24-09-2013, 04:22 PM
RCC BOX CULVERT - METHODOLOGY AND DESIGNS INCLUDING COMPUTER METHOD
RCC BOX CULVERT.doc (Size: 1.33 MB / Downloads: 58)
ABSTRACT
Culverts are required to be provided under earth embankment for crossing of water course like streams, Nallas across the embankment as road embankment can not be allowed to obstruct the natural water way. The culverts are also required to balance the flood water on both sides of earth embankment to reduce flood level on one side of road thereby decreasing the water head consequently reducing the flood menace. Culverts can be of different shapes such as arch, slab and box. These can be constructed with different material such as masonry (brick, stone etc) or reinforced cement concrete.
Since culvert pass through the earthen embankment, these are subjected to same traffic loads as the road carries and therefore, required to be designed for such loads. This Paper deals with box culverts made of RCC, with and without cushion. The size, invert level, layout etc. are decided by hydraulic considerations and site conditions. The cushion depends on road profile at the culvert location. The scope of this Paper has been further restricted to the structural design of box. The structural design involves consideration of load cases (box empty, full, sur- charge loads etc.) and factors like live load, effective width, braking force, dispersal of load through fill, impact factor, co-efficient of earth pressure etc. Relevant IRC Codes are required to be referred. The structural elements are required to be designed to withstand maximum bending moment and shear force. The Paper provides full discussions on the provisions in the Codes, considerations and justification of all the above aspects on design. Proper design covering these aspects has also been given in the Annexure. To our knowledge, these matters have neither been covered in any text book nor in any special publication at one place.
INTRODUCTION
It is well known that roads are generally constructed in embankment which come in the way of natural flow of storm water (from existing drainage channels). As, such flow cannot be obstructed and some kind of cross drainage works are required to be provided to allow water to pass across the embankment. The structures to accomplish such flow across the road are called culverts, small and major bridges depending on their span which in turn depends on the discharge. The culvert cover upto waterways of 6 m (IRC:5-19981) and can mainly be of two types, namely, box or slab. The box is one which has its top and bottom slabs monolithically connected to the vertical walls. In case of a slab culvert the top slab is supported over the vertical walls (abutments/ piers) but has no monolithic connection between them.
CO-EFFICIENT OF EARTH PRESSURE
The earth can exert pressure, minimum as active and maximum as passive, or in between called pressure at rest. It depends on the condition obtained at site (Terzaghi4 and Gulati5). For example in case of a retaining wall where the wall is free to yield and can move away from the earth fill the pressure exerted by the earth shall tend to reach active state and thus be minimum. As to reach active state only a small movement is required which can normally be achieved in case of a retaining wall, also before failure of the wall by tilting, the back fill is bound to reach active state. The wall thus can safely be designed for active pressure of earth, with co-efficient applicable for active pressure. In case of an anchored bulk head, the earth pressure on the anchor plate will tend to achieve passive state because the anchor plate is dragged against earth and large displacement can be allowed, one can consider passive co-efficient for the design of anchor, of course, some factor of safety need be taken as required displacement to achieve passive state before the bulk head gives way may not be practical. In cases where the structure is constructed before back fill earth is placed in position and the situation is such that structure is not in a position to yield on either side.
EFFECTIVE WIDTH
Effective width in the run of culvert (length across span) is expected to be affected by a moving live load. This width plays a significant role as far as consideration of live load in the design of culvert. Where however, there is large cushion the live load gets dispersed on a very large area through the fill and the load per unit area becomes less and does not remain significant for the design of box, particularly in comparison to the dead load due to such large cushion. In case of dead load or uniform surcharge load the effective width has no role to play and such loads are to be taken over the entire area for the design.
Effective width plays an important role for box without cushion as the live load becomes the main load on the top slab and to evaluate its effects per unit run for design as a rigid frame, this load is required to be divided by the effective width. As such evaluating effective width correctly is of importance. The relevant IRC Codes, other Codes, books, theory/concepts are at variance as far as effective width is concerned and requires discussions at some length.
