# IRC 21 1987 PDF

IRC Road Bridges Section III Plain Cement Concrete - Download as PDF File .pdf), Text File .txt) or read online. March, August, September, IRC MEMBERS OF THE .. Committees of the IRC and revised in and in in the light of their. IRC Road Bridges Section III Plain Cement Concrete.,ii. -*r*hL*.: o. al. EE9. o. ;t. a. o. 'tt. 3. FI. z. E. o. z. resourceone.info a. gE! G,X. € =;{ d. H6.

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IRC: STANDARD SPECIFICATIONS AND CODE OF PRACTICE FOR ROAD BRIDGES SECTION: III CEMENT CONCRETE (PLAIN. Shitola Sharan NOTATIONS IRC: Es = modulus of Elasticity of steel Ec IRC and revised in and in in the light of their recommendations. IRC: STANDARD SPECIFICATIONS AND CODE OF irc road bridges section iii plain cement concrete Documents STANDARD SPECIFICATIONS AND CODE OF PRACTICE Part-I)pdf c.

The minimum diameter of transverse reinforcement i. Further, there are no provisions on the need for confinement of concrete in vertical members.

Also, possible buckling of longitudinal reinforcement is not considered. The incomplete treatment of shear design and of transverse reinforcement questions on the performance of such Indian bridge piers under the expected strong seismic shaking.

The minimum area of such reinforcement vide Clause Such reinforcement is to be distributed on both faces of the wall: Again, here also, there are no provisions on additional intermediate ties or links to hold together the transverse hoops on the outer and inner faces of the hollow RC pier.

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When the bars are located on the periphery of a circle, a complete circular tie is to be used. No other special seismic design aspects are addressed. Thus, the Indian codes advocate only flexural strength design; ductility design is not addressed at all; it is not ensured that the shear capacity of the pier section exceeds the shear demand when plastic moment hinges are generated during strong shaking.

The country is now classified into four seismic zones Figure 1. Further, the Indian Road Congress came up with interim measures 5 to be read with the revised zone map Clause As per Clause Clause One of the most important and welcome changes enforced through the interim provisions is with regard to the procedure for seismic force estimation.

The S a g value depends on the type of soil namely rocky or hard soil, medium soil and soft soil and the natural period T of the structure. But, the interim provisions recommend a single value of 2. This factor is to be used for all components of the bridge structure.

However, the bearings do not have redundancy in them and are expected to behave elastically under strong seismic shaking. Therefore, designing the bearings for a much lower seismic force than that it should carry from superstructure to piers is not desirable. In advanced seismic codes, the R factor for design of connections is generally recommended to be 1.

This interim provision needs to be revised immediately from the point of view of safety of bridge bearings. With the enforcement of the interim provisions, the prescribed seismic hazard of structures in the country has changed significantly. For single pier bridge vibration unit BVU , for most of the normal construction practice in India, piers tend to be slender in the direction of traffic or the longitudinal direction L , and stiffer in the direction T transverse to that of the traffic Figure 2.

Thus, in general, the natural period of piers is different in the longitudinal and transverse directions. Foe example, the single pier BVU under consideration has natural period of 1. Thus, as per the interim provisions, the S a g values for the longitudinal and transverse directions are 0.

The design seismic coefficient for the bridge in different seismic zones in the country calculated as per the IRC: In general, the design lateral force on piers in their transverse direction as per the Interim provisions is about twice those as per IRC: Now, consider bridges in the two metropolitan cities, namely Delhi and Madras. Thus, the design seismic coefficient for the single pier BVU in Delhi changes from 0.

## IRC 21-2000_Concrete - IRC 21-2000 STANDARD SPECIFICATIONS...

For bridges in Madras, the design seismic coefficient for the single pier BVU changes from 0. Hence, bridges in Madras become deficient as per the Interim provisions. In addition, there are special mandatory and recommended measures in the Interim provisions for better seismic performance of bridges.

These include ductile detailing, dislodgement prevention units, and isolation units. However, these are beyond the scope of this paper and hence not discussed. Capacity Design for Bridge Piers The capacity design philosophy warrants that desirable ductile modes of damage e. Under strong shaking, inelasticity in bridges is admissible only in the piers. Further, for strong seismic shaking, since it may not be economically viable to design a structure for elastic response, this inelasticity is deliberately introduced in piers but with adequate ductility.

Under these overstrength conditions, if the shear demand on the pier exceeds its design shear capacity, undesirable brittle failure for the whole structure may result. Thus, if capacity design of bridge piers is conducted, the piers are designed for shear corresponding to the overstrength flexural capacity of the pier.

