Drift Capacity of Lightly Reinforced Concrete Columns

Transcription

1 Australian Earthquake Engineering Soiety Conferene, Perth, Western Australia Drift Capaity of ightly Reinfored Conrete Columns A Wibowo, J Wilson, NTK am, EF Gad,, M Fardipour, K Rodsin, P ukkunaprasit 4, ) Faulty of Engineering and Industrial Siene, Swinburne University of Tehnology, Hawthorn,, Australia ) Department of Civil & Environmental Engineering, University of Melbourne, Parkville,, Australia ) Department of Civil Engineering, King Mongkut s University of Tehnology, North Bangkok, Thailand 4) Department of Civil Engineering, Chulalongkorn University, Bangkok, Thailand ABSTRACT A lightly reinfored olumn is ommonly believed to be relatively brittle with a very low drift apaity. A researh projet has been undertaken to investigate ollapse behaviour of suh olumns. The effet of variation of axial load ratio and longitudinal reinforement ratio on flexural, yield penetration, and shear displaement as omponents of the drift apaity were observed. Interesting outomes showed that lightly onfined reinfored onrete was able to sustain gravity axial load onsiderably greater than the ode reommendations. Moreover, the present shear preditions available are defiient for lightly reinfored olumns, sine they tend to overestimate the nominal shear strength of the olumn. Keywords: Drift apaity, axial load ratio, reinfored olumn tests, light onfinement reinforement, seismi performane

2 INTRODUCTION. Bakground ightly reinfored onrete olumns are prevalent in many old buildings and ommon in urrent detailing pratie in the regions of lower seismiity. This type of struture is believed to have a very low lateral load and drift apaity from a onventional design perspetive. However, many post earthquake investigations (Otani,997, Wibowo et.al 8) show that the primary ause of reinfored onrete building ollapse during earthquakes is the loss of vertial-load-arrying apaity in ritial building omponents leading to asading vertial ollapse, rather than loss of lateral-load apaity (Ghannoum et.al, 8). For example, an existing building may ontain an overall weak lateral-fore resisting system that is suseptible to shear (point a) and a olumn that is suseptible to axial failure (point b), as an be seen in Figure (Moehle, 8). But even though the struture is upgraded by adding new shear walls and hene stiffened and strengthened by the new wall, the building remains vulnerable to loss of vertial-load-arrying apaity if drifts are not ontrolled to be less than the drift at axial load failure (point b). Therefore, an investigation and laboratory study of non dutile olumns has been undertaken to examine the drift apaity and the primary parameters that ontribute to the loss of olumn axial-load apaity.. Outline of the paper This paper desribes the seismi performane assessment of non-dutile olumns based on results from laboratory testing. The experimental testing of four olumn speimens has been undertaken by Swinburne University of Tehnology in ollaboration with The University of Melbourne and Chulalongkorn University. All four olumns were subjeted to quasi-stati yli lateral load. The speimens represent some of the most ommonly found detailing buildings in developing ountries and/or in low-to-moderate seismi regions. These olumns are haraterized by: moderate aspet ratio, lightly reinfored, limited lateral onfinement and moderate axial load ratio. SPECIMEN DESIGN Four olumn speimens were designed to represent a prototype of the non-dutile reinfored onrete olumns of old buildings in low-to-moderate seismi regions. The two parameters varied were the axial load and longitudinal steel reinforement ratio. The speimens were 7 mm antilever olumns with a height (to the appliation of lateral load) of mm. All speimens had Grade 5 reinforing bars with two speimens reinfored with four N, and the other two speimens reinfored with four N6 (longitudinal reinforing ratio of.56% and % respetively). In all ases, R6 stirrups were used at mm spaing orresponding to a transverse reinforement ratio of.7 whih is less than minimum lateral reinforement required by AS6. All perimeter ties had 5 o hooks with just half of the required length of urrent design odes. The onrete over was mm, whilst the speified onrete ompressive strength and steel yield stress were MPa, 56 MPa for main bars and 6 MPa for stirrups, respetively (details are presented in Table and Figure ). TEST SETUP The drift apaity of onrete olumns is made up of flexural, yield penetration, and shear omponents whih were measured using VDTs and strain gauges. The axial displaement was also measured to detet loss of axial-load apaity. Displaements were measured using eighteen linear variable displaement transduers (VDT), as shown in Figure a. The arrangement of

