rc design and detailing

Design and Detailing of RC Buildings / 1

EARTHQUAKE RESISTANT DESIGN AND

DETAILING OF RC BUILDINGS

Yogendra Singh Professor, Department of Earthquake Engineering, IIT Roorkee

1. INTRODUCTION

As brought out in the previous Chapters, the structures are to be designed to have sufficient strength and ductility for safety against earthquake forces. Both strength and ductility are important for seismic safety. The current codal practice of design of RC buildings is based on a linear analysis and Limit State Design philosophy. The effect of ductility is considered in the form of a response Reduction Factor, which is used to reduce the earthquake forces for design.

The RC members are to be designed for three actions: (i) Axial Force, (ii) Shear Force, and (iii) Bending Moment. Beams are generally monolithic with slabs and these are not designed for axial load. On the other hand, the columns are to be designed for an interaction of axial load and bending moment. The design for Shear is independent.

Concrete is known to be brittle material. Typical to brittle materials, it has much lower strength in tension, than in compression. The behaviour of concrete can be greatly enhanced by confining it. The ductility of concrete can be significantly improved by proper detailing of the reinforcement. This Chapter deals with important aspects of the design and detailing of RC buildings.

2. STRENGTH AND OVER-STRENGTH

If we test 100 cubes of same batch of concrete, they will not give the same strength. Similarly if we test 100 rods of steel of same grade or test 100 beams made of same concrete and same steel, they will fail at different loads. This is due to inherent variability of strength of materials. To take this into account we consider a lower than average strength of materials in design. Our code defines this as Characteristic Strength. It is the estimate of strength below which not more than 5% samples will fall. Further a partial factor of safety (1.15 for steel and 1.5 for concrete) is used to estimate the design strength. Therefore, it is clear that the actual strength of a member is higher than the force for which we have designed as per our current design practice. This higher strength is termed as over-strength and it is kept as reserve strength in case of gravity and wind load. In case of earthquake load, this strength is also utilized to resist the earthquake forces. In fact, the forces resulting


from the earthquake are much larger than the actual strength of the members and the members yield under such forces. Our normal linear analysis procedure can not predict the behaviour of structures for yielding members and we require non-linear analysis procedures. However, there are some simplified procedures, which can be used to approximately predict the non-linear behaviour from the linear behaviour. The response reduction factor given in our code is one such procedure which takes into account the over-strength and ductility.

2.1 Shear Strength of Beams and Columns The seismic performance of reinforced concrete frame buildings in past earthquakes demonstrates that loss of axial load carrying capacity due to shear failure in columns, is one of the most common causes of the building damage and failure. In a well designed column subjected to seismic actions, the contribution of shear deformation to the total deformation of column may be even less than 10% (Lehman and Moehle 2000), however, shear deformation becomes as significant as 40% of the total deformation (Sezen 2002) when the columns are designed only for gravity loads without considering seismic detailing requirements. The shear failure in columns is a brittle mode of failure (Fig. 1), which is considered as a force controlled mode. It implies that the member cannot undergo any plastic deformation (points B and C coincide in Fig. 1) and for satisfactory performance of the structure, the shear force should be controlled within the expected capacity of the member. Therefore, estimation of shear capacity of columns is an important issue in simulation of seismic behaviour of RC frames.

Fig. 1 Generalized force-deformation behavior of a typical RC member to define performance

limit states under shear.

Extensive research on this front over the past decades has revealed that the shear strength (Vn) of a column can be considered to have distinct contributions from concrete (Vc) and transverse reinforcement (Vs). Contribution of concrete in shear strength is rather complex and is influenced by several factors including axial compressive force, column aspect ratio and deformation ductility demand (Priestley


et al. 1994; Sezen and Moehle 2004; Erduran and Yakut 2007). A number of models are available for evaluation of shear strength of RC columns. Table 1 summarizes a few of the available models, which are simple to use and are applicable for the common range of building parameters.

