CHAPTER 9 CONCLUSIONS

Transcription

1 78 CHAPTER 9 CONCLUSIONS uctlty and structural ntegrty are essentally requred for structures subjected to suddenly appled dynamc loads such as shock loads. Renforced Concrete (RC), the most wdely used constructon materal, possesses consderable mass per unt cost, excellent fre-resstance characterstcs and can also absorb large amount of energy, f provded wth proper detalng. However, one of the dsadvantages of concrete s the possblty of spallng/scabbng when t s subjected to shock loadng, whch weakens the core and affects the ntegrty of the structure. Among the alternatve systems of constructon, Laced Renforced Concrete (LRC) and Steel-Concrete Composte (SCC) constructon are found to possess propertes that are promsng for shock resstant structures. In ths research work, a new form of Steel-Concrete Composte (SCC) system s proposed, after analyzng the lmtatons of exstng systems. In addton to ths, an equvalent stress-stran relatonshp for the analyss of Laced Composte Systems such as Laced Renforced Concrete (LRC) and Laced Steel-Concrete Composte (LSCC) s derved. A smplfed approach to solve problems of equally renforced RC / LRC structural elements subjected to flexure s proposed. Equatons for obtanng the equvalent stress and stran characterstcs for equally

2 79 renforced LRC beams under flexure have been derved retanng the moment-curvature characterstcs. The proposed approach s able to predct the peak load and ductlty factors satsfactorly for LRC beams. Laced Renforced Concrete (LRC) has proven performance aganst blast loadng. Steel-Concrete Composte (SCC) can be consdered as an alternatve materal n vew of complex detalng requrements n LRC storage structures partcularly at the jonts and also concretng poses problems. Performance of SCC under blast loadng s studed usng a smplfed model to see the sutablty of SCC as an alternate materal to LRC. Steel-concrete composte panels can be constructed rapdly and can be best alternatve for laced renforced concrete for blast resstant constructon. However, the weldng of studs and placng concrete n between the parallel plates etc., wll pose problems n constructon. A system that can combne the advantage of steel-concrete composte together wth lacng appears to be the best choce. A new Laced Steel-Concrete Composte (LSCC) system s proposed. It conssts of two thn steel cover plates connected usng lacngs and cross rods, and flled wth concrete. Ths method of fabrcaton avods weldng n total. onotonc load testng under four pont flexure on two specmens, one wth 45 lacng and another wth 60 lacng, are conducted under dsplacement control mode. Expermental results ndcate that both the beams exhbt almost smlar strength performance, whle the one wth 60 lacng perform better n terms of deformaton. Falure of the beams could not be acheved due to constrant n test setup on maxmum dsplacement of actuator, the translaton of the roller and the possblty of slppage of roller. At ths stage, maxmum support rotatons acheved by LSCC beams wth 45 and 60 lacngs are 3 and 6 respectvely.

3 80 Reverse cyclc load has been appled on two specmens of LSCC beams. Both the specmens are found to exhbt almost smlar behavour. axmum load attaned under both saggng and hoggng moment condtons are found to be nearly equal. The envelope of cyclc load dsplacement response ndcates softenng response after reachng a peak value. Load drop n LSCC-60-C specmen s found to be about %, whle n LSCC-45-C specmen t s about 6%. Spallng of concrete cover s a common problem wth RC structural element subjected to reversed cyclc loadng, because each element comes alternatvely n compresson and tenson. But, n LSCC beams, steel cover plates prevent the spallng of concrete core also. Energy absorbed by the beam n each load cycle s calculated from the load deformaton curves. Among the three load cycles at a level, t s always the frst cycle whch s found to absorb more energy. Energy absorbed n the second and n the thrd cycles are found to be n the range 70-95% and 65-85% respectvely of that absorbed n frst cycle for both the specmens. However, energy absorbed s estmated to be nearly the same for the load appled n the upward as well as n the downward drectons. Fnte element model wth sold, plate and lnk elements representng concrete core, cover plates and lacngs respectvely s generated for numercal analyss. Behavour of concrete s smulated through concrete damaged plastcty model. ult-lnear and b-lnear stress stran curve are used for representng the cold formed steel and hgh strength deformed bars used for cover plates and lacngs respectvely. Loaddsplacement response obtaned from numercal nvestgaton are n close agreement wth that of expermental results. Smplfed materal model proposed for LRC s extended for LSCC beams. Ths smplfes the entre task of modelng. Hysteretc model wth pnchng s used to obtan the

