Overview of Precast concrete frame

272 JOURNAL OF STRUCTURAL ENGINEERING Vol. 38, No.3, AUGUST-SEPTEMBER 2011

Journal of Structural EngineeringVol. 38, No. 3, August-September 2011 pp.272-284 No.38-26

Seismic performance and design of precast concrete building structures: an overview

R. K. Khare*, ,, M. M. Maniyar**, S.R. Uma*** and V. B. Bidwai*, Email: [email protected]*Civil Engineering & Applied Mechanics Department, SGS Institite of Technology & Science, 23, Park Road, Indore (MP), 452 003, India.

**Sardar Patel College of Engineering, Munshi Nagar, Andheri(W), Mumbai, 400 058, India.***Earthquake Engineer, Institute of Geological & Nuclear Sciences, Lower Hutt, New Zealand

Received: 03 March 2008; Accepted: 31 October 2010

Seismic performance and behaviour of precast concrete structures which were not designed and detailed as per existing provisions in relevant standards was very poor during past earthquakes while the buildings constructed and designed incorporating seismic design concepts performed remarkably well. A brief review of seismic performance and design of precast concrete systems is presented to seek for the ways to improve and develop construction of precast concrete struc-tures in India. This paper brings together the historical perspective on the performance of precast concrete structures so that lessons can be learnt to avoid the poor performance of these systems. An extensive literature on experimental stud-ies has been also reported here to demonstrate the improved seismic performance of precast concrete systems. Further, a review and comparison of International code provisions on the design and construction of precast concrete systems is presented to help in developing the provisions and practice of these systems in Indian perspective. Identifi cation of areas that need revision or attention in the current IS Code provisions are attempted in the light of International practice.

KEYWORDS: Seismic performance; precast concrete structures; earthquakes; seismic design concept; design and construction.

Precast concrete is signifi cantly being used in earthquake resisting structures in many parts of the world. Main advantages of incorporating precast concrete in construction are the possible increased speed of construction, high quality of precast units, improved durability, reduction in site labour and formwork, and more importantly, social and environmental benefi ts. Future prospects of these structures are high as having no damage during earthquakes by using post-tensioning with or without energy dissipating devices.

Due to the lack of understanding of the basic na-ture of seismic behaviour, the precast concrete struc-tures were viewed with scepticism in seismic regions1. Some countries considered the use of precast concrete in earthquake resisting structures with suspicion be-cause of their bad performance in major earthquakes. Examples of poor behaviour of precast concrete build-ing structures during 1976 Tangshan (China), 1985 Mi-choacan (Mexico), 1988 Armenian, 1994 Northridge and 1999 Kocalli earthquakes due to improper design

and detailing of ductile element, inadequate diaphragm action, poor joint and connection details, inadequate separation of non-structural elements and inadequate separation between structures are presented in the state-of-the-art report by Park and co-workers (fi b, 2003). Damage to precast school buildings at Gujarat in 2001 Bhuj earthquake is another example of failure due to the poor connections between structural elements. It is re-ported2 that roof planks resting on the beam shifted due to inadequate bearing area and lack of positive anchor-age. A monolithic behaviour of frames, and diaphragms action of fl oors could not be achieved due to poor con-nections.

The fast economic growth of the country in recent past and the need of infrastructural development em-phasize to use precast concrete structures. Advantages of precast concrete construction from Indian point of view, in addition to earlier mentioned are uniformity of construction, planned and well managed cities. Stan-dardisation of precast concrete elements will also be

JOURNAL OF STRUCTURAL ENGINEERING 273 Vol. 38, No.3, AUGUST-SEPTEMBER 2011

able to control the non-engineered practice of Rein-forced Concrete construction.

