Behavior of Hollow Composite Steel concrete Members under Long term Axial Compression Procedia Engineering 172 ( 2017 ) 711 – 714 Available online at www sciencedirect com 1877 7058 © 2017 Published b[.]
Available online at www.sciencedirect.com ScienceDirect Procedia Engineering 172 (2017) 711 – 714 Modern Building Materials, Structures and Techniques, MBMST 2016 Behavior of hollow composite steel-concrete members under longterm axial compression Antanas Šapalasa*, Gintas Šaučiuvėnasb, Konstantin Rasiulisc a, b,c Department of Steel and Timber Structures, Vilnius Gediminas Technical University, Saulėtekio al 11, Vilnius LT-10223, Lithuania Abstract Research results related to the behavior of hollow composite steel-concrete members under short-term loading are well known Such members consist of two materials — steel and concrete with very different properties They have different rheological behavior Steel rheological behavior can be neglected at normal temperatures, but deformations are significant for concrete creep Concrete creep phenomenon is well examined, but such knowledge cannot be applied directly to a composite member Permanent actions are responsible for a large portion of external actions acting on compressed structural members Research of composite members under long-term actions is necessary to ensure the reliable design of such structures © by Elsevier Ltd This is an openLtd access article under the CC BY-NC-ND license ©2017 2016Published The Authors Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of MBMST 2016 Peer-review under responsibility of the organizing committee of MBMST 2016 Keywords: composite; steel; concrete; long-term loading Introduction Compressed structural members from steel or reinforced concrete are the most popular in framed structural systems Both types have certain advantages and disadvantages Steel structures are light but thin-walled, therefore, inclinable to local buckling Besides, the fire resistance level of steel structures is low While concrete structures are heavy, but have no buckling issues and are significantly fire resistant It is possible to receive all advantages of steel and concrete while avoiding the disadvantages This can be done by making one structural composite member In the case of compression, it can be assumed that longitudinal deformations of steel shell and concrete core are the same, but, * Corresponding author Tel.: +37067241760 E-mail address: Antanas.Sapalas@vgtu.lt 1877-7058 © 2017 Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of MBMST 2016 doi:10.1016/j.proeng.2017.02.090 712 Antanas Šapalas et al / Procedia Engineering 172 (2017) 711 – 714 because Poisson’s ratio is different, radial stresses will appear in the steel–concrete contact surface These radial stresses will arise from the radial compression of the concrete core Experimental investigations into bearing capacities of compressed composite steel–concrete members show that due to complex stress–strain conditions in the steel shell and the concrete core, the composite effect can increase the strength of the hollow section from to 25 % [1,2,3] Combining different materials in composite cross section arise new positive properties [4,5,6] Many scientific publications are related to the behavior of concrete-filled steel tubes with a solid concrete core and only some of them have a hollow concrete core Almost all experimental tests are executed with short term loading or with members with solid concrete core [7] Concrete is one of the materials, from which the composite cross section is made It is sensitive to long-period loading It is necessary to investigate time-dependent stresses and deformations to ensure the safe design of compressed steel-concrete composite members and experimental investigations are the most reliable for such a purpose Experimental investigations of long-term deformations of composite members Steel tubes with the external diameter of 250, 200 and 140 mm, the wall thickness of mm, and the yield strength of 260 MPa have been used for the production of test specimens The hollow concrete core has been formed by filling a steel tube with fresh concrete mix and spinning it on a rubber belt centrifuge Eight groups of test samples were produced consisting from nine Fig Shape of the cross-sections of a composite steel–concrete member specimens with same geometrical and physical properties For the control of the concrete strength on the day the external long-term compression was applied, and on the final day of long-term observations, the circular hollow core and prismatic (100x100x400mm) concrete specimens were tested Prismatic test samples were compacted by vibration, and circular hollow core samples were centrifuged All of the test specimens were stored in the same room at the same temperature and moisture conditions For the comparison, concrete members have been produced at the same time from the same concrete mix and with the same centrifuging technology as the composite members The external steel shell was removed from some of the specimens in one test group just before testing with the short-term static load or just before applying long-term loading on them Such preparation of concrete tubular test samples ensures same hardening conditions for composite as well as concrete members Three composite and three concrete circular concrete members from each group were placed in the equipment for long-term axial compression The control composite and concrete members were tested with the short-term static load for axial compression on the same day The intensity (the ratio between the long-term load and the ultimate load) of long-term external load η was in the range 0.5–0.8 Long-term observations of compressed members under the constant axially applied load lasted from 100 to 400 days Longitudinal deformations were measured during the entire observation period Members were unloaded after these observations and