3 VDT was used in order to measure axial displaement (no. 8), total lateral displaement (no. -5), flexural displaement (no. 6-) and shear deformation (no. -7), whilst sixteen strain gauges were installed on the reinforement to measure the longitudinal and transverse strains (Figures b). Three strain gauge loations were used; one level for heking yield penetration length; the seond level at the footing-olumn surfae for measuring maximum strain needed for yield penetration; and the third level was at the middle of predited plasti hinge length. The axial load was applied and maintained using a hydrauli jak, whilst the lateral load was applied using an atuator with ton loading apaity (Figure 4). The displaement ontrolled loading sequene onsisted of drift-ontrolled mode at drift inrements of.5% until reahing % drift, and then followed by drift inrement of.5%. Two yles of loading were used in eah drift ratio to ensure that the hystereti behaviour ould be maintained. Disrete load stages were defined where lateral loading was held onstant whilst VDT and strain gauge measurements were taken, rak patterns reorded, and visual inspetions made. The test ended when the olumn lost the apaity to resist axial load rather than when peak lateral loading apaity of speimen was redued by %. ateral oad upgraded struture existing struture or omponent lateral failure a b Drift Vertial oad axial apaity urve axial failure olumn axial load demand Drift Figure. Ilustration of strength and deformation demands for existing buildings (Moehle, 8) Figure Geometry and reinforement details of olumn speimens (a) VDTs Figure Instrumentation (b) Strain Gauges

4 Table Basi Property of Column Speimens Spe. Dimension (mm) a (mm) AR ρ V Main Rebar ρ H Ties (@mm) n f (MPa) Hook type S % 4N.7 % R6@.. 5 o S 7 4. % 4N6.7 % R6@.. 5 o S 7 4. % 4N6.7 % R6@ o S % 4N.7 % R6@ o Notation : a is shear span whih is a lear-height of a olumn in this ase, AR is the shear span-to-depth ratio defined as shear span divided by the depth, n is the axial load ratio (ratio of the axial load to axial load-arrying apaity or A f ). ρ H is the longitudinal reinforement ratio. ρ V is the lateral reinforement (A sh /bs). A sh = total area of transverse reinforement; s = tie spaing; and b = olumn setion width. Figure 4 Setup of oading Test 4 EXPERIMENTA RESUTS Speimen S with.56% rebar ratio and % axial load ratio was able to sustain a maximum drift of 5% before gravity load ollapse with lassial plasti hinge formation at the base of the olumn and a rigid body roking mehanism as shown in Figure 5(a). Suh desirable behaviour is assoiated with yield penetration at the base allowing the joint to open and lose rather than raking and spalling of the onrete above the base. In ontrast, speimens S and S with almost twie the longitudinal reinforement tolerated lower maximum drifts of.5% and.5% for axial load ratio of % and 4%, respetively. While the analytial formulae predited flexural failure, the large tie spaing ( mm) in these speimens lead to bukling of longitudinal bars (φ6 mm) and an abrupt transfer of axial load from the steel bars to the onrete. This triggered shear failure due to the deterioration of onrete strength during the yli loading as an be seen in Figure 5(b) and 5(). Meanwhile speimens S4 and S, both with an axial load ratio of 4% responded in a similar fashion with a maximum drift ratio of.5%, despite the different rebar ratio. A detailed desription of the hystereti behaviour of all speimens is provided in Wibowo, et.al (), whilst an analysis of the omponents ontributing to the drift apaity is presented in the next setion.

