Capacity assessment and strengthening of pre-stressed concrete girders: A case study

Transcription

1 Capacty assessment and strengthenng of pre-stressed concrete grders: A case study C. Maraveas 1), Z. Fasoulaks 2) 1), 2) Dept. of Structural Eng., C. MARAVEAS PARTNERSHIP-Consultng Engneers, Athens, Greece 1) School of M.A.C.E., Unversty of Manchester, UK 1) c.maraveas@maraveas.gr ABSTRACT The present case study nvestgates the capacty assessment and subsequent strengthenng of pre-stressed concrete grders and provdes addtonal nformaton on the strengthenng measures taken for the whole of the structure nvolved. It s based on an exstng concrete buldng constructed n Structural modfcatons had to be made to ensure a proposed load-bearng capacty upgrade due to change of use. The buldng contans concrete grders pre-stressed wth the post-tensonng method, the mechancal and geometrc propertes of whch were not avalable, although t was known that the two-span grders were ntended to meet hgh-capacty demands. Nonetheless, an accurate smulaton of the structure was essental. For ths purpose, non-destructve methods had to be employed, whle safety factors were ntroduced n accordance wth the correspondng code provsons, n order to mnmze structural uncertantes. Subsequently, fnte element models were created to evaluate the response of the structure under the new loadng condtons, and the necessary modfcatons were proposed. 1. INTRODUCTION Structural behavor assessment and strengthenng demands have always been a great challenge for cvl engneers, especally wth ncreased structural complexty. Consequently, creatve solutons and sophstcated methods have evolved through the years, provdng engneers wth the best possble results n that partcular feld. The relable evaluaton of the load-bearng capacty s of crucal mportance for extendng the lfe of structural elements. Thus, a subsurface radar applcaton was used for determnng the geometry of the steel tendons nsde the grders, whereas the materal propertes were estmated through a combnaton of destructve and nondestructve methods. The grders under examnaton were subsequently smulated usng commercal fnte element software. For a more detaled response evaluaton, the smulaton was conducted for fve dfferent constructon stages. The specfcatons of Eurocode 2 were also taken nto consderaton. 1) Techncal Drector 2) Structural Engneer

2 The strengthenng process nvolved ncreasng the cross sectons of most load bearng elements and applyng steel plate attachments, n order to satsfy both ultmate and servceablty lmt state desgn crtera. Fnally, the addton of new stff structural members was deemed necessary for the mprovement of the buldng's behavor under sesmc loadng. 2. STRUCTURE DETAILS AND DESIGN PRINCIPLES The constructon of the buldng under examnaton, whch s stuated near the center of Athens, Greece, dates to Orgnally, t was used as a prntng and bookbndng factory. Its new use s by the Praeus Technologcal Insttute for educatonal purposes, whch ntroduced the demand for ncreased lve loads due to the presence of large crowds. 2.1 Geometry and structural system descrpton Investgaton results The total surface of the buldng s approxmately 1870m 2 (rectangular layout wth approxmate dmensons of 31.00m by 60.00m). It conssts of a basement and three addtonal levels wth correspondng heghts of 5.00m, 6.25m, 4.20m and 3.70m. The structural system conssts of concrete frames wth pre-stressed two-span twn grders (prmary beams) n the transverse drecton and smply supported pre-stressed grders (secondary beams) n the longtudnal drecton. The maxmum lengths of the beams are 15.15m and 9.95m for each drecton respectvely, whereas the crosssecton of the examned grders vares from 20/60 cm to 35/80 cm. The geometry of the ground floor formwork s presented n Fg Fg. 1 Plan vew of the ground floor formwork