Countries like Japan, Canada, Italy, Chile, Mexico, New Zealand and USA, which are well known for high seismicity, adopt precast concrete construction practices. In these countries, the design and construction practices are usually supported by the results from experimental investigations. This aspect of experimental investigation due to known or unknown reasons is not to the desired level in India, although premier institutes like IITs and IISc are making their best effort in this direction. Testing being an essential part to understand behaviour of structures needs to be strengthened in the country. Detailed guidelines for designing structures explaining the codal provisions are also needed to be prepared.

Recent experimental investigations have mainly focussed on developing techniques to reduce damage in structures using precast elements. For example, with reference to New Zealand, University of Canterbury conducted tests to design connection details between hollow-core fl oors with beams and walls35 that could sustain up to 6% of inter-storey-drift. Also, innovative techniques such as damage avoidance design (DAD) and self-centring technique used post-tensioning systems and developed rocking walls and columns6 that performed with no damage even up to 4.7% drift level.

From the above discussion it is clear that the precast concrete structures have shown good performance under seismic conditions. The advancements and advantages of precast concrete structures make such structural systems a promising one to be advocated in Indian context. Precast concrete building structures include beam, column, frame, slab panels, folded plate or shell, stairs and wall panels. These structures can be very well designed as gravity load and seismic load resisting systems. It can potentially prove to be an appropriate structural system with additional research and development of design guidelines. The main objectives of this paper are:i. Reporting a historical perspective on the

fronts of seismic behaviour of precast concrete construction, developments in codal provisions and experimental research.

ii. Discussing on international code provisions supported with the experimental studies on precast components and their connections.

iii. Identifying the areas within Indian standards where potential improvement can be made to enable the earthquake resistant precast concrete construction.

HISTORICAL PERSPECTIVE

The Pioneering Efforts on Understanding the Behaviour of Precast Concrete Structures

Literature explaining the seismic behaviour of precast concrete structures is reported in following. These papers help in understanding the development and improvement in seismic resistant precast concrete construction.

Some insight into the behaviour and earthquake resistance of a large number and variety of precast concrete structures subjected to severe earthquakes, for the fi rst time, was provided by Fintel7. He reported after the observed seismic damage that occurred in the earthquake of Bucharest, Rumania 1977. A state-of-art report on seismic resistance of precast concrete structures was fi rst presented by Hawkins8, in which results of analytical and experimental studies concerning the seismic resistance of precast concrete structures and their sub-assemblages were reviewed and research needs for building industries were identifi ed. However, the lessons learnt in the above ways need to be translated into design guidelines to help construction of improved structures. Englekirk9 emphasized the need of a design standard supported by a good technical data base without which precast concrete buildings can not be economically feasible in regions of high earthquake intensity. The evolution of the precast industry in seismically active regions of the United States and other parts of the world, with an emphasis on the need to develop technology compatible with precast concrete construction, are discussed and presented by Englekirk10. It was reported that precast concrete construction is extensively used and being promoted in Japan on high rise buildings even though Japan is not having a specifi c national design standard on precast concrete structures. The technical justifi cation for precast systems in Japan is provided by experimental studies. Further research on the behaviour of precast concrete elements and structures under seismic loading is reported in references1115.


Modifi cations in Codal Provisions to Improve Seismic Performance

Design of precast members and connections need to include loading and restraint conditions from initial fabrication to end use in the structure, including form removal, storage, transportation, and erection. Precast concrete elements shall be connected to other precast members, cast in place or steel elements or to the foundation structure to ensure that effective load paths for the transfer of forces to primary lateral force resisting systems can be developed. Forces and deformations occurring in and adjacent to connections are included in the design. Tolerances are very important issue in the construction of precast concrete structures in particular in seismic resistant construction. The steps of manufacturing and erection process govern the design criteria. Any mistake in erection may lead to damage of the precast element. Further knowledge of displacement compatibility in relation to seismic separation and the protection of brittle elements is important to ensure that the structure will behave in the intended manner. To limit the possibility of progressive collapse and to obtain a monolithic action, structural integrity is taken care of in precast concrete structures by means of longitudinal and transverse ties connecting members to a lateral load resisting system. Forces shall be permitted to be transferred between members by grouted joints, shear keys, mechanical connectors, reinforcing steel connections, reinforcing topping, or a combination of these means. The adequacy of connections to transfer forces between members is determined by analysis or by test. In designing a connection using materials with different structural properties, their relative stiffnesses, strengths, and ductilities are considered. Provisions related to seismic design considerations are continuously being improved and incorporated in different international standards. Development in the codal provisions and guidelines of American and New Zealand construction practice are briefl y discussed in the following.