Table 1 Overview of shear strength models of RC columns considered in the present study

2.2 Beam-Column Joints

Beamcolumn joints, particularly in frames not designed for earthquake actions, have been damaged in past earthquakes. Behavior of beam-column joints in frames subjected to lateral loading is a complex phenomenon, as a number of parameters affect the strength of the joints. Further, there is significant difference in the mechanism of shear resistance in case of exterior and interior beam-column joints. Shear strength of beam-column joints is mainly influenced by compressive strength of concrete, joint aspect ratio, amount of longitudinal reinforcement in beams

Model reference Vc Vs

FEMA-356 (2000)

2


connected to the joint, and axial force in column. Numerous studies have been carried out in the last decade to evaluate shear strength of the RC beam-column joints and several models of exterior and interior joints have been proposed. Table 2 provides the overview of the shear strength models of RC beam-column joints. Considering uncertainties regarding role of transverse reinforcement in failure mechanism of joints, the joint shear strength models prescribed in some of the codes/documents, viz., FEMA-356 (2000); (ACI-352R-02 2002); Eurocode-8 (2004), assume that the internal forces in the joint are to be transferred by diagonal compression strut of concrete core alone. The model proposed by Hegger et al. (2003) considers the maximum number of parameters influencing the shear strength of joints, including the role of transverse reinforcement, and is applicable for all types of joints.

Table 2 Overview of shear strength models of RC beam-column joints

Model reference Interior Joint Exterior Joint

FEMA-356 (2000) cj'cjn hbfV Park and Mosalam

(2012) cj'cjn hbf.V 0830

Hegger et al. (2003) cj

'cjn hbf.V 250

Eurocode-8 (2004) cjc

cgccnj hbf

fAP

ffV

25016.0

1250

14.0'

'''

80% of interior strength where, is nominal strength coefficient based on joint geometry and amount of transverse reinforcement; 1, 2, 3, are coefficients to account for anchorage efficiency in beam reinforcement, axial force in column, and slenderness of joint, respectively; bj is effective width and hc is depth of joint.

Unlike, the joint strength models of Eurocode-8 (2004), ACI-352R-02 (2002) and FEMA-356 (2000), the model in NZS-3101:Part1 (2006) requires considerable amount of transverse reinforcement in the joint to transfer the tensile forces and therefore not applicable to the non-ductile gravity designed buildings, where no transverse reinforcement is provided in the joint region. Indian Standard (BIS 1993) provides some detailing guidelines for beam-column joints, but does not provide any model for estimation of joint shear strength.

cj'cnj hbf.V 250321


(a)

(b)

Fig. 2 Shear resistance mechanism of beam-column joint in: (a) bare and (b) infilled frame


Figure 2 shows the shear resistance mechanism of a beam-column joint in bare and infilled RC frames. Assuming that the tension (T) in beam reinforcement is equal to the compressive force (C) in beam, the joint shear force (Vjh) of bare and infilled frame can be represented as in Eqs. (1) and (2), respectively.

cjh VTCV

(1)

cosRVTCV cjh

(2)

where, Vc is the shear force in column, and R cos is the shear force exerted by the infill. It is evident from the figure that strut action of infill results in increased shear in column, which in-turn results in reduced shear force in the joint. 3. DESIGN FOR DUCTILITY

As mentioned earlier, the Response Reduction Factor, used in the design of structures depends on ductility of the structure. The ductility of structures, in tern, depends on the ductility of individual components and structural configuration, including relative strength of different components and redundancy. These two aspects of ductile design are described below.

3.1 Ductile Design of Individual Components

The ductility of structure depends on the ductility of individual components. In RC members, the ductility of components can be enhanced in flexure but there are limitations on ductility in axial action and shear action. In flexure, the ductility can be achieved by making under-reinforced sections and by providing proper confinement at the locations where maximum moments are expected and the component is expected to yield. The member ends near the joints are the most probable locations of yielding under earthquakes. Further, it should be ensured that the member should yield in flexure and not in shear or axial action. This can be ensured by providing higher strength in shear and axial action, than that required for yielding of the member in flexure.