3 Inverted-T beams wth a heght of up to 3.00m were postoned along the structure s permeter n order to receve the horzontal and sesmc loads, functonng as concrete walls, as the structure lacked them. Ths created the danger of possble short columns. In terms of load transferrng, durng the nvestgaton of the structure t was determned that the slabs were smply supported on the secondary beams. Thus, the unform dstrbuton of vertcal floor loads on the secondary beams and subsequent transfer on the prmary beams as concentrated loads s justfed. The orgnal desgn specfcally mentoned that the Morand post-tensonng method was employed for pre-stressng the beams. However, any other nformaton about the tendons was absent. Durng the vsual n stu nspecton, t was determned that steel renforcement bars of varous dameters and numbers were used as tendons, nstead of mult-strand cables. In addton, after examnng the anchorng postons, t resulted that the prmary beams were frst constructed as smply supported and then were stressed only from one end (the one at the permeter of the buldng). All ths led to the concluson that, n fact, the proposed pre-stressng method had not been mplemented, and nstead an equvalent one had been used. Further nvestgaton establshed the capacty of the tendons at the tme of constructon. Vsual nspecton of the buldng revealed that the concrete (at least at the permeter frames of the buldng) was n unfavorable condton due to envronmental factors, whch mples that the outer steel renforcement bars had not been adequately protected aganst corroson; t was reasonable to assume that the stuaton was no dfferent for the nternal frame steel renforcements. Specal menton must also be made to the observed secondary beam crackng. Ths fact was attrbuted to ether nadequate pre-stressng or tendon force relef over tme. The aforementoned deteroraton rate of the steel renforcement bars was promptly evaluated va tests, estmatng ther dynamc varance, as the subsequent corroson of the new steel renforcements would be crucal for the safety of the structure. In Fg. 2, one can observe (a) the anchorage type at the exteror vew of the buldng and (b) an nteror vew of the frst floor, where the structural members are clearly vsble. In Fg. 2a, steel plates can also be notced, whch contrbute as shear renforcement on the frst floor external jonts, most lkely due to some knd of falure aganst sesmc actons n the past. Fg. 2 (a) Exteror and (b) nteror vews of pre-stressed concrete grders

4 2.2 Geophyscal applcaton For further assessment of the tendon geometry, other non destructve methods had to be appled. To ths end, a georadar method was utlzed, whch conssts of transmttng low frequency electromagnetc sgnals whch gve notfcaton for each dentfed nterface that separates areas that has a dfferent delectrc constant. The reflected energy proporton depends on the varance of the delectrc constant. Subsurface radar has become an establshed nvestgatve tool for assessment of concrete structures, whle t has been proved useful for locatng steel tendons n prestressed concrete; n ths study t was appled accordng to ASTM specfcatons. Before the devce s placed over a beam under examnaton, ts last poston s confrmed through an nvestgaton on the transverse drecton. The georadar devce s presented n Fg. 3. Fg. 3 Detecton of tendons usng modern geophyscal methods For the detecton of the tendons along the entre beam length, a contnuous recordng was preferred over selectve pont recordng. In Fg. 4, one can dentfy the tendon pathway along the pre-stressed grder. The aforementoned procedure was repeated for a large number of beams for each floor (approxmately 120 tests n total).

5 Fg. 4 Georadar method results along pre-stressed grders n graphcal form (not to scale) (Top: secondary beam / Bottom: Prmary beam)