A brief history of building code provisions for precast/prestressed concrete in the United States was presented by DArcy, et al.16, in which it was report-ed that the fi rst set of specifi c design provisions ever developed in the United States for precast concrete structures in regions of high seismicity appeared in NEHRP17 Recommended Provisions. The NEHRP17

provisions presented two alternatives for the design of precast lateral-force-resisting systems: one, emulation (same as) of monolithic reinforced concrete connection and the other, use of the unique properties of precast concrete elements interconnected predominantly by dry connections (jointed precast). Uniform Building Code(1997)18 adopted monolithic emulation option for frames only. For emulation of the behaviour of mono-lithic reinforced construction, two alternatives were provided: structural systems with wet (ductile) con-nections and those with strong (elastic) connections. The design provisions for precast structures in high seismic regions were expanded in NEHRP (FEMA, 2001) Provisions19. The seismic-force resisting system for high seismic regions suggested in NEHRP (FEMA, 2001) provisions19 are special moment resisting frames and special structural walls with superior type dry con-nections. The ACI 318-0220, introduced design provi-sions for precast concrete structures located in regions of moderate to high seismic risk or assigned to interme-diate or high seismic design categories. Provisions for non-emulative (jointed precast) design of precast wall systems were not included in ACI318-0220.

A perspective on the seismic design of precast concrete structures in New Zealand is presented by Park21. Until 1995, the New Zealand concrete design standard did not include seismic design provisions covering all aspects of precast concrete structures. Design provisions of UBC 199718 for precast structures in regions of high seismicity supported by an example of a 12-story precast frame building using strong connections were discussed by Ghosh et al22. Hawkins and Ghosh presented the NEHRP recommended provisions23 for seismic regulations for precast concrete structures with detailed seismic behaviour of special moment frames, special structural walls, diaphragms and their connections. Trends and developments in the use of precast reinforced concrete in New Zealand for fl oors, moment resisting frames and structural walls of buildings with aspects of design and construction, particularly the means of forming connections between precast concrete elements were discussed and presented by Park21.

Experimental Research to Improve Seismic Performance

After observing the failures in Northridge, a multi-stage study was undertaken at the University of Canterbury,


to determine whether New Zealand designed and built structures have similar problems, and if so, to what extent these problems exist and what can be done about them. At fi rst, an extensive study that examined the seismic demands on a variety of precast concrete multi-storey buildings was examined by Matthews3. Experimental studies were then performed in two stages to determine the inter-storey drift capacities of multi-storey RC buildings with precast concrete hollow-core fl oors. A series of large scale experiments were conducted on a full scale super-assemblage in order to ascertain the inter-storey drift corresponding to various damage states. Stage 1 of the experimental study examined the then-existing precast concrete detailing practice in New Zealand, as recommended by the New Zealand concrete standard NZS3101:1995.

The collapse of hollow-core units during the tests by Matthews3 in stage 1 fl agged issues over the performance of existing precast concrete frame structures with hollow-core fl ooring structural systems. In stage 2, Lindsay4 and MacPherson5 tested and reported the improved performance of similar super-assemblage incorporating the fl oor-frame connection details as recommended in Amendment No. 3 to the NZS3101:1995.

