101 Civil Engineering Dimension, Vol. 17, No. 2, September 2015, 101-107 CED 2015, 17(2), DOI: 10.9744/CED.17.2.101-107 ISSN 1410-9530 print / ISSN 1979-570X online Experimental and Theoretical Analysis of Hollow Steel Columns Strengthening by CFRP Keykha, A.H. 1*, Nekooei, M. 2 , and Rahgozar, R. 3 Abstract: The need for strengthening and retrofitting is well known and extensive research is progressing in this field. The reasons for strengthening and retrofitting are numerous: increased loads, changes in use, deterioration, and so on. In recent years, the use of carbon fiber reinforced polymer (CFRP) for strengthening has shown to be a competitive method, both regarding structural performance, and economical aspects. Extensive research has been carried out in this field. However, most of the research has been undertaken on concrete structures and for confinement, flexural, and shear strengthening. Limited research has been carried out on steel structures strengthened with CFRP. This paper presents axially loaded steel columns strengthened for increased load. The topic is studied theoretically and through laboratory tests. The theory covers analytical methods. Carbon fiber reinforced polymers has been used to strengthen the columns. The tests have been undertaken on full scale specimens, non- strengthened for reference, partially strengthened and fully strengthened. Keywords: CFRP; experimental; strengthened; steel column; theoretical analysis. Introduction In recent years, the use of CFRP for strengthening and retrofitting of steel structures has been consi- derably developed. A variety of strengthening or rehabilitation techniques such as section enlarge- ment, external bonding of steel plates and fibers has been proposed to strengthen metallic structures. Though the external bonding of steel plate was successful in practice, it posed some problems such as adding of self-weight, requirement of heavy lifting equipment to place the plates in position, difficulty in shaping and fitting in complex profiles and compli- cation in bonding/welding, besides added plates are susceptible to corrosion which leads to an increase in future maintenance costs. Fiber reinforced polymer composites represent a new and promising solution to the shortcomings of several traditional materials and upgrading techniques and has a great potential to integrate into the bridge infrastructure [1]. CFRP materials have high tensile strengths; most of them stretch to relatively high strain values before providing their full strength. CFRP also has a higher strength to weight ratios and impact resistance, offer greater design flexibility and better resistance to chemicals and corrosion [2]. 1,2 Department of Civil Engineering, Science and Research Branch, Islamic Azad University, Tehran, IRAN 3 Department of Civil Engineering, Shahid Bahonar University of Kerman, Kerman, IRAN . * Corresponding author; e-mail: ah.keykha@yahoo.com Note: Discussion is expected before November 1 st 2015, and will be published in the “Civil Engineering Dimension” volume 18, number 1, March 2016. Received 29 July 2015; revised 31 July 2015; accepted 10 August 2015. The other advantage of CFRP over the traditional method is its applicability and the capacity to cover areas in quick succession and therefore used in underwater applications [3]. No heavy instruments are necessitated for bonding CFRP over steel surface. Due to their low weight and good tailor ability, rapid execution can be achieved. Zhao et al. [4] investigated the improved web crippling behaviors of rectangular hollow section (RHS) strengthened by CFRP, several types of strengthening were adopted, such as wrapping CFRP sheeting outside the RHS or applying CFRP plates outside or/and inside the RHS. It was found that the CFRP strengthening significantly increases the web crippling capacity especially for those with large web depth to thickness ratios. The design models are proposed to predict the increased capacity for CFRP strengthened RHS subjected to transverse end bearing force. Silvestre et al. [5] carried out an experimental and numerical investigation on the nonlinear behavior and load carrying capacity of CFRP strengthened cold-formed steel lipped channel column. 19 short and long fixed ended lipped columns were strengthened with carbon fiber sheets (CFSs) bonded at different outer surface locations (web, flange or lips) and having the fibers oriented longitudinally or transversally, the results of the experiment showed that the presence of the single CFS may increase the load carrying capacity by up to 15% for the short column strengthened at column web and flanges with transverse CFS, and 20% for the long column strengthened over whole outer surface with transverse CFS.