Designing structures against extreme loads

Tailor Made Concrete Structures – Walraven & Stoelhorst (eds)© 2008 Taylor & Francis Group, London, ISBN 978-0-415-47535-8

Seismic behaviour of precast column to foundation joint

G. MetelliUniversity of Brescia, Brescia, Italy

P. RivaUniversity of Bergamo, Bergamo, Italy

ABSTRACT: This paper aims at presenting the results of two experimental tests concerning the cyclic behaviourof a dissipative column-to-foundation connection for precast concrete elements. The joint is characterized bythe use of high strength threaded steel bars in grouted sleeves and by steel support elements which allow aneasy assemblage and centering of the column, meanwhile ensuring its stability before the grout injection. Thetests allow to compare the response of cast-in-place connections against pocket foundation and grouted sleevesolutions.

1 INTRODUCTION

Warehouses and commercial malls in Italy are gener-ally built using precast reinforced concrete elements.The typical structural layout consists a moment resist-ing frames with plastic hinges occurring at columnbase and beams hinged to the columns. The founda-tions are usually made of isolated precast cup-footings,in which the columns are inserted and grouted in-situ.Such a structural layout is extremely cost effective,and sensibly reduces the construction time. How-ever, its effectiveness is seriously hampered when itis intended for construction in seismic areas, particu-larly if Capacity Design (CD) based codes are adoptedin the design process (Eurocode 8, 2003, OPCM3274, 2003). Adopting a CD approach, the foundationbase becomes easily very large, also for medium-sized columns. Hence, cast-in-situ mat foundationswith the column-to-foundation connection done bymeans of grouted sleeves become often more con-venient than isolated footings. However, the seismicresponse of such column-to-foundation connections isnot well documented. In fact, while a large amount ofexperimental tests have been carried out on columnssubjected to cyclic loading (e.g. CEB, 1996), fewexperimental results concerning the cyclic responseof grouted sleeve column-to-foundation connectionsare available in the literature (Riva, 2006).

2 THE CONNECTION SYSTEM

As shown in Figures 1–2, the Edilmatic connectionsystem for column-to-foundation joints is composed

Figure 1. Edilmatic connection system for column-to-foundation joints in precast reinforced concrete structures.

Figure 2. The assembled Edilmatic joints before the groutinjection.

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of the following main components: threaded bars withbushes (1) embedded in the column and tied to thecolumn reinforcement; plastic ducts (2) deactivate thebond of longitudinal high strength steel bars for alength of 200 mm in order to provide adequate duc-tility and dissipative capacity of the joint in case ofcyclic action; threaded bars (3) tightened to the col-umn bushes allow the bending actions to be transferredfrom the column to the foundation by the bond shearstress developing in grouted and corrugated sleeves(4); during the assembling phase, the column must besupported by steel S355 support plates (5) which arefixed to studs (6) embedded in the foundation pocket;these plates and studs allow an easy assemblage andcentering of the column by means of a system of nutsand washers, meanwhile ensuring its stability beforethe grout injection (7) in the sleeves. The cast of thehigh performance grout is carried out in two phases:during the first phase, the grout is cast inside thesleeves while the pocket is cast in a second phase, afterthe removal of the support plates and after 12 hourscuring of the grout cast in the sleeves.

3 TEST SET UP AND RESULTS

The response of two columns (section 450 × 450 mm,height 3200 mm) connected to a foundation (section600 × 800 mm, length 2000 mm) by using the Edil-matic system, was investigated under a cyclic tophorizontal displacement history. The 4M27 and 8M18specimens are characterized by having four or eightgrouted sleeves for high strength M27 or M18 threadedsteel bars with an anchorage length, designed accord-ing to experimental results of pull-out tests, equal to 15times the bar diameter. The mechanical characteristicsof materials are shown in Figure 1.

For all the tests, the axial force, equal to 600 kN,was first applied by means of two hydraulic jacks.A cyclic horizontal displacement of increasing ampli-tude was then applied at the top of the column by meansof a 1000 kN electromechanical screw jack having a500 mm maximum stroke.

As shown in Figures 3–4, the three specimens haveexhibited a good behaviour in terms of strength andductility. The specimen 8 M18 has reached a maxi-mum load and bending moment slightly larger thanthose of specimen 4 M27, as well as a slightly higherductility, having reached a 5.5% drift against the 5%drift of the 4 M27 specimen. Ii is observed that thedrift at failure are much larger than the maximum driftcommonly accepted for prefabricated columns undera design seismic event, equal to 2.5%.

The collapse of the column-foundation joint wasdue to the failure of the threaded studs for cyclesof amplitude higher than 5% drift, value that resultsbeyond the specified code limits for the ultimate state.

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Figure 3. Test results for 4 M27-a joint, removing theleveling plate.

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-� -5 -� -� -� -1 0 1 � � � 5 �

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Figure 4. Test results for 8 M18 joint, removing the levelingplate.

It is worth pointing out that in tested connectionsthe damage is localized in the concrete cover at thecolumn base and in the pocket grout. As a result, verylittle damage may be observed in the column outside ofthe base section, thus allowing an easier post-seismiccolumn repair.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the support ofEdilmatic srl (Pegognaga, Italy) for financing thisresearch project on the seismic behaviour of joint inprecast concrete structures and Mozzo Prefabbricati(Verona, Italy) for the production of concrete elements.The cooperation of Nicola Barzi and Consuelo Beschiin carrying out the tests are gratefully acknowledged.

REFERENCES

Eurocode 8: Design of structures for earthquake resistance –Part 1: General Rules, Seismic Actions and Rules forBuildings, PrEn 1998-1, European Committee for Stan-dardization, December 2003.

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OPCM 3274 2003. First elements concerning general crite-ria for the seismic classification of the Italian territoryand seismic code provisions (in Italian), and followingmodification of O.P.C.M. n◦ 3431 (3 maggio 2005).

Riva, P. 2006. Seismic behaviour of Precast Column-to-Foundation Grouted Sleeve Connections. Proceedings of

the International Conference on Advances in Engineer-ing Structures, Mechanics, and Construction, Universityof Waterloo, Waterloo, Canada, 14–17 May 2006.

CEB Bullettin No.231 1996. RC Frame under EarthquakeLoading. London: Thomas Telford Ltd.

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