In major earthquake events of high seismicity, a performance objective for buildings and structures is to ensure life safety and continuous operations after strong ground shaking. Structural components of buildings must also satisfy serviceability limit states and member strength limit state requirements. Widespread damage and post-earthquake operational problems have been observed in the recent earthquakes. Damage Avoidance Design (DAD) philosophy is one approach whereby higher performance objectives at different level earthquakes can be achieved without causing any structural damage to the constructed facilities. Such a conceptual design approach was proposed by Mander and Cheng24 for bridge substructures whereby rocking columns form the seismic resistance mechanism. Hamid25 adopted that approach of structural fl exibility and prestressed unbonded tendons in precast hollow-core walls for industrial/warehouse facilities. Precast prestressed hollow-core wall panels were designed, constructed and tested in the laboratory so that the outcomes are applicable to seismic environment with minimal damage to the structures.

An account of rocking structures with and without prestressed unbonded tendons was presented by Hamid25. Holden et al.26, Sudarno27, Liyanage28 and Ajrab et al.29 have used and adopted their approach to design and construct precast reinforced wall panels. The experimental results carried out by Sudarno27 and Liyanage28 showed that slender precast wall panels may loose their stability at 0.4g and collapse under a Maximum Considered Earthquake (MCE). This issue is of concern for designers, developers, builders, precast manufacturers, and territorial authorities.

Hamid25 reported that most of the previous studies focused on direct-displacement approach and unbonded post-tensioned tendons precast wall panels using spiral reinforcements and transverse reinforcement bars. Limited studies on the application of rocking structures in solid reinforced concrete precast wall panels using the DAD philosophy26, 29 are available. Hamid25 investigated the overall seismic behaviour of precast hollow core walls without horizontal reinforcing bars. In particular, the connection interface between wall-foundation, the most effi cient energy dissipators and the combination of seismic and non-seismic wall as a rocking wall system were investigated. An alternative way of using hollow core units as precast wall panels with the concept of DAD in tilt up construction was provided.

On the basis of all these experimental work conducted at University of Canterbury, New Zealand, the modifi cation in NZS3101:199530 were suggested which were incorporated in NZS3101:200631.

CODAL PROVISIONS RELATED TO THE SEISMIC PERFORMANCE OF PRECAST CONCRETE SYSTEMS

The provisions of American, New Zealand and Euro codes and guidelines related to the seismic performance of precast concrete systems are discussed in this section. These provisions have demonstrated that how the seismic performance of precast systems can be improved. This study will help in framing the codal provisions and guidelines in Indian perspective.

Failure of precast concrete buildings in 1964 Alaska, 1976 Tangshan, China, 1988 Armenia, 1994 Northridge, 2001 Bhuj and 2008 Wenchan china earthquakes was mainly due to collapse of fl oors for some or other


reasons. One of the main reasons of collapse of fl oors were loss of seat due to failure of support system, poor connections, excessive deformation of support system (beam elongation) and deformation incompatibility between the support and fl oor. Typical detail of the damage of seat of a fl oor resting on wall or beam support due to the movement is shown in Fig. 1.

Movement Crack in topping

Cast in place reinforced concrete topping

Spalling at end of precast concrete

floor unit

Spalling of cover concrete

Precast concrete beam

Fig. 1 Damage of seat due to movement of fl oors

After this damage takes place, fl oors without topping fall due to their own weight. Earthquake vertical accelerations add on to this action. Floors with topping are also failed during these earthquakes when the top reinforcement could not transfer the shear force from the precast fl ooring to the supporting beam as shown in Fig. 2.

Fig. 2 Failure of fl oors due to inability of topping to transfer shear stress

A possible solution to avoid these failures can be by providing the suffi cient seating incorporating the effect of all possible movements into account. Fig. 3 shows such detail of required bearing length at the support suggested by NZS 3101: 200631.

Figures 4(a) and 4(b) show the alternative special reinforcing to transfer shear force and support precast concrete fl oor units in the event of loss of bearing. Continuous bottom reinforcement will transfer shear force to beam in case of loss of seating. The other alternative with hanger stirrups in the vicinity of support can also help in the transfer of shear force. Another possibility to support the fl oor unit in such event can be to design the fl oor as a T-beam by providing an additional tie at the middle to transfer fl oor load by providing

continuous reinforcement through the support. In this case the unit will need transverse reinforcement also to behave as a T-beam.