In the capacity design of piers, the important items that come into play are design transverse reinforcement, concrete confinement by transverse reinforcement, shear strength of confined concrete, and stability buckling of longitudinal reinforcement. In countries like Japan, New Zealand and USA, the design of the bridge pier for seismic conditions is a paramount step in the entire process of bridge design practice.

In the capacity design approach, the following procedure is adopted in the above mentioned international codes, in general. First, through an elastic analysis under the specified loads, the bending moments and axial loads at all critical sections are determined, and the members designed for the combined effects of axial load and bending moment. Second, the potential plastic hinge locations and the preferred collapse mechanism are identified.

The overstrength flexural capacities of the plastic hinges are determined based on the actual reinforcement provided and the properties of actual material used. Third, the structure is re- analysed assuming all potential plastic hinges to have developed their overstrength flexural capacities. The associated axial load, shear force and bending moment in all structural components other than those with the plastic hinges are determined; these members are designed for these forces.

It is clear that this capacity design approach for shear design of substructures may not be possible for substructures of the wall-type; it is not possible to generate the flexural hinge even under the extreme seismic shaking.

Detailed studies are required to address the seismic design of wall-type substructures. The flexural overstrength of the structure, which in turn results in higher flexural overstrength-based shear demand, should be based on realistic properties. The flexural overstrength is caused due to many factors.

One of them is due to the materials used in construction having strengths higher than the nominal strengths employed in design. For instance, the actual tensile yield strength of steel is higher than its characteristic yield strength used in design f y , and the actual compressive strength of concrete is higher than the characteristic compressive strength f ck. On the other hand, seismic shear capacity is to be conservatively determined based on the nominal material strengths only 10, i.

In resisting shear, concrete carries significant part of the total shear force, particularly in large concrete cross-sections and those carrying vertical compressive loads, such as those of bridge piers. In general, the shear force capacity Vc offered by a concrete section depends on the shear strength of both concrete and longitudinal steel; shear strength improves with concrete grade and amount of tension steel though through dowel action.

## IRC 21- 1987 Road Bridges Section III Plain Cement Concrete

The shear strength of concrete itself depends on the level of confinement provided by transverse reinforcement, and on the imposed curvature; it increases with increase in volumetric ratio of transverse steel and with decrease in curvature In RC structures, the actual constitutive stress-strain relations of concrete and steel significantly affect the seismic response of the structure.

Transverse reinforcement causes a confining pressure on concrete resulting in an enhancement of its strength and strain capacities 21, 22, 23; this, in turn, causes an increase in the load carrying capacity of member. In capacity design, since the maximum flexural overstrength-based shear demand decides the ductile response of the structure, the actual constitutive relations of cover and core concretes must be used considering the confinement action of transverse reinforcement in the analysis 10, Under confinement, the maximum strain in concrete may be as high as 15 to 20 times the maximum strain of 0.

Transverse reinforcement in RC piers serves a three-fold purpose, namely for a providing shear strength, b confining the core concrete and thereby enhancing its strength and deformation characteristics, and c controlling the stability of the longitudinal reinforcement bars. The first two functions have been discussed earlier.

Regarding the third one, literature reports that inelastic buckling of longitudinal reinforcement in compression can be prevented by limiting the maximum spacing of transverse reinforcement bars to within six times the nominal diameter of longitudinal reinforcement 25, 26, This limit is generally recommended within the potential plastic hinge region Table 6.

However, the limit is relaxed outside the potential plastic hinge region, only if design calculations are made in line with design lateral force obtained as per Eq. Also, different codes prescribe minimum amount of transverse reinforcement in plastic hinge zones. Pushover Analysis The review of the Indian code provisions for RC pier design in light of the international seismic design practices, and importance of employing the capacity design concept in bridge design necessitates checking the seismic safety of piers designed as per the existing Indian standards.

The lateral strength and deformation characteristics of such piers can be determined by conducting, monotonic displacement-controlled experiments on prototype or model specimens. However, in India, the infrastructure required to perform experimental studies is still limited and expensive. Thus, a displacement-based pushover scheme is developed that would provide sufficient insight into the full response, i.

This does not accurately model the spread of inelasticity both along the member length and across the cross-section.

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Hence, a distributed plasticity model is required, which is described below. As an RC section is composed of both concrete of two types, namely the confined and unconfined and longitudinal reinforcing steel, the section is further discretised into separate concrete and steel fibres Figure 5. Such a general approach of discretising RC sections into a number of discrete fibres was long adopted to accommodate general geometric irregularities and geometric and material non- linearities, and to capture the complex stress distribution across the cross-section under any loading condition 30, Also, procedures for obtaining tangent stiffness matrix of a segment discretised into such discrete fibres was presented earlier 32, For analysis involving material and geometric nonlinearity, incremental equilibrium equations between incremental stress resultants and incremental deformations, i.