tested for axial compression with a static load Some of the composite members were tested in the long-term loading equipment with the purpose to investigate 713 Antanas Šapalas et al / Procedia Engineering 172 (2017) 711 – 714 the influence of unloading on the ultimate strength of composite members Creep measure curves of concrete and composite specimens for different loading ratio values were built based on long-term observation results Fig Average creep deformations: — composite members η=0.56; — comparative samples for (1) concrete member; — composite members η=0.77; — comparative samples for (2) concrete member Fig presents creep measure diagrams for composite members subjected to long-term loading with intensity 0.56 and 0.77 and comparative concrete members The creep measure has been calculated as: C (t , W) Hcr (t , W) , Vc (W) (1) where Hcr (t , W) — creep deformations after the time period (t W) , Vc (W) — longitudinal stresses in the hollow concrete core at the time moment W when the long-term load was applied Accompanying concrete tubular members were subjected to long-term loading on the same day One of the tasks of conducted experiments was to find out the influence of the external steel shell on long-term deformations of members Tubular concrete specimens were subjected to a long-term load, which was able to induce longitudinal deformations with the same value as the initial deformations in composite long-term loaded members As demonstrated by the curves presented in Fig 2, long-term deformations of tubular concrete members were more than two times greater than those of composite members The intensity of the long-term external load also had an effect on the value of final creep deformations The increase in the external load by 27 % increased final deformation by about 40 % In a real situation, columns in a building would be subjected to permanent loads; and later, imposed loads would start acting In a critical situation, overloading due to the increase of imposed loads can appear The full unloading before it is impossible In our tests, we unloaded members subjected to long-term loading and then tested them till the ultimate load in the hydraulic testing machine One task of our tests was to investigate the influence of unloading on the bearing capacity of long-term loaded composite members In one group of specimens with similar properties, three of them were tested till the ultimate load directly in the long-term loading equipment, and other three were unloaded and tested in the hydraulic testing machine In one group of specimens with similar properties, three of them were tested till the ultimate load directly in the long-term loading equipment, and other three were unloaded and tested in the hydraulic testing machine The average 714 Antanas Šapalas et al / Procedia Engineering 172 (2017) 711 – 714 longitudinal deformations of unloaded and not unloaded specimens are presented in Fig It can be stated that unloading has no influence on ultimate longitudinal deformations of test specimens Fig Average longitudinal deformations of composite members unloaded (1) and tested without unloading (2) Test results of unloaded and not unloaded composite members demonstrated that deformations and ultimate fracture load is also the same for both types of specimens Conclusions Based on the results of long-term observations it can be stated that: The external steel shell has a great influence on the concrete creep process Depending on the intensity of the applied external load, the difference in creep deformations of concrete and composite specimens can reach 40 % Unloading of members subjected to long-term loading has no influence on the bearing capacity and deformations of members due to the testing until collapse The external steel shell has a positive influence on the hardening of the internal concrete core, significantly reducing the shrinkage and creep deformations References [1] Min Yu, Xiaoxiong Zha, Jianqiao Ye, Chunyan She, A unified formulation for hollow and solid concrete-filled steel tube columns under axial compression, J Engineering Structures (2010) 1046–1053 [2] A.K.Kvedaras, A.Kudzys, Tubular composite beam-columns of annular cross-sections and their design practice, J Steel and Composite Structures (2010) 109-128 [3] C.D.Goode, A.Kuranovas, A.K.Kvedaras, Experimental data analysis of concrete-filled steel columns, The 9th international conference "Modern building materials, structures and techniques": selected papers (2007) 546–553 [4] G.Marčiukaitis, J.Valivonis, B.Jonaitis, J.Kleiza, R.Šalna, Determination of stiffness of the connections of composite steel and concrete bridge deck by the limit permissible deflections, The Baltic Journal of Road and Bridge Engineering 6(3) (2013) 193–200 [5] T.Skuturna, J.Valivonis, The statistical evaluation of design methods of the load-carrying capacity of flexural reinforced concrete elements strengthened with FRP, Archives of Civil and Mechanical Engineering (2015) [6] T.Skuturna, J.Valivonis, Design method for calculating load-carrying capacity of reinforced concrete beams strengthened with external FRP, Construction and Building Materials 50 (2014) 577-583 [7] L.H.Inchinose, E.Watanabe, H.Nakai, An experimental study on creep of concrete filled steel pipes, J Constructional Steel Research 57 (2011) 453-466 ... members Three composite and three concrete circular concrete members from each group were placed in the equipment for long- term axial compression The control composite and concrete members were... One of the tasks of conducted experiments was to find out the influence of the external steel shell on long- term deformations of members Tubular concrete specimens were subjected to a long- term. .. short -term static load for axial compression on the same day The intensity (the ratio between the long- term load and the ultimate load) of long- term external load η was in the range 0.5–0.8 Long- term