6 2.3 Eurocode specfcatons In order to perform the capacty assessment, the specfcatons of the Eurocodes were used, namely (EN 1992) for both renforced and the pre-stressed concrete members of the structure, whle the mposed loads followed the specfcatons of (EN 1991). In addton to the self-weght of the structure, dstrbuted dead loads of 0.50 kn/m 2 were consdered on the slabs. The lve load s 6.00 kn/m 2, for area Category C3 per (EN 1991), snce the structure s ntended to meet hgher capacty demands than those t was orgnally desgned for. The mechancal propertes of the concrete were determned from laboratory tests on samples taken from the feld. Furthermore, non destructve methods were combned for the concrete beams and columns. As a result, the concrete slab and columns were categorzed as C20/25, whle for the beams the propertes corresponded to category C30/37. For the smulaton of the tendons, hgh strength steel was recommended, namely wth characterstc yeld/ultmate stress values of 800/1050kN respectvely, usng tensle forces equvalent to 55% of the ultmate stress. Fnally, steel renforcement bar category was specfed as B500c. For the desgn checks the followng load combnatons were used, as defned n (EN 1990). Eq. 1 ndcates the combnaton of actons other than sesmc: S d γ G, jgk, j " " γpp" " γq,1q k,1" " γq, ψ0, Qk, (1) j where + mples to be combned wth, the summaton symbol Σ mples the combned effect of, G k,j denotes the characterstc value k of a permanent acton j, P ndcates the relevant pre-stressng actons and Q k, refers to the characterstc value k of a varable acton. The sesmc actons were combned accordng to Eq. 2 (EN 1998), whle the combnaton of the horzontal components of the sesmc actons s defned n Eq. 3: S d Gk, j " " Ed" " ψ 2, Qk, (2) j d E X 0. EY or Ed EY 0. 30E X E 30 (3) where E d s the desgn value of the sesmc acton and ψ 2, s the combnaton coeffcent for a quas-permanent acton, taken equal to E X and E Y are the horzontal sesmc actons along the X and Y drecton of the structure (longtudnal and transverse respectvely). In addton, servceablty requrements were taken nto account, by combnng actons (Eq. 4) accordng to (EN 1990): S d Gk, j " " P" " Qk,1" " 0, Qk, (4) j

7 3. NUMERICAL ANALYSIS The exstng structure was smulated as a three-dmensonal model, usng beam elements for the beams and columns. Apart from the overall structure, the separate smulaton of sngular pre-stressed beams was also deemed necessary. In ths case, the columns were ncluded for up to half the level heght above and below the grders, n order to smulate the approprate boundary condtons for the beam ends. In Fg. 5, 3D vews of both models are presented. Partcular attenton was gven to the numercal analyss of pre-stressed concrete. For the accurate computaton of the nternal forces, several load cases had to be taken nto consderaton. In partcular, 5 load cases representng constructon stages were chosen, actvatng n each one the correspondng elements, n order to consder tmedependent deformatons of the concrete,.e. shrnkage and creep. Sesmc loads were not taken nto consderaton for these desgn checks, snce the sesmc response of the global structural model dd not exceed 33% of the one derved from the combnaton defned n Eq. 1, accordng to the numercal analyses. Fg. 5 3D vews of the models used to smulate a) the overall structure and b) a twospan pre-stressed grder

8 4. STRUCTURAL EVALUATION OF THE EXISTING BUILDING In order to detect the crtcal areas of the concrete members, the stress dstrbuton was frst obtaned from all the load cases. Subsequently, the code-based crtera were appled to evaluate the exstng resstance (flexural, shear, and axal) of concrete members and perform the requred adequacy checks. Results showed that capacty was exceeded n many concrete members due to lack of flexural or shear renforcement, whle slab thckness was found nadequate aganst shear and punchng shear. 5. STRENGTHENING PROPOSAL Based on the analyses results, the strengthenng strategy conssted of two parts, the strengthenng of exstng load bearng members (both pre-stressed and renforced), and the addton of new stff members n approprate places. All concrete columns along the buldng permeter were coated wth renforced concrete jackets of mnmum 25cm thckness, n order to mprove resstance aganst sesmc loads. For the same purpose, 10cm thck jackets were appled on the nternal columns. The prmary pre-stressed beams were strengthened by ncreasng the adjacent slab thckness va renforced jackets n areas around columns, thus mprovng ther T-beam functon. Ther flexural strength was also enhanced locally by attachng steel plates around them. The ncrease n slab thckness also solved the problem of slab nadequacy aganst shear and punchng shear forces. The permeter nverted-t concrete beams functonng as concrete walls were partally demolshed and reshaped as beams wth a much shorter web heght and greater wdth, n order to avod short column effects. Thus unavodably emerged an even greater need for horzontal load bearng elements than before, whch was addressed by ncreasng the cross sectons of the columns and constructng new concrete starway cores, whch contrbute to the redstrbuton of the sesmc forces. The adequate nteracton between the exstng and added concrete and renforcement bars was ensured wth the use of dowels, whle new renforcements were also welded wth the old ones where possble, thus securng cooperaton between them and proper transfer of forces. The modfed structural members wth addtonal steel renforcements were checked n accordance wth the mnmum requrements of (EN 1992) specfcatons. Concrete qualty C30/37 and renforcng steel qualty B500c was used for all the addtons. The modfed cross-sectons of two permeter columns of the ground floor are presented n Fg. 6.