These incremental equations are combined to form the incremental equilibrium equation of a segment. Finally, the incremental equilibrium equation of the entire pier is obtained by assembling those of its segments. Large displacements and small strains are considered in the analysis. Each fibre is treated as a two-nodded axial member with no flexural property.

P is the axial load positive for tensile load and Et is the tangent modulus of elasticity of the material at the prevailing strain level. In Eq. Here, the shear response was assumed to be uncoupled from the axial load and bending effects, and hence, the linear superposition was conducted even under non-linear and inelastic conditions. In bridge piers of large cross-sections, shear deformation contributes significantly to the overall deformation response of the pier.

Hence, it is important to include the same. The stiffness matrix derived is applicable for a general frame member that may be a slender one with predominant flexural behaviour, or a stocky one with significant shearing behaviour. The incremental equilibrium matrix equation of the whole pier is formed by assembling those of all its segments. Symbolically, if [K ]t is the complete global tangent stiffness matrix of the pier, [K ]ts is the global tangent stiffness matrix of the segment s from Eq.

In RC structures, the two different materials, namely reinforcing steel and concrete, require two different material constitutive law models. Moreover, core concrete fibres are confined and the cover concrete unconfined. Also, the longitudinal and transverse steels can be of different grades and amounts.

Transverse steel affects the confinement of the core concrete and influences the axial stress-strain relation of the core concrete. On the other hand, longitudinal steel plays a direct role in the axial, bending and shear resistance of the section. A model representing the virgin stress-strain curve for HYSD bars, developed through regression analysis of experimental data from uniaxial tensile tests is used Brief descriptions of some of the constitutive law models of concrete available in literature are discussed elsewhere Of the different constitutive models available, the analytical model that is applicable to hollow sections also is used in this study 35; this model is an extension to an earlier model The falling branch as defined by the original single equation 37, 36 is too flat and is seen to be above the experimental uniaxial stress-strain data.

During pushover analysis of the pier, initially all the fibres are in compression under the action of gravity load. As the pier tip is displaced horizontally, the curvature at any section is gradually increased; the compressive strain in some fibres increases, while in others, it decreases and eventually becomes tensile unloading in compression and subsequent loading in tension. At a certain curvature, spalling of cover concrete occurs, which results in redistribution of stresses within the section.

There is possibility of unloading and reloading of both concrete and steel fibres. However, for the purpose of a monotonic pushover analysis, exhaustive hysteretic models for material stress- strain curves may not be required; simple loading, unloading and reloading rules are therefore used.

The following are the salient features of the hysteretic stress-strain model of steel used in this study: Likewise, a simple hysteretic stress-strain model of concrete is used in this study. The load-carrying capacity of compression reinforcement in RC compression members is significantly affected by the unsupported length of the longitudinal bars between the transverse ties that are expected to provide lateral support and thereby prevent buckling of longitudinal bars.

Displacement Based Pushover Analysis Procedure An analytical procedure is developed to assess the inelastic drift capacity of cantilever circular and square, solid and hollow RC piers bending in single curvature. The pier is subjected to a monotonically increasing displacement in increments at its tip in one transverse direction until its final collapse.

The force required to sustain the specified displacement is calculated considering the strength of the material, the deformation of the pier and the progression of internal cracking. From this, the overstrength shear demand, drift capacity and displacement-ductility of the RC cantilever pier bending in single curvature, are extracted. Then, a small displacement increment is imposed at the tip of the cantilever pier. Corresponding to this tip displacement, an initial deformed profile is assumed.

Usually, the deformed shape of an elastic cantilever with only bending deformations considered under the action of a concentrated load at the tip, is a good first approximation.

Pushover analysis involves iterative computations due to the nonlinearities in the constitutive relations of the materials and due to geometric effects. Modified Newton-Raphson Method is used for the iterations.

The above internal resistance calculation procedure is repeated with additional displacement increments until the pier reaches failure. Thus, the full lateral load-lateral displacement response is traced.

Piers of typical 5 m height are designed as per the strength design methodology outlined in IRC: The approximate initial choice of section size cross-sectional area and probable load on the piers are taken from field data of existing bridge piers. In this study, a 2-lane superstructure is considered.

The weight of the superstructure is taken as Hence, for a span of 40 m, the piers are subjected to a superstructure gravity load of kN. The lateral and vertical seismic loads on the piers are calculated as outlined in IRC: The nomenclature used to designate bridge piers studied is described as follows. The first character i. The second character i. The third character i. The fourth character i. The fifth set of numbers in the investigation on effect of slenderness i.