BRAKING FORCE
This is another area where opinion of the designers vary in two ways firstly, whether braking force caused by moving loads shall deform the box structure and should therefore be considered in the design of box. Secondly, if it is to be considered what effective width should be taken to obtain force and moment per unit run of box. Of course the braking force will affect the global stability and change the base pressure to some extent. The IRC Code is silent as far as box is concerned. It will be in order to neglect effect of braking force on box having large cushion. In such situation the braking effect will be absorbed by the cushion itself and no force will be transmitted to the box beneath. Question will, however, arise up to what cushion height no braking force need be taken. This height generally is taken to be 3 m. Thus no braking force for cushion height of 3 m and more and full braking force for no cushion, for intermediate
IMPACT OF LIVE LOAD
Moving loads create impact when these move over the deck slab (top slab). The impact depends on the class and type of load. The IRC:6-2000 Code gives formula to obtain impact factor for different kind of loads by which the live load is to be increased to account for impact. The box without cushion where the top slab will be subjected to impact is required to be designed for live loads including such impact loads. Any such impact is not supposed to act on box with cushion. Hence no such impact factor shall be considered for box with cushion. The impact by its very nature is not supposed to act at lower depth and no impact is considered for the bottom slab of the box. It does not affect the vertical walls of the box and not considered in the design.
SHEAR STRESS
The box is designed for maximum moment for its concrete section and reinforcements. It is checked for shear at the critical section and if it exceeds permissible shear stress for the size of section; mix of concrete and percentage of reinforcements, the section has to be increased to bring shear stress within the permissible limit. Alternatively, the reinforcement can be increased to increase allowable shear strength. The third option is to provide stirrups to counter excess shear stress. This may have to be adopted in situation where thickness of slab cannot be increased due to certain restrictions. The top and bottom slabs are needed to be checked for shear. The vertical walls carry much less loads and shall be normally safe in shear, therefore, there is no need to check in shear. To make safe in shear one or any combination of increasing size, increasing tension reinforcement and/or providing shear stirrups can be adopted It is important to note that IRC:21-20006 under Clause304.7.1 has given table 12B. Permissible shear stress in Concrete for checking section for shear stress. The values given here have been drastically reduced compared to similar provision in previous Codes and practices. It is observed that the shear may govern the design of the section, in particular, box with large cushion.
DISTRIBUTION REINFORCEMENTS
The Code IRC:21-20006, in Clause 305.18 provides for distribution reinforcements. The distribution reinforcement shall be such as to produce a resisting moment in direction perpendicular to the span equal to 0.3 times the moment due to concentrated live loads plus 0.2 times the moment due to other loads such as dead load, shrinkage, temperature etc.
In box, moment due to live loads and dead loads are obtained considering both the loads together. It, therefore, becomes cumbersome to separate these two moments to apply above provision of the Code to calculate distribution reinforcements. To make it convenient and easy a combined factor for both the loads, based on weighted average in proportion of their magnitude, can be worked out to apply for the design. This has been adopted in the typical design provided in Annexure.
DESIGN OF TYPICAL BOX
Based on the above discussions and clarifications design of a typical box covering all above mentioned points are presented as Annexure. The box of 3 m x 3 m without cushion and with 5 m cushion have been given. Various load cases have been given for the maximum design moments. The box has also been checked in shear and shear reinforcement provided as required. The relevant parameters are mentioned in the design. Detailed design of single cell box culvert with and without cushion have been given. Basically, there is no difference in design of multi cell box having two, three or more cells. The bending moment is obtained by moment distribution considering all the cells together for different combination of loading and design of section accomplished for final bending moments for that member. Shear force and resulting shear stress have to be checked for members independently as done in case of single cell. A drawing furnishing details of the box based on detailed design and general arrangement for site of work as usually required for construction has also been given as Annex D.