standards are based mainly on the American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specifications, 2nd edition, 1998, with modifications to Ethiopian conditions, requirements and applicable laws of the Federal Democratic Republic of Ethiopia. As a supplement to these standards, for structures not mentioned here (such as pre-stressed bridges, large bridges, certain steel bridges, composite bridges, aluminum bridges, etc.), the AASHTO LRFD Specifications may be used but only together with Chapter 3: Load Requirements in these Specifications. The ERA Bridge Design Manual is written for the practicing engineer and describes current, mandated and recommended practice in selected aspects of bridge engineering. These are based on mathematical calculations, and the Ethiopian terrain, experience and research, and have the support of the ERA. 2.5 SITE SELECTION AND DATA COLLECTION Site selection It may not be possible always to have a wide choice of sites for a bridge. This is particularly so in case of bridge in urban areas and flyovers. For river bridges in rural areas, usually a wider choice may be available. For selection of a suitable site for a bridge, the investigating engineer should make a reconnaissance survey for about one km on the upstream side and one km on the downstream side of the proposed bridge site and should journey along the road for about one km on either side of the road from the bridge site in order to __________________________________________________________________________ Fundamentals of bridge design – CEng 5121 – Chapter 1 Instructor: M. K. Chandrasekar 2 School of Civil Eng., IoT., Hawassa University form the best suited and economical alignment of the road with the suitable bridge site by considering the topographical features and soil conditions. To the extent possible, it is desirable to align the bridge at right angles to the river, i.e. to provide a square crossing, which facilitates minimum span length, deck area and pier lengths, with accompanying economies. Further, a square crossing involves simpler designs and detailing. Sometimes, a skew crossing which is inclined to the center line of the river at an angle different from a right angle has to be provided in order to avoid costly land acquisition or sharp curves on the approaches. A skew bridge usually poses more difficulties in design, construction and maintenance. Following are the factors to be carefully considered while selecting the ideal site for a proposed bridge. 1. Connection with roads: The bridge site shall be such that, as far as possible, the roads leading to the bridge on either side may have a shorter component along the obstruction. The bed of approaches connecting ends of bridge with the roads should be dry and hard. The approaches at the ends of the bridge site should be such that they do not involve heavy expenditure. 2. Firm embankments: Firm high and solid embankments at the abutment sides could guard the bridge at the time of heavy floods and they do not allow the course of river to alter. 3. Foundations: The nature of the soil at the bridge site should be such that good, proper and economical foundation can be provided for the bridge 4. Material and labor: Material and labor shall be available at a least possible expense at the site. 5. Square crossing: Square crossing is preferred as the advantage has been stated earlier. 6. Straight stretch of river: Straight stretch of river ensures smooth and uniform flow of water without any whirling. This does not cause much disturbance during construction and also does not cause much problems of maintenance. 7. Velocity of flow of water: It is better if the bridge site is so selected that the velocity of water flow is to the acceptable limit to avoid scouring and silting. 8. Width of the river: It is quite evident that the width of river indicates length of the bridge. It is desirable to have minimum width of river at the bridge site. The smaller the width of river, the cheaper will be the bridge construction cost. Economic considerations to be made for bridge design Structural types, span lengths, and materials shall be selected with due consideration of projected cost. The cost of future expenditures during the projected service life of the bridge should be considered. Regional factors, such as availability of material, fabrication, location, shipping, and erection constraints, shall be considered. If data for the trends in labor and material cost fluctuation are available, the effect of such trends should be projected to the time the bridge will likely be constructed. Cost comparisons of structural alternatives should be based on long-range considerations, including inspection, maintenance, repair, and/or replacement. Lower first cost does not necessarily lead to lowest total cost. Data Collection Preliminary general data: 1. Index map: A map showing the proposed location of the bridge, the existing means of communication, the general topography of the area and the important towns and villages in the area. 