Fig. 3 Required bearing length at the support of a member in relation to its clear span (NZS 3101:Part 2:2006)

Fig. 4(a) Alternate continuous reinforcement through the beam at the level of bottom of fl oor to support precast concrete fl oor units

Fig. 4(b) Alternate continuous reinforcement through the beam at the level of bottom of fl oor and in the topping of slab fl oor to support precast concrete fl oor units

Codal provisions related to these failures have improved much after the 1994 Northridge earthquake. A comparison of the clauses related to the issues responsible for failure of precast concrete structures during earthquakes is attempted in following sub-sections. The clauses common to precast and cast-in-place concrete structures are not considered here.

Precast Concrete Floor Systems

A few common types of pre-cast concrete fl oors used in New Zealand are discussed: (i) fl at slab fl oor (ii) hollow-core concrete slab fl oor and (iii) double-tee fl oor. Flat slab fl oors (Fig. 5) can provide economic


solutions up to 6 m span. It consists generally of a series of 75 mm thick precast, prestressed concrete slabs with a reinforced concrete topping. The slabs are usually 1.2 m or 2.4 m wide, and require 75 mm end seating.

Fig. 5 Cross-section of a precast fl at slab fl oor (Ref: IB 76, 2004).

Figure 6 shows a section of a precast, prestressed concrete hollowcore fl oor panel with continuous longitudinal voids to reduce self-weight. These fl oor slabs can span up to 18 m (at 400 mm depth) and provide a working platform immediately after being positioned. Hollowcore slabs are generally un-propped during the casting of the topping. Concrete topping on precast fl oors can be of about 65mm to 75mm.

Fig. 6 Typical section of hollow-core concrete slab fl oor (Ref: IB 76, CCANZ, 2004)

Another type of precast fl oor used for long spans is a double tee unit consisting of two prestressed ribs with an integral fl oor connecting top slab (Fig. 7). The ribs can vary in depth from 200 to 600 mm, and the units are generally 2.4 m wide, although units may vary in size depending on the manufacturers. Double Tees typically span up to 19 m, and provide a safe platform, directly after placing, for subsequent work.Three types of support for precast concrete hollow-core or solid slab fl ooring units seated on precast beams, identifi ed by the New Zealand Guidelines are given in Park24. It is desirable to resist the relevant design forces by providing adequate connections by means of reinforcement and shear transfer mechanisms from

precast concrete diaphragms to components of the vertical primary lateral force resisting systems.

Fig. 7 Typical section of a Double Tee fl oor (Ref: IB 76, CCANZ, 2004)

Connections and Bearing

The codes permit a variety of methods for connecting members in plane and out of plane. These are grouted joints, shear keys, mechanical connectors, reinforcing steel connections, reinforced topping, or a combination of these. Codes suggest a minimum bearing length after considering for tolerances to be as the clear span/180 from the edge of the support to the end of the precast member. However this length should not be less than 50mm for solid or hollowcore slabs and 75mm for beams or stemmed members as per ACI 318-0832. NZS 3101 has a small change that for hollowcore slab this length is 75mm. Codes have suggested to have a clear distance of 15mm from the unarmored edges and make allowances for concrete cover. Required length of bearing at the support of a member in relation to its clear span is illustrated in Fig. 3.

Connections that rely solely on friction caused by gravity forces are not permitted by codes. NZS 3101 suggests in particular for hollow-core fl oor that the fl oor should be mounted on low friction bearing strips with a coeffi cient of friction less than 0.7 and a minimum width of 50mm.