5 (a) Speimen S (b) Speimen S () Speimen S (d) Speimen S4 (e) The envelope urve of all speimens (normalized to f MPa) Figure 5 Results of Experiment Test (f) Hysteresis Curve of eah speimen Table Main Parameters resulted from the test S S S S4 Maximum ateral oad (kn) Drift at maximum load (%) ateral load at 8% of peak load (kn) Drift at 8% of peak load (%) Axial failure lateral load (kn) Axial failure drift(%) Drift dutility DRIFT COMPONENTS This setion provides drift apaity omponent measurements based on experimental results. The lateral olumn displaement an be determined as the sum of the flexural, shear, and yield penetration omponents. The flexural displaement onsists of elasti and plasti omponents. And the measurement obtained for flexural, shear and yield penetration omponents an be seen in Figure Flexural Behaviour The vertial VDT (no. 6-) were used to measure flexural displaement (refer Setion ), where the average urvature of the segment an be estimated via: β f f ϕ = = () V δ δ H V

6 where v = height per eah segment, h = distane between flexural VDTs, δ f = vertial VDT measurement. The top lateral displaement of flexural omponent for eah VDT segment an be obtained using: Vi = ϕ ( x xdx = ( δ f δ f ) fi. i ) H Whilst, the displaement of upper segment without a VDT transduer an be alulated using () ϕ i fi = = Vi E I where V = lateral load; i = length of segment. () 5. Shear Behaviour The measured shear deformation sh an be estimated from the diagonal VDT, suh that ( δ δ ) ( δ δ ) + D s s s s sh = seξ = (4) v where δ s = diagonal VDT measurement, D = ross setion depth (parallel to lateral loading diretion). v 5. Yield Penetration Behaviour The yield penetration effet was obtained from strain gauge and VDT measurements at the olumn base interfae as shown in Figure 6() and showed good agreement. Slip displaement of tensile steel at gap opening an be estimated as follows (Sezen and Moehle,): ε ( ) sg s slip = 8 f ' E d b = ε sg 8 f s f d whilst, shortening displaement of ompressive steel an be obtained via ' b δ s = ε (6) sg olumn where ε sg and ε sg are strain gauge reading at tensile and ompressive steel respetively. Hene, neutral axis depth at olumn base interfae an be estimated via = δ s ( d d' ) + d' slip+ δ Slip rotation θ slip is given by s slip θ slip = (8) d (5) (7)

7 fl sh sh V4 v4 δ f β δ f V V V δ f β H δ f v v v δ s ζ D δ s V slip θ slip sg sg d d d D d H Figure 6 Measurement obtained from transduers and strain gauges: (a) flexure, (b) shear, and () yield penetration As a omparison, by assuming a roking mehanism within the first setion of olumn, an upper bound of slip displaement an be determined from vertial VDT measurement at the first level (no. 8 and ) as shown in Figure 6(). Neutral axis depth at olumn base interfae an be estimated via: δ f = δ + δ f f H d Whilst, the slip rotation of tensile steel an be obtained using: slip δ f δ f θslip = β = () d Hene, the related slip displaement an be alulated using: slip = β( d ) δ d f H + () (9) The top displaement of the olumn an be alulated from the produt of the slip rotation θ slip and the olumn heights assuming rigid body rotation. =θ () yp slip olumn 5.4 Drift Capaity Assessment Figure 7 shows the various omponents of lateral drifts of olumn for all speimens as a funtion of lateral load in order to qualitatively indiate the modes of failure. The flexural displaement is presented in two parts, within and outside the predited plasti hinge length p. All speimens show dominant flexural behaviour due to the large aspet ratio of 4, but only speimen S (ρ v =.56% and n=.) has a onentrated plasti deformation within the predited plasti hinge length area, whilst the other speimens developed a plasti hinge over a signifiant length up to the seond stirrup. This implies that for non-dutile olumns, the standard plasti hinge length preditions developed for dutile olumns are inadequate. Interestingly, speimen S4 (ρ v =.56% and n=.4) experiened a similar total failure drift apaity ompared with S (ρ v =.% and n=.4), but with different harateristis. S4 responded with a onsiderable flexural displaement omponent, whilst the shear omponent of S was