Because there is no provision for shear design of piers or compression members in IRC: However, provisions for shear design in beams and slabs are outlined in IRC: The overstrength based shear demands of these eight piers are estimated from their monotonic lateral load-displacement responses. Also, the nominal design shear capacities of the sections are computed as per IRC: Next, the effect of pier slenderness on overall response is investigated.

A set of eight piers is designed for two slenderness ratios, namely 2 and 6. In all piers, the cross-sectional area is kept at approximately 4. Pushover analysis is performed for all the twelve piers to compare the overstrength shear demand with the nominal shear capacity at the critical sections.

Then, the effect of level of axial load on the overall response of piers is investigated. For this, a 10 m long solid circular pier of diameter 2 m is designed for superstructure gravity load of kN and lateral load of kN.

The pier has nominal transverse reinforcement in the form of circular hoop of diameter 8 mm at mm centres. The pier is then subjected to axial compression loads of 0. The circular hoops in the pier are the enhanced to 12 mm diameters at mm centres, and the lateral load-deformation response for the three axial load levels are obtained for these additional transverse reinforcement type also analysis cases CSWP, CSWP and CSWP In all numerical studies, concrete cover of 40 mm and concrete grade of 40 MPa are used.

All studies are performed for major axis bending, i. For all the piers, since the resultant tension due to direct compression and bending under design loads exceeds permissible stress given in IRC: Results The results of the pushover analyses are shown in Figures 11 to In these, the flexural overstrength based shear demands in the piers are normalised with respect to the design shear force and are plotted against the percentage drift capacity of the piers.

The investigation with different cross-section shapes or geometries shows that in all cases, the flexural overstrength based shear demand is more than 2. This is primarily due to the safety factors used in the design. Also, in all cases, the shear demand is more than the shear capacity of the sections Table 7 , implying possible shear failure in these piers.

Further, short piers with slenderness ratio of 1. Hollow sections have larger section dimension and therefore draw more lateral force. In piers with circular cross-section, this increases the overstrength-based seismic shear demand without any appreciable increase in deformability. In piers with rectangular cross-section, the pier with hollow cross-section shows increased deformability, apart from the expected increased shear demand Figure This is due to the IRC: This forces additional intermediate ties in both directions in the hollow rectangular sections, which enhance the effective confinement of concrete compare volumetric ratio of transverse reinforcement in Table 8 and therefore increase the maximum strain that concrete can sustain.

This also results in increased deformability of the hollow rectangular section compared to the solid rectangular section with only nominal transverse reinforcement. On the other hand, piers with solid cross-sections with design transverse shear reinforcement have better post-yield behaviour in the form of enhanced deformability and displacement ductility. This signifies the importance of transverse reinforcement on the overall response of piers.

IS: — specifications for storage of materials. IS: — compressive strength test for cement mortar cubes.

IS: — specifications for SSC super sulphated cement. IS: — specifications for HAC for structural use high alumina cement. S: — specifications for masonry cement. IS: — chemical analysis and tests on cement. IS: ; ; SP 23 — codes for designing concrete mixes. IS: — methods of sampling and analysis of concrete. IS: — ultrasonic testing of concrete structures. IS: — specifications for concrete batching plant.

IS: — tests on water samples IS: — specifications for plywood formwork for concrete. IS: — specifications for concrete admixtures. IS: — specifications for bricks for masonry work. IS: — methods of sampling of bricks for tests. IS: — methods of testing of bricks. IS: ; ; — mild steel of grade I. IS: ; — mild steel of grade II. IS: — specifications for hard drawn steel wire fabric for reinforcing concrete. IS: — specifications for plain hard drawn steel wire fabric for prestressed concrete.

IS: — specifications for high tensile strength steel bar for prestressed concrete.

IS: — specifications for steel for general purposes. IS: — specifications for rolled steel made from structural steel.Each fibre is treated as a two-nodded axial member with no flexural property. The reinforcement provided to resist the bending moments determined in Clause Managing Director, Gammon India Ltd. But as the ratio decreases, a proportionately smaller value shall be taken.

January, First Revision: Regarding the third one, literature reports that inelastic buckling of longitudinal reinforcement in compression can be prevented by limiting the maximum spacing of transverse reinforcement bars to within six times the nominal diameter of longitudinal reinforcement 25, 26, The pier is then subjected to axial compression loads of 0.

One of the most important and welcome changes enforced through the interim provisions is with regard to the procedure for seismic force estimation.

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