Structural Integrity

Structural integrity is necessary to improve the redundancy and ductility in structures. This also helps to avoid collapse of the structures in the event of damage to major supporting element or an abnormal loading event by maintaining overall stability. Codes suggest provisions for precast concrete structures to achieve structural integrity to the same extent as of monolithic structures. Tension ties are provided in the transverse, longitudinal


and vertical directions and around the perimeter of the structure to effectively tie precast concrete elements together. This will also achieve the diaphragm action of the fl oor and a seismic load path in the structure. Recommendations are made for minimum provisions of geometry and reinforcement detailing of horizontal and vertical ties by the codes to achieve these actions. A typical arrangement of tensile ties is shown in the Fig. 8.

Fig. 8 Typical arrangement of tensile ties in precast concrete fl oors

Diaphragm Action

Precast concrete fl oor could not transmit in plane force induced by earthquakes to lateral load resisting system adequately and failed during past earthquakes. Codes have dealt with the design of precast concrete diaphragms similar to the cast-in-place diaphragms. Design and detailing provisions for both un-topped and composite diaphragms with topping are given in the codes. Codes have suggested the minimum thickness of topping to be 50mm for 20mm cover and 25MPa strength of concrete. It is further needed to be increased depending on the size of reinforcement and clear cover to be used.

ACI 318-0832 recommends minimum thickness of topping slabs placed on precast concrete or roof elements, acting as structural elements and not relying on composite action to be 67.5mm. NZS 3105 relates the minimum thickness of topping with the diameter of bars used. Minimum thickness of topping for 6, 10, 12 and 16 mm stirrups, ties or spirals used is 50, 75, 90 and 105 mm respectively. It is also suggested that if the cover is greater than 20mm then the thickness of topping should be increased by the amount of additional cover.

Deformation compatibility of fl ooring systems

Elongation of plastic hinge regions in beams result in the deformation incompatibility of fl oors with the support system. This phenomena is much observed in the collapse of hollow-core fl oors in 1988 Armenian

and 1994 Northridge earthquakes. To overcome this incompatibility issue and avoid the brittle failure, NZS 3101 have suggested for the precast fl oor systems to be designed to have adequate ductility. The code has suggested the connection details that have performed well in analytical and experimental investigations.

Precast Concrete Frame and Wall Systems

Codes have suggested the design and detailing of these systems to be same as cast-in-place system with taking particular care in designing the connection to emulate similar behaviour. Precast concrete frame systems composed of concrete elements with ductile connections are expected to experience fl exural yielding in connection regions. ACI 318-0832 has recommended the reinforcement provisions and type of mechanical splices to achieve the monolithic behaviour of connections.

The arrangement commonly used in New Zealand for strong column-weak beam designs with the objective to achieve behaviour emulating a monolithic structure is presented by Park21. Arrangement of precast members for constructing moment resisting reinforced concrete frame are divided into three systems. The precast concrete beam elements of System 1 are placed between the columns and the bottom longitudinal bars of the beams are anchored by 90-degree hooks at the far face of the cast-in-place joint core. For System 2, the vertical column bars of the column below the joint protrude up through vertical ducts in the precast beam unit, where they are grouted, and pass into the column above. The columns of the precast elements of System 3 are connected by longitudinal column bars which protrude into steel sleeves or ducts in the adjacent elements and are grouted. The beams are connected using cast-in-place joint at mid span. Capacity design procedure for these three systems ensures that yielding of the column bars at the connections is kept to a minimum. Figure 9 shows a further system using pretensioned prestressed concrete U-beams and cast-in-place reinforced concrete. Figure 10 shows a typical cross-section of such a beam and the arrangement of cast-in- place beam along with pre-cast fl ooring.

Connections between the precast panel and the cast-in-place foundation system are the most critical connections in precast concrete structures. In tall buildings, other wall panel-to-panel connections are also equally important. Horizontal joints in panel-to-


panel connections are a combination of grout and spliced vertical reinforcing bars for monolithic behaviour. The grout provides continuity for compressive forces across the joints and bars provide continuity for tensile forces. Fig. 11 shows the joint where vertical reinforcement is made continuous with the lapped bars in conduit.