3.2 Ductile Design of Structural System

A structure can yield in a variety of modes depending on the relative strength of various components and joints and structural configuration. As some of the members have to yield under earthquake, redundancy of structural system is very important. The structure with higher degree of redundancy can afford to have larger


number of plastic hinges before collapse and therefore it will exhibit higher ductility. On the other hand a determinate structure will become unstable on the formation of first plastic hinge, without showing much ductility.

The local failure mechanism resulting due to formation of plastic hinges in columns prior to those in beams causes brittle failure of structure and should be avoided. This can be avoided by designing the columns to be stronger than the beams. Failure of joints is another cause of poor seismic performance of structures. If the joints fail in shear which is a brittle mode of failure and if joints fail prior to yielding of components, the ductility can not be achieved. This requires proper detailing of the reinforcement in joints.

3.3 Capacity Design Concept

The capacity design is the art of avoiding failure of structure in brittle mode. This can be achieved by designing the brittle modes of failure to have higher strength than ductile modes. In a RC building this can be achieved by following the following design sequence:

(i) First design the beams in flexure for the moments obtained from the analysis for Gravity, Wind and earthquake Loads.

(ii) Calculate the provided flexural strength of beams and the corresponding shear strength requirement.

(iii) Design the beams for higher of the shear obtained above in (ii) and that obtained from analysis.

(iv) Calculate the flexural strength requirement of the columns by considering the strength of beams joining the columns. The combined flexural strength of columns joining at a node must be higher than the combined flexural strength of beams joining at the node.

(v) Design the columns for the higher of the moment obtained in (iv) above and that obtained from analysis.

(vi) Design the columns for the shear force higher of that obtained from the flexural capacity and obtained from analysis.

4. HOW CAN WE MAKE RC STRUCTURES DUCTILE ?

Concrete is known to be brittle material, i.e. it fails suddenly when subjected to load. But concrete can be made ductile when confined by reinforcement. Fig. 3 shows the behaviour of unconfined and confined concrete. It can be seen that confinement not only increases the strength of concrete, but it tremendously increases the ductility of concrete. The confinement of concrete is obtained by providing stirrups, as shown in Figs. 4-5. Here, it is very important, that stirrups should be hooked at 1350 into the


core concrete, otherwise these stirrups open up under force due to earthquake and the confining action is not available (Fig. 6).

Fig. 3 Behaviour of Confined and Unconfined Concrete

Fig. 4 Confining concrete by hoops / stirrups

Fig. 5 Effect of spacing of hoops / stirrups


Fig. 6 Role of anchorage of hoops / stirrups

Fig. 7 Variation in ductility of steel with strength

An important issue in detailing of RC structures is the grade of steel used for reinforcement. As seen from Fig. 7, the ductility of steel decreases with increase in strength. Therefore, it is important that the minimum ductility of steel is ensured.


According to IS 13920, the steel having total elongation more than 14.5% only can be used for reinforcement in RC structures in Seismic Areas.

Fig. 8 Capacity Design concept explained through an analogy with a chain

Fig. 9 Capacity shear in beams

The most important issue in ductile design of RC structures is avoiding the failure in brittle modes. This is ensured through capacity design. Fig. 8, shows a chain, which has one ductile link, while all other links are brittle. This chain is subjected to load P


at the ends, as shows in the Fig. Now, the question is, whether the failure of chain will be brittle or ductile? This can be answered, if we know whether the ductile link is going to fail first or a brittle link. If the capacity of all brittle links is higher than the ductile link, the failure of the chain will be ductile, otherwise it will be brittle. This concept is used in making a structure to behave in a ductile manner by designing all the brittle modes to have higher strength than the ductile modes. In case of RC members, shear is known to be a brittle mode of failure. It can be avoided by designing the beams for the higher of the two: (i) factored applied shear, as obtained from analysis, and (ii) capacity shear as obtained using the procedure shown in Fig. 9.