4 8 load-dsplacement response analytcally. Energy absorbed s calculated from the load-dsplacement response and s found to be n close agreement wth expermental results. Ths model s extended untl 25% degradaton n strength s acheved to get the cumulatve energy absorbed by the LSCC beams. A model to predct the shear strength of LSCC beams s proposed based on the observaton of stran varaton n lacngs. uctlty of LSCC beams s comparatvely more than LRC and conventonal RC beams. Comparson of responses between LSCC beams and SCC beams wth other form of connectors shows the advantage of LSCC beams. LSCC beams exhbt comparatvely a better performance under cyclc load. Cyclc ductlty factors of LSCC beams are substantally hgher than that of LRC beams wth and wthout fbres. Shear strength of LSCC beams are relatvely hgher than that of LRC beams. Engneerng parameters for evaluatng the cyclc behavour of LSCC beams are suggested. Based on the expermental, analytcal and numercal studes carred out on LRC and LSCC beams n ths nvestgaton, followng conclusons are made: A smplfed approach for analyss of laced renforced concrete (LRC) structural elements s formulated. Equatons for obtanng the equvalent stress and stran characterstcs for equally renforced LRC under flexure have been derved retanng the moment-curvature characterstcs. The proposed approach and smplfed materal model usng equvalent stress-stran curve s able to predct the peak load and ductlty factors satsfactorly for LRC beams. The

5 82 stress-stran characterstcs can be easly adapted n any fnte element software usng mult-lnear nelastc sotropc materal model. The proposed approach s extended for solvng a LRC slab subjected to unform dstrbuted loadng. The model can be used for ordnary renforced concrete by modfyng the ultmate stress and stran values. Ths model can be extended for steel-concrete composte flexural elements. Equatons for dervng the equvalent stress-stran curve are: ' ' b ' 2 k d 2 ' 2 d d d d From the above relatons, equvalent stress, s evaluated. Equvalent stran, s calculated from 2 New user frendly laced steel-concrete composte (LSCC) system possessng the essental propertes for blast resstant constructon, namely, ductlty and structural ntegrty, s proposed. LSCC system comprses of thn steel cover plates provded wth apertures / perforatons, through whch renforcements n the form of lacngs are ntroduced and held n poston wth the help of transverse / cross rods, after whch concrete s flled n between the cover plates. Ths method of fabrcaton avods weldng n total. Expermental nvestgatons on two LSCC beams are carred out under monotonc and reverse cyclc loads.

6 83 Support rotatons of more than 6 are acheved by LSCC beams uctlty of LSCC beam wth 60 lacng s found to be more than that of beam wth 45 lacng, whle ther moment carryng capactes are nearly equal. Under reverse cyclc loadng, both the specmens are found to exhbt almost smlar behavour. axmum load attaned under both saggng and hoggng moment condtons are found to be nearly equal. Envelop of cyclc load-dsplacement response ndcates softenng after reachng a peak value The pnchng model for hysteretc behavour wth the formulaton proposed by Ibarra et al (2005) s able to completely predct the cyclc behavour of the LSCC beams. The followng engneerng parameters are suggested for evaluatng the cyclc behavour of LSCC beams: c = -0.0 s = 0.25 = 250 c =.25 d = 0.3 f = 0.7 c = y 3.5

7 84 LSCC beam s found to have hgh rotatonal capacty as compared to that of LRC and RC beams and steel-concrete composte (SCC) beams wth other form of connectors. It s observed from the varaton of lacng stran wth load n cyclc load reversals that only tensle stran are regstered for both postve and negatve dsplacements. Based on the above observaton a model for shear resstance of LSCC beams has been proposed, whch ndcates that LSCC beams are unlkely to fal n shear even under low shear span to depth ratos. The shear strength, V u where 4 d t w f u 3 f t bd Vu 2nA d = depth of web, t = w f = u thckness of web, s gven by s f y sn bdf t cos ultmate stress of cold-formed steel, f t = tensle stress = 0.3 fck, f = ck cube compressve strength of concrete, b = wdth of the beam, d = depth of the beam, A = s f = y area of cross-secton of lacng, yeld stress of lacng steel, = angle of nclnaton of lacng, and n = no. of parallel planes of lacngs Thus, a new form of steel-concrete composte beam s proposed and expermentally evaluated. LSCC system s found to possess the

8 85 essental propertes, namely, hgh ductlty, support rotaton and structural ntegrty for resstng suddenly appled dynamc loads such as shock loads, whch are the man objectves of the present research. work: Followng are the contrbutons made n the present research Smplfed approach for analyss of laced composte system such as LRC, LSCC usng an equvalent stress-stran characterstcs derved based on the moment-curvature relatonshp. Evoluton of new user frendly laced steel-concrete composte system possessng the essental propertes for blast resstant constructon, namely, ductlty and structural ntegrty. Suggeston of engneerng parameters to analytcally evaluate the cyclc behavour of LSCC beams. Evaluaton of shear strength of LSCC beams. The followng are the suggestons for future research: Parametrc study on LSCC beams wth varyng dameter of lacng and thckness of plates have to be carred out. Some more specmens have to be tested to generate desgn gudelnes. LSCC needs to be developed for planar elements by sutable ntegraton technques. needs to be explored. Applcaton of LSCC beams for dfferent loadng condtons