Fig. 9 A structural system involving precast pretensioned prestressed concrete U-beams and cast-in-place reinforced concrete (Ref: CAE, 1999)

Fig. 10 Typical cross-sectional view of composite construction using precast shell beam (Courtesy: website: www.stresscrete.co.nz)

Fig. 11 Section at a joint with lapped splices in large conduit (Ref: ACI 550, 2009).

Over lapping bars in grout conduit are extended for full-height through the structural element. ACI 318-08 recommends welded and lapped splices to be located more than 2 times the fl oor thickness away from the face of the wall. Overlapping bars can also be made continuous by splicing bars with a threaded coupler. In such case wall panel is fi rst erected and held high. Loose vertical bars in the panel being erected are spliced to protruding bars from below. Panel is then lowered to correct elevation and conduit is grouted by gravity fl ow from top or optional grouting port from bottom of panel. Special mechanical splices Type 2 are recommended to be used by ACI 318-08. A typical mechanical splice for connection of walls and fl oors is shown in Fig. 12. Many other possible connection details are available in the report ACI 55033.

Fig. 12 Section at a joint with mechanical splices in large conduit (Ref: ACI 550, 2009).

Ideal locations for monolithic connections in precast concrete frame systems are the sections of minimum moments or point of infl ection. H-shaped and cruciform frame systems have connections at the points of infl ection likely to occur under lateral loading. Connections are typically similar to wall and fl oor connections shown in Figs. 6 and 7 by replacing wall and fl oor by column and beam respectively. In that case all the reinforcement crossing beam and column will be spliced. A typical column-to-column connection through conduits installed in a beam is shown in Fig. 13. Conduit diameter should be two to four times the bar diameter for tolerance in fi eld erection. This connection can also be modifi ed by cast-in-place closure of beam at the joint of beam and column.


Figuer 14 shows a typical connection of such type. These connections are detailed to resist the earthquake forces and deformation emulating cast-in-place detailing of beam-column joints.

Fig. 13 Column-to-column connection through conduits installed in a beam (Ref: ACI 550, 2009).

Fig. 14 Connection at beams and columns with cast-in-place closure (Ref: ACI 550, 2009).

Precast concrete systems composed of elements joined using strong connections are recommended to be designed using capacity design concept. Examples of strong connections for beam-to-beam, beam-to-column and column-to-footing are illustrated by ACI 318-08. These connections are intended to experience fl exural yielding outside the connections.

Precast concrete wall systems are restricted the yielding to reinforcement in connections between wall panels or wall panels and the foundation. The connections that are not designed to yield are recommended to develop over strength using capacity design concept.

The ductile-jointed hybrid connections in precast systems are permitted by codes with proper fi eld, ana-lytical or experimental evidences of good performance.

COMMENTS ON IS 11447: 1985 AND RECOMMENDATIONS TO INDIAN CODES OF PRACTICE

The code (IS11447:1985)34 only discusses the seismic design provisions of fl oors and load bearing walls in large panel prefab structures. Some salient points related to seismic design provisions as given in IS11447 are as under:1. Code suggests designing the large panel prefab

structures which is stable for all possible situations of precast construction.

2. Joints between members are recommended to be designed to resist the forces acting on them without excessive deformation and cracking. They should also be able to accommodate the deviations (tolerances) in the dimensions of the panels during production and erection.

3. It is recommended to provide the tie-beams at each fl oor level along all structural walls and along the perimeter of the building to obtain a monolithic action of walls and fl oors, and to limit the possibility of progressive collapse. Tie-beams may be designed as monolithic ones constructed at site during assembly or hidden ones constructed by connecting the bars placed on the fl oor panels.

4. Analysis and design of fl oors and walls is recommended to be performed as per IS 1893:200235.

The above provisions indicate to achieve the structural integrity and monolithic behaviour of large panel prefab structures. Studying the code and comparing with other international standards it is found that following points need attention for reconsideration or revision to achieve the good performance in seismic zones:

1. The code permits the large panel prefab up to 6m width which can be revised for large spans by enhancing the minimum provisions.