Fig. 10 Strong column and weak beam design concept

In a RC frame building, two common modes of failure are possible (Fig. 10). In the first mode of failure columns of one storey yield and building fails in a local mechanism. On the other hand, in the second mode of failure, all the beams yield first than the columns. This type of failure mechanism is called global mechanism. It is obvious that the second mode of failure provides much larger ductility than the first mode. This can be achieved by designing the beams of the building weaker than the columns. Weak beam and strong column design is the most important concept of building design.


4. SPECIAL REINFORCEMENT DETAILING FOR DUCTILITY

As discussed in the previous section, ductile buildings can be designed even with concrete, which a non-ductile material. This can be achieved by providing proper amount of steel reinforcement at proper location. The following sections describe the reinforcement detailing for ductility

4.1 Anchorage and Splicing of Reinforcement

Joints are subjected to very large earthquake forces and it has been observed that the beam reinforcement pulls out of columns and the building collapses. To avoid this, the code IS: 13920 recommends that the beam reinforcement should be anchored into columns by a length ld + 10 (Fig. 11). The increase of 10 to the development

length is to take into account the loss of bond due to cracking of concrete during earthquake.

Similarly, care should be taken in splicing the reinforcement. The splicing should not be done near the beam column joints as these locations are subjected to high bending moments and concrete may crack and bond may be lost at these locations. Further, the code require that not more than 50% of reinforcement should be spliced at one location.

5.2. Special confining reinforcement

As discussed above, it is the confinement of concrete, which makes it ductile. Code requires special confining reinforcement at the location where moment hinges are likely to occur. The diameter and spacing of these hoops special confining reinforcement is to be calculated according to codal requirements, but in no case this spacing of stirrups should be more than 100 mm for columns and it should not be more than 150 mm for beams. Figs. 12 -13 summarize the requirements of special confining reinforcements of and columns.

5.3 Reinforcement in Shear Walls

Shear walls are similar to a wide column and these have reinforcement grid, generally on both faces. These walls resist large shear forces and bending moments and the reinforcement should be provided to resist both shear and bending moment. The code requires that if the stress in the shear wall exceeds 0.2 fck then these should be provided with boundary elements. These boundary elements are similar to

Fig. 11 Anchorage of beam reinforcement


columns but monolithic with shear walls (Fig 14). The width these boundary elements may be same as the thickness of the shear wall or it may be more.

Fig. 12 Arrangement of stirrups Fig. 13 Special confining reinforcement

Fig. 14 Boundary Elements Fig. 15 Reinforcement at openings


Special care is required at openings in the shear walls. Concentration of stresses takes place near opening. To take care of this, additional reinforcement (Fig. 15) should be provided around the openings. In case of coupled shear walls, the coupling beams are subjected to very high shear forces. Due to reversal of stresses under earthquake conditions, the concrete in coupling beams gets crushed. To take care of the shear force, diagonal reinforcement should be provided (Fig. 16) in the coupling beams. This diagonal reinforcement should be anchored by 1.5 times the full development length, into the shear wall concrete.

5.4 Detailing requirements in special conditions

There are two commonly found conditions in RC buildings, which need special attention in detailing. First, whenever, there is an abrupt change in stiffness of members, special confining reinforcement should be provided. Two such cases are encountered when the shear wall is supported on columns (Fig. 17) or columns are supported on shear wall (Fig. 18). The first case is not desirable from earthquake safety point of view and must be avoided. In both the conditions, the columns should be provided with special confining reinforcement throughout the length.

Fig. 17 Shear wall on columns Fig. 18 Columns on shear walls

Fig. 16 Reinforcement in coupling beam


The second case is whenever, there is a possibility of short-column effect, due to partial infill or a mezzanine floor (Fig. 19), the columns should be provided with special confining reinforcement throughout the length.

6. PRECAUTIONS DURING CONSTRUCTION

For satisfactory performance of buildings, during earthquake, construction supervision is also equally important. Several failures have been observed due to faulty construction.