2. A contour plan of the stream showing all the topographical features for a sufficient distance ‘d’ on either side of the site to check how it influences the location, design and approaches. i) d = 100m for a small bridge or culvert or when the catchment area is less than 3 sq.km ii) d = 300m for less important bridges or when the catchment area varies from 3 to 15 sq.km iii) d = 1500m for important bridge or when the catchment area is more than 15 sq.km __________________________________________________________________________ Fundamentals of bridge design – CEng 5121 – Chapter 1 Instructor: M. K. Chandrasekar 3 School of Civil Eng., IoT., Hawassa University Preliminary survey The objective of the preliminary survey is to study more than one alternative bridge sites. First the probable bridge sites are located in the map and thereafter these sites are visited to collect certain preliminary data for choosing the best site. If all the requirements as stated as preliminary data below are not satisfied there may be compromise for the less important ones. Preliminary data are to ascertain whether or not the following requirements are fulfilled. Some compromise for the less important ones is permissible. (i) The channel should be well defined and narrow. (ii) The river course should be stable and has high and stable banks. (iii) The river shall have large average depth compared to localized maximum depth to ensure uniform flow. (iv) The bridge site shall be far away from the confluence of large tributaries especially at upstream. (v) If the river meanders, the nodal point of meandering shall be the suitable bridge site. (vi) It is preferred to have straight approach road and square crossing. (vii) Site shall be approachable for transporting materials and labor. (viii) Site shall require shortest approach roads. (ix) To a maximum extent curves shall be avoided. (x) Site shall be such that there shall be no need of costly river training works. (xi) Site shall be sound from geological consideration. Detailed river survey Ideal bridge site shall be selected finally, based on the preliminary data collected from the different alternative bridge sites. Once this is done, detailed river survey for this selected site shall be conducted on the items as outlined below. (a) A survey plan of the stream showing the topographic feature of both upstream and downstream of the proposed site up to 500m on either side depending on the size of the stream. Information such as the name of the river and the road, approximate outline of the banks, direction of flow of water, alignment of approaches, name of the nearest town, reference to the bench mark and its reduced level, lines and identification numbers of the cross-section and longitudinal section taken within the scope of the plan shall be recorded in the drawing. (b) Following hydraulic data shall be collected at cross sections of river, one along the proposed bridge site and few more both at upstream and downstream at 50 m intervals. (i) The bed line upto the top of the banks, the ground line to a sufficient distance beyond the edge of stream (ii) Low water Level (LWL) (iii) Ordinary flood level (OFL) (iv) Highest Flood Level (HFL) (v) Maximum surface velocity at bridge site (vi) Scour depth determination (c) Longitudinal section of the stream showing the lowest bed level for a distance of 500 meters both on upstream and downstream of the bridge site for the determination of the bed slope. (d) Nature of streams namely, alluvial with erodible bed or quassi alluvial i.e. fixed bed but erodible banks or rigid with non-erodible bed and banks, perennial, seasonal or tidal. (e) Nature of bed and bed materials like clean bed, rifted bed, bed with weeds, silty, sandy or with gravels and boulders or rocky etc. (f) Catchment area and run off data (for the estimation of design discharge) (g) Geological data: Information about seismic conditions of the site and already occurred earthquakes, quicksand conditions, bearing capacity of soil and soil investigations. __________________________________________________________________________ Fundamentals of bridge design – CEng 5121 – Chapter 1 Instructor: M. K. Chandrasekar 4 School of Civil Eng., IoT., Hawassa University Bore holes for soil investigations shall be located along the center line of the proposed bridge at each pier and abutment foundations. In order to get some information about the change in soil strata, on either side of the center line of the bridge few more borings are also taken in upstream and downstream sides at a distance equal to around 1/10 to 1/15 of the length of the bridge. 