9 Addtonal renforcements Concrete jacket Exstng column Steel renforcement cooperaton va welds Exstng renforcements Steel renforcement cooperaton va welds Exstng beam Exstng renforcements Exstng beam 0.95 Addtonal renforcements 2.01 Concrete jacket Fg. 6 Cross-sectons of modfed concrete columns In Fg. 7, two examples of prmary pre-stressed beam strengthenng are llustrated. In partcular, one can see the concrete renforcement of the concrete slab n the area around the column for a frst floor beam (Fg. 7a) and the beam strengthenng by use of steel plates for a ground floor beam (Fg. 7b). Dowels Concrete jacket Steel renforcement cooperaton va welds a) Exstng concrete slab Exstng concrete Exstng pre-stressed beam Steel plate Exstng column Addtonal renforcng steel Steel plates Modfed column Exstng column b) Prmary pre-stressed beams Steel plates Connecton wth bolts Fg. 7 Cross-sectons of pre-stressed concrete beams of a) frst floor and b) ground floor after strengthenng modfcatons

10 6. CONCLUSIONS The nvestgaton n ths study focused on the structural response and strengthenng of pre-stressed concrete grders. The capacty assessment of the exstng structure nvolved destructve methods for the evaluaton of concrete mechancal propertes, whereas the tendon geometry was specfed through a geophyscal method. In addton, non-destructve methods proved qute useful for determnng the corroson rate of the exstng steel renforcements and tendons. Numercal analyses followed wth approprate smulatons of members and boundary condtons, whle the European Standards were mplemented n order to check the structural elements. Several load stages were taken nto account for each analyss. The results demonstrated that the structure was n need of serous strengthenng n order to cope wth ts new use requrements. The proposed modfcatons conssted of addton of new structural members and strengthenng of exstng ones, by ncreasng concrete cross sectons and addng steel renforcements and plates. REFERENCES EN 1990 (2002), Eurocode Bass of structural desgn, CEN, Brussels. EN 1991 (2002), Eurocode 1 Actons on structures, CEN, Brussels. EN 1992 (2004), Eurocode 2 Desgn of concrete structures - Part 1-1: General rules and rules for buldngs, CEN, Brussels. EN 1998 (2003), Eurocode 8 Desgn of structures for earthquake resstance - Part 1: general rules, sesmc actons and rules for buldngs, CEN, Brussels. Earthquake Plannng and Protecton Organzaton, Greek Code for Sesmc Interventon, Greece. Earthquake Plannng and Protecton Organzaton, Greek Sesmc Code, Greece. Hurst, M.K. (1998), Pre-stressed concrete desgn, Second Edton, E&FN Spon, New York. Larson, N., Gomez, E.F., Garber, D., Bayrak, O. and Ghannoum, W. (2013), Strength and servceablty desgn of renforced concrete nverted-t beams, Center for Transportaton Research, Unversty of Texas, Austn. Telford, W.M., Geldart, L.P. and Sherff, R.E. (1990), Appled geophyscs, Second Edton, Cambrdge Unversty Press, New York. Bungey, J.H., Mllard, S.G. and Shaw, M.R. (1997), Radar assessment of posttensoned concrete, Proceedngs of the 7th Int. Con. on Structural Faults and Repar, Ednburgh, UK, vol. 1. Jaeger, B.J., Sansalone, M.J. and Poston, R.W. (1996) Detectng vods n grouted tendon ducts of Post-Tensoned Concrete Structures Usng the Impact-Echo Method, Struct. J., 93(4),