The most important point during construction is the construction joint. To avoid failure at the construction joints, shear keys should be provided at construction joints. Before placing the new concrete, the surface of old concrete should be thoroughly cleaned by water jets. Wooden blocks should be used for making shear keys and these blocks should be removed after initial sitting of concrete. These blocks should never be left in place.

Splicing of reinforcement during construction is very important. As discussed above, not more than half of the reinforcing bars should be spliced at the location and splicing should be avoided near the joints.

Anchorage of stirrups in the most important factors on which the safety of building depends. In no case the stirrups should be anchored at 90o as these open up during earthquake and confinement is lost

Alignment of columns is also very important as any eccentricity will give rise to high bending moments in columns.

There is considerable congestion of reinforcement at the joints. Compaction of concrete at joints is a difficult task and honeycombed concrete at joints is

Fig. 19 Short-column effect


quite common. Special care should be taken to compact the concrete at joints, as joints are the highly stressed parts of a building.

REFERENCES 1. ACI 352R-02. 2002. Recommendations for Design of Beam-Column Connections in

Monolithic Reinforced Concrete Structures. Detroit, Michigan, American Concrete Institute.

2. ATC 40, 1996, Seismic Evaluation and Retrofit of Concrete Buildings, Applied Technology Council, California.

3. Erduran, E., and Yakut, A. 2007. Vulnerability Assessment of Reinforced Concrete Moment Resisting Frame Buildings. Journal of Structural Engineering; American Society of Civil Engineers (ASCE), 133 (4):576-586

4. Eurocode-8. 2004. BS EN 1998-1: Design of Structures for Earthquake Resistance- Part 1: General Rules, Seismic Actions and Rules for Buildings. Brussels, Belgium, European Committee for Standardization (CEN).

5. FEMA-356. 2000. Prestandard and Commentary for the Seismic Rehabilitation of Buildings. Washington, DC, U. S. A., Federal Emergency Management Agency.

6. Hegger, J., Sherif, A., and Roeser, W. 2003. Nonseismic Design of Beam-Column Joints. ACI Structural Journal, 100 (5):654-664.

7. IS 13920-1993, Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces Code of Practice, Bureau of Indian Standards, New Delhi.

8. IS 1893-2002, Criteria for Earthquake Resistant Design of Structures, Part 1 General Provisions and Buildings, Bureau of Indian Standards, New Delhi.

9. IS 4326-1993, Earthquake Resistant Design and Construction of buildings Code of practice, Bureau of India Standards, New Delhi.

10. IS 456-2000, Plain and Reinforce Concrete Code of Practice, Bureau of Indian Standards, New Delhi.

11. Key, David, 1988, Earthquake Design Practice for Buildings, Thomas Telford, London.

12. Lehman, D. E., and Moehle, J. P. 2000. Seismic Performance of Well confined Concrete Bridge Columns, PEER Rep. 98/01 University of California at Berkeley, Pacific Earthquake Engineering Research Center.

13. NZS-3101:Part1. 2006. Concrete Structures Standard, Part 1, Design of Concrete Structures. Wellington, New Zealand, Standards Association of New Zealand.

14. Paulay T., and Priestley, M.J.N., 1992, Seismic Design of Reinforced Concrete and Masonry Buildings, John Wiley & sons, Inc., New York.

15. Penelis, George G., and Kappos, Andreas J., 1997, Earthquake Resistant Concrete Structures, E & FN Spon.


16. Priestley, M. J. N., Verma, R., and Xiao, Y. 1994. Seismic Shear Strength of Reinforced Concrete Columns. Journal of Structural Engineering; American Society of Civil Engineers (ASCE), 120 (8):2310-2329.

17. Sezen, H. 2002. Seismic Behavior and Modeling of Reinforced Concrete Building Columns, University of California, Berkeley.

18. Sezen, H., and Moehle, J. P. 2004. Shear Strength Model for Lightly Reinforced Concrete Columns. Journal of Structural Engineering; American Society of Civil Engineers (ASCE), 130 (11):1692-1703.

rc design and detailing

Documents