2.6 ECONOMIC SPAN DETERMINATION Economic span is one for which the total cost of the bridge is minimum. For the most economic span, the cost of the superstructure equals the cost of substructure, with the following assumptions. The cost of the superstructure of one span is proportional to the square of the span. Csp = ks2 The spans are of equal length. The cost of each abutment is the same. The cost of each pier is the same. The cost of railings, parapet, flooring etc is proportional to the total length of the bridge and taken as K’L. A = Cost of approaches, B = Cost of two abutments including foundations, C = Total cost of the bridge, L = Total linear water way s = length of one span, n = number of spans, P = cost of one pier including foundation Total cost of superstructure = n.k.s2 Total cost of bridge C can be written as: C = A + B + (n – 1) P + nks2 + K’L For minimum cost of bridge, dC/ds should be equal to zero. Substituting n = L/s, differentiating and equating to zero, C = A + B + (L/s – 1) P + (L/s)ks2 + K’L = A + B + (L/s)P – P + Lks + K’L 2 2 dC/ds = - LP/s +Lk = 0; - P/s + k = 0 or P = ks2 2 But ks = Cost of superstructure of one span 1. 2. 3. 4. 5. Therefore for an economic span, the cost of super structure of one span = cost of substructure of the same span. Hence economic span can be computed as s = √(P/k) 2.7 GENERAL DESIGN REQUIREMENTS (from ERA design manual) Free (clear) opening is the face-to-face distance between supported components. It shall be measured perpendicular to the supports. If the supports are not parallel, the free opening is the least distance between them, see Figure 1-1. Total (overall) bridge length is the distance between the rear ends of the wing walls or abutments. It shall be measured parallel to the alignment between the rear ends of the wing walls or abutments. Span length should be:  For simple spans: the distance center to center of supports but need not exceed clear span plus thickness of slab.  For members that are not built integrally with their supports: the clear span plus the depth of the member but need not exceed the distance between centers of supports. Span length should give the placing of the piers regardless of type or dimensions selected at a later stage. It is normally measured at the alignment and given as stations. Theoretical span length is the distance between the center of bearings. At the abutments or at special wide piers it will be better to give the dimension from face to face of the pier or abutment front wall. __________________________________________________________________________ Fundamentals of bridge design – CEng 5121 – Chapter 1 Instructor: M. K. Chandrasekar 5 School of Civil Eng., IoT., Hawassa University Span length Alignment of Road Free opening Overall length of bridge Figure: 2.1 Definition of Bridge Dimensions MINIMUM DIMENSIONS The minimum dimensions listed in Table 1-1 shall be used in bridge design and construction. Footing depth Stem and Head wall/ballast wall of abutment thickness Bearing shelf of abutment or pier Wingwall thickness Pier columns in water, thickness Pier walls thickness Concrete deck depth, excluding any provision for grinding and sacrificial surface Concrete deck for pedestrians bridge Dimension  0.25 m  0.25 m  0.40 m  0.20 m  0.50 m  0.30 m  175 mm  150 mm Table 1-1: Minimum Dimensions In designing expansion joints in bridge decks, an additional 20-mm space shall be added to the calculated expansion. If the total height of the substructure exceeds 8 m, then 30 mm shall be added for exceptional movements such as settlement. Edge beams of bridge decks shall not be less than 0.35 m wide and 0.4 m deep. Arch barrels of stone masonry shall have a thickness at the crown of not less than 0.5 m unless otherwise shown in the detail design and after testing of the material. __________________________________________________________________________ Fundamentals of bridge design – CEng 5121 – Chapter 1 Instructor: M. K. Chandrasekar 6 School of Civil Eng., IoT., Hawassa University WIDTH OF BRIDGE DECK The width is to be measured between the inside of the railings  or the curbs. Total width of bridge is defined as the distance between the inside of the outer railings including walkways, island/refuge and similar. If the width will vary along the bridge all dimensions should be given. A listing of bridge configurations and corresponding widths are given in the following table. Application Two-lane in “urban” area Two-lane in “rural” area Single Lane Pedestrian Overpass Table 1-2 Table of Bridge Widths Width (m) 10.30 7.30 4.20 3.0 The dimensions of 7.30m for a two-lane bridge are based on trucks with widths of 2.6m meeting, providing 0.7m clearance between vehicles and at the sides, the greater clearance allowing a higher average speed. At higher design speed, and/or in the vicinity of densely populated areas, a bridge allowing for the shoulder width should be considered. Here the bridge width becomes 10.30 meters (7.30 meters plus 2 x 1.5 m shoulders). This allows for opposing trucks and pedestrians to meet safely. This width is recommended for bridges nearer than 5 km to a town/village of at least 10,000 inhabitants. All dimensions are valid regardless of the length of the bridge, due to safety reasons. For pedestrian overpasses, the minimum width is 3.0 m, which can accommodate three pedestrians, or a bicycle and a pedestrian in width. PEDESTRIAN LANE WIDTHS Segregated