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Glass-fibre reinforced plastic pipes are widely used to convey fluids for various purposes. They offer a number of distinct advantages over conventional metals, such as high specific strengths, high specific moduli, superior corrosion resistance and low coefficient of thermal expansion. However, their behaviour under lateral quasi-static and impact loading are still not well known. The research programme described in this thesis was designed to characterise the performance of 55° winding angle GRP pipes, subjected to lateral quasi-static and impact loading. Two approaches: experimental tests and finite element analysis, were used to investigate the behaviour of the GRP pipes. The experimental investigation was started with diametral compression of short GRP pipes to examine the structural behaviour and failure mechanisms. Subsequently, lateral indentation tests were conducted on rigid-foundation supported or simply supported specimens using two different indenter geometries: line-ended and flat-ended. Furthermore, low-velocity impact tests were performed under similar conditions as those for indentation tests in order to characterise the response of the GRP pipes and to identify the correlation between the two forms of loading. The pipes exhibited multi-mode failure mechanisms, resin cracks, delaminations and fibre breakage. It is found that delamination, which resulted in significant loss in stiffness and strength, was the most significant mode of failure for the GRP pipes. A good correlation in behaviour was identified between quasi-static indentation and its energy equivalent low-velocity impact when the global bending stiffness of the GRP specimens were high. Specimens with span S 10.5D i, where Di is the internal diameter of the pipe, are considered to have high bending stiffness, while simply supported specimens with S10.5D i have low bending stiffness. Irrespective of the support conditions and loading type, specimens with high bending stiffness followed a failure mechanism sequence: local resin failure, delamination and the fibre breakage. However, the large global bending experienced by low bending stiffness specimens resulted in a change of failure mechanism, only local damage and surface tensile cracks were observed.

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lass fibre reinforced plastic (GRP) structural profiles, in standard shapes and sizes are now being commercially manufactured by the process of pultrusion. GRP profiles are light weight, posses higher specific strengths and are more durable than the conventional metal or concrete counterparts. GRP pultruded profiles have open or closed cross-sections comprising thin composite walls of low elastic moduli. Stability failure has been identified as the main cause of failure for these profiles when subjected to compressive stresses, as it may occurs at stresses much lower than the ultimate strengths. Therefore, the load carrying capacities of composite compression members mainly depends upon stability criteria. The conventional stability analyses for the prediction of buckling loads are not considered adequate as the GRP material is orthotropic and its behaviour is different from steel (non-yielding). The existing guidance for the design of composite members under compression ignores the presence of geometrical imperfections inherited in the pultruded profiles, whilst, experimental evidence suggests considerable loss of stiffness due to the imperfections particularly in the intermediate column heights. The design guidance provided by the manufacturers gives empirical equations based on data obtained from experiments on specified profiles. A universal design curve based on the experimental results of concentrically loaded GRP columns has been developed and presented. However, conducting a vast experimental study is not always feasible. The need to develop a procedure, predicting failure load numerically for the development of a design curve for GRP columns has been recognised. Two GRP box-sections (closed square cross-sections) have been investigated for failure/buckling loads using experimental and numerical methods. In the experimental phase, specimen columns of various heights have been concentrically loaded in compression to measure the failure loads. Experimental results have been compared with the theoretical predictions made using classical methods and the equations given by the design manuals. Based on the experimental and analytical failure loads, an experimental design curve has been derived. In the numerical study, 3-dimensional full scale finite element models representing experimental configuration of the composite columns, have been analysed using both linear and nonlinear solutions. Imperfections of known amplitudes have been included parametrically to establish the sensitivity of the failure loads towards imperfections. Imperfect model have been calibrated for the estimation of imperfection amplitude present in the profiles using experimental data. Using the numerical and analytical data, a design curve has been derived establishing interaction coefficients for each profile. The numerical design curve is compared with the experimental design curve for the validation of the numerical procedure adopted in this study. Effects of perforations (circular holes) on the buckling stiffness of GRP box-section columns have also been investigated. Holes are drilled in the walls of profiles and tested experimentally to measure the loss in the buckling loads. Finite element models of columns with holes have been developed and analysed for buckling loads. Comparisons of experimental and numerical results are plotted. For use in the numerical representation of the composite columns, mechanical properties of the orthotropic GRP material of the both sections have been established analytically and experimentally. In-plane shear properties have been measured by physically testing standard sized coupons, extracted along the length of profiles. However, short coupons were available in the transverse directions due to dimensional constraints. Short coupons, similar in geometry to the standard coupon, but smaller in size, have been validated for performance using finite element analyses and comparing the outcomes with the models of standard coupons. Both standard and short coupons have been used for the experimental measurement of the in-plane shear properties. Compression properties have also been measured experimentally. Ultimate failure/buckling loads of the composite columns depend upon their heights, material properties, and the cross-sectional dimensions. These factors have been combined into one characteristic parameter 'λ', the slenderness ratio. As the later two factors are constant for a particular box-section profile, the ultimate loads depend upon column heights. Four types of failure modes; global, local, modal interaction and material failure have been observed. The loss in the buckling stiffness is minimal for smaller circular holes, provided the interval between holes is not less than 20 times the diameter of the holes. For bigger holes and an inter hole spacing of 10time the diameter, a loss of 30% have been measured. Finite element representation of pultruded columns adequately predicted the numerical failure loads and failure modes for most of the column heights.