8 larger. It seems that an inrease in the longitudinal reinforement redues the flexural omponent of the total displaement regardless of the value of the total displaement itself. Displaement due to shear is signifiant in speimens with higher longitudinal rebar ratio (S and S) sine the olumns have higher moment apaities and hene higher applied shear fores. On the other hand, the smaller effet of shear deformation in speimen S was expeted due to the smaller amount of both main rebar ratio and axial load ratio. Overall for all speimens, the inrease of axial load ratio resulted in a redution of total displaement, with a greater redution for olumns with smaller main rebar ratio (ompared S/S4 and S/S). arge yield penetration drift ourred on speimens S and S due to smaller axial load ratio, with a larger effet on S (smaller amount of main reinforement) ompared S. In ontrast, both speimen S and S4 experiened only slight yield penetration drift due to the higher magnitude of axial load ratio. ateral oad (ton) S (ρv=.56% ; n=.) flexural - within p flexural - outside p shear yield penetration total displaement ateral oad (ton) ` S (ρv=.% ; n=.) flexural - within p flexural - outside p shear yield penetration total displaement Drift (%) Drift (%) ateral oad (ton) Drift (%) S (ρv=.% ; n=.4) flexural - within p flexural - outside p shear yield penetration total displaement Figure 7 Variation of displaements due to flexure, shear and yield penetration ateral oad (ton) flexural - within p flexural - outside p shear yield penetration total displaement Drift (%) S4 (ρv=.56% ; n=.4) 6 CONCUSION Experimental researh on non-dutile olumns has been undertaken by Swinburne University in ollaboration with University of Melbourne and Chulalongkorn University. Four lightly onfined onrete olumns have been tested with variation of main rebar ratio and axial load ratio parameters. The drift omponents of flexure, shear and slip and their effet on overall behaviour have been presented in this paper, based on the experimental results. REFERENCES FEMA NEHRP guideline for the seismi rehabilitation of buildings. Federal Emergeny Management Ageny. Washington DC, USA. Ghannoum, W.M., Moehle, J.P., and Bozorgnia, Y. 8. Analytial Collapse Study of ightly Confined Reinfored Conrete Frames Subjeted to Northridge Earthquake Ground Motions. Journal of Earthquake Engineering, Vol, pp 5 9.

9 am, S.S.E., Wu, B., Wong, Y.., Wang, Z.Y., iu, Z.Q. and i, C.S.. Drift Capaity of Retangular Reinfored Conrete Columns with ow ateral Confinement and High-Axial oad. Journal of Strutural Engineering, ASCE, Vol. 9(6), pp Moehle, J.P. 8. Earthquake Collapse Risk of Older Conrete Buildings. Pro. uis Garia Symposium. Bogota, Colombia, May 8. Otani, S RC Building Damage Statistis and SDF Response with Design Seismi Fores. Earthquake Spetra, Earthquake Engineering Researh Institute, Vol. 5, No., pp Priestley, M.J.N., Verma, R., and Xiao, Y Seismi shear strength of reinfored onrete olumn. Journal of Strutural Engineering, ASCE, Vol. (8), pp.-9. Sezen, H., and Moehle, JP,. Bond-slip behavior of reinfored onrete members, fib-symposium: Conrete Strutures in Seismi Regions, CEB-FIP, Athens, Greee. 4. Sezen, H. and Moehle, J.P. 4. Shear strength model for lightly reinfored onrete olumns. Journal of Strutural Engineering, ASCE, Vol. (), pp Wibowo A, Wilson J, Gad EF, and am NTK.. Collapse Modelling Analysis of a Preast Soft-Storey Building in Melbourne. Engineering Strutural Journal, Elsevier. Speial Issue: earning Strutural Failures, Vol. (7), July, pp Wibowo A, Wilson J, Fardipour M, am NTK, Rodsin K, ukkunaprasit P, Gad EF.. Seismi Performane Assessment of ightly Reinfored Conrete Columns. Pros. Of Australasian Conferene on the Mehanis Strutures and Materials, Melbourne, Australia, 7- Deember, paper no. 68. Wibowo, A., Kafle, B., Kermani, A.M., am, N.T.K, Wilson, J.., Gad, E.F. 8. Damage in the 8 China Earthquake. Pros. of Australian Earthquake Engineering Soiety Conferene, Ballarat, Australia, - November.