2. Minimum specifi cations for anchorage, bearing and topping are less than the other international codes studied.

3. In high seismic zones only solid slab concrete panels are recommended and their connection


is permitted between ribs only. While the other codes of practice allow using composite or non-composite topping slab (Figs. 4 to 8) reinforced and detailed to provide for a complete transfer of forces to the lateral-force-resisting system.

4. Specifi cations for joints detailing and ties are not suffi cient when compared to other national codes of practice. Detailed study in last section is helpful in specifying provisions to Indian standard in this regard.

5. Deformation compatibility provision for fl ooring system needs to be incorporated in the Indian standard in line with NZ standards.

6. Statement, Joint which rely on friction only due to the vertical forces should be avoided in seismic regions is must and should be incorporated in Indian standard.

7. Welding in seismic joints is not recommended in high seismic zones and should be avoided; however mechanical connections with ductile welds are permitted with appropriate over strength.

8. Clarity in all the provisions with a commentary explaining the behaviour is needed to be incorporated in the code.

9. A guideline for connection details for high seismic regions emulating cast-in-place construction is needed to be prepared.

10. IS: 4326 and other precast codes in India are there for small precast components only.

11. IS 13920 suggests that Precast and/or prestressed concrete members may be used only if they can provide the same level of ductility as that of a monolithic reinforced concrete construction during or after an earthquake.

12. There is no code for the construction of precast frame buildings in India. Beams, columns and moment resisting frames are now the part of precast construction and need to be included in the Indian code of practice.

It is suggested to include a chapter on general provisions on precast concrete element and structures in IS 456: 2000 and special provisions on seismic design of precast concrete elements and structures in IS 1893: 2002 and IS 13920: 1993, as is the practice in ACI 318 08.

CONCLUSION

Failure of precast concrete buildings during past earthquakes has raised a question mark in the construction of precast concrete buildings in seismic areas. A review of seismic performance and behaviour of precast concrete structures indicates that the buildings constructed and designed incorporating seismic design concepts performed remarkably well. This paper summarises the historical perspective on the performance of precast concrete structures and an extensive literature on experimental studies to demonstrate the improved seismic performance of precast concrete systems. A brief review of provisions on the design and construction of precast concrete systems in American, New Zealand and Euro codes and practices is presented to help in developing the provisions and practice of these systems in Indian perspective. It is concluded that the provisions in IS 11447: 1985 for seismic design of large panel prefab buildings are insuffi cient in general and from the earthquake resistant point of view. It is recommended to include a chapter on general provisions on precast concrete element and structures in IS 456: 2000 and special provisions on seismic design of precast concrete elements and structures in IS 13920: 1993. As a future scope of this paper, each precast concrete element can be dealt separately for studying the seismic performance, design and international codes of practice. Need of experimental investigation, is also felt to study the seismic performance of the structural element in Indian environment. Based on the experimental investigations and experiences on seismic performance a state-of-the-art report can be prepared for the analysis and design of precast concrete structures in the country.

ACKNOWLEDGEMENT

The fi rst author wishes to thank National Programme on Earthquake Engineering Education (NPEEE) of Ministry of Human Resource Development, Government of India, for awarding him a fellowship for conducting this research. He also wishes to express his gratitude to S. G. S. Institute of Technology and Science, Indore, India and University of Canterbury, Christchurch, New Zealand for providing him all the necessary facilities.


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2. Earthquake Spectra, 2002. Bhuj, India Earthqauke of 26 January, 2001 Reconnaisance Report, Earthquake Spectra, Supplement A to Vol 18, July 2002.

3. Matthews, J. G., Hollow-core fl oor slab performance following a severe earthquake, Ph. D. Thesis, University of Canterbury, Christchurch, New Zealand, 2004.

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(Discussion on this article must reach the editor before November 30, 2011)

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Overview of Precast concrete frame