pedestrian lane (footway) protected by a barrier (railing) with end treatment having protection for both pedestrians and vehicles is recommended from safety point. Such one pedestrian lane shall not be less than 1.5 m wide. To be safe, a pedestrian walkway should permit two pedestrians to meet comfortably, which translates to 2 x 0.6 m width plus 0.3m clearance equals 1.5m. For safety considerations, the height of the railings along the footways shall be 1.5m by means of a top rail made of steel pipes. OVERALL LENGTH OF BRIDGE AND SPAN LENGTH For smaller bridges, the ground slopes at the abutments will often affect the overall length of the bridge. Therefore it could be more economical to use some sort of retaining wall at the abutments such as stone masonry walls, gabions and reno mattresses, concrete block wall, etc. This will make the bridge shorter and, most probably, more economic. Underpasses require a certain width for safety reasons, unless guardrails are applied alongside the underpass road. The distance from the roadway edge to the front wall of the abutment or the pier shall be 1.5m Bridges over rivers will normally require a certain opening of hydraulic reasons. Especially at high design water velocity, the basic rule is: with more constriction due to numerous and thicker piers, the backwater increases. Therefore, a longer span length could be preferable. It should be remembered that in some cases a footpath for local residents or a trail for wildlife and/or livestock exists along the waterline near the structure. Whether __________________________________________________________________________ Fundamentals of bridge design – CEng 5121 – Chapter 1 Instructor: M. K. Chandrasekar 7 School of Civil Eng., IoT., Hawassa University they should be elevated above the high water level all the time of the year depends on the frequency of use. Sometimes aesthetical reasons require a larger opening than necessary. DESIGN DEPTH OF SUPERSTRUCTURE AND FREE BOARD HEIGHT The clear height of roads shall normally be 5.1 meters for underpasses. Design depth of superstructure is the thickness of the superstructure excluding the pavement thickness. Normally it will be measured in middle of the span. The design depth is a very important parameter for the construction work. A minimal depth is not always the most economical solution, although it sometimes has an aesthetical value. If a thin superstructure is selected, a deflection analysis should be made. The waterway below the superstructure must be designed to pass the design flood and the floating debris carried on it. This should apply even after several years of sedimentation under or downstream of the bridge. Therefore, the freeboard above the design water level should not be less than in Table 1-3 below. Discharge Q (m3/s) Vertical clearance (m) 0 - 3.0 0.3 3.0 - 30.0 0.6 30 to 300 0.9 1.2  300 Table 1-3 Vertical Clearance at Design Flood Level (DFL) MINIMUM CLEARANCE ABOVE WATER, ROADS AND RAILWAYS Bridges above water shall normally have a minimum clearance height according to Table 1-3 unless a refined hydraulic analysis has been made. For arched structures, the clearance shall be measured at the quarter points of the span. Above roadways, the clearance shall be at least 5.1 m. Light superstructures (i.e. timber, steel trusses, steel girders, etc) above roadways shall have a clearance height of at least 5.3 m. Underpasses for pedestrians and bicycles should not be less than 2.4 m. For cattle and wildlife underpasses should be designed as the normal height of the actual kind of animal plus 0.5 m and for horse-riding the clear height should not be less than 3.4 m. Bridges above railways shall have a clearance height of at least 6.1 m - if not otherwise stated - to facilitate future electrification. CROSSFALL For adequate dewatering while minimizing the use of materials for the bridge deck, a crossfall of 2% (1 to 50), as well as longitudinal slope/grade of 1% (1 to 100), should be provided. Sometimes this is not achievable at transitions to skews. In these cases a close cooperation with the road designer should be established in order to find an acceptable solution. DEFLECTION In the absence of other criteria, the following deflection limits shall be considered for concrete, and/or steel construction:  Vehicular load, general ......................................................... .................... Span/500  Vehicular and/or pedestrian loads ........................................ .................... Span/800  Vehicular load on cantilever arms ......................................... .................... Span/300  Vehicular and/or pedestrian loads on cantilever arms ......... .................... Span/400  Uplift of a free end of the bridge deck …………………………….. 5 mm. (ex:. A span of 16.4m may not deflect more than 16400/500=32mm, due to vehicular loads) __________________________________________________________________________ Fundamentals of bridge design – CEng 5121 – Chapter 1 Instructor: M. K. Chandrasekar 8