Αντικείμενο της παρούσας Διατριβής Διπλώματος Ειδίκευσης είναι η κατά βάση πειραματική διερεύνηση της αποτελεσματικότητας περίσφιγξης ορθογωνικών υποστυλωμάτων με μεγάλο λόγο πλευρών, ενισχυμένων με μανδύες ινοπλισμένων πολυμερών και σύνθετων υλικών ανόργανης μήτρας. Το πειραματικό πρόγραμμα διεξήχθη στο Εργαστήριο Μηχανικής και Τεχνολογίας Υλικών του Τμήματος Πολιτικών Μηχανικών του Πανεπιστημίου Πατρών. Το πρόγραμμα αυτό, περιελάμβανε δύο σειρές δοκιμίων. Η πρώτη περιελάμβανε έξι δοκίμια και η δεύτερη δέκα. Η κατηγοριοποίηση σε σειρές έγινε με βάση το λόγο πλευρών των δοκιμίων. Έτσι, την πρώτη σειρά αποτέλεσαν δοκίμια με λόγο πλευρών (3:1), ενώ τη δεύτερη δοκίμια με λόγο πλευρών (4:1). Ένα δοκίμιο από κάθε σειρά δοκιμάσθηκε χωρίς ενίσχυση και αποτέλεσε μέτρο σύγκρισης για όλα τα υπόλοιπα. Τρία δοκίμια από κάθε σειρά ενισχύθηκαν με τρείς στρώσεις FRP και με θυσάνους. Ακόμα, ένα δοκίμιο από κάθε σειρά ενισχύθηκε με δύο στρώσεις FRP, χωρίς χρήση θυσάνου. Επίσης, δύο υποστυλώματα από τη δεύτερη σειρά ενισχύθηκαν με δύο στρώσεις FRP, θυσάνους και δύο επιπρόσθετες στρώσεις FRP τύπου U, στις δύο μικρές πλευρές. Τέλος, ένα δοκίμιο από κάθε σειρά ενισχύθηκε με τέσσερις στρώσεις TRΜ, ενώ άλλο ένα με τέσσερις στρώσεις TRΜ και με θυσάνους. / The subject of this thesis is the experimental investigation of the effectiveness of confining rectangular columns with large aspect ratio, reinforced with fiber reinforced polymers and with tensile reinforced mortars. The experimental program was conducted at the Laboratory of Engineering and Technology of Materials in Civil Engineering, University of Patras. This program is consisted of two sets of samples. The first included six small scale columns and the second ten. The categorization in series was based on the aspect ratio of the specimens. So specimens with aspect ratio (3: 1) were included in the first series, while the second included specimens with aspect ratio (4: 1). One specimen from each series was tested without any reinforcement and became the comparison for all the rest. Three specimens from each series were reinforced with three layers of FRP and anchors. Still, a sample from each series was reinforced with two layers of FRP, without use of anchors. Also, two columns of the second series of amplified with two layers FRP, anchors and two additional layers of FRP type U, in the two smaller sides. Finally, a sample from each series was reinforced with four layers TRM, while another one to four layers TRM and anchors.

Earthquakes are one of the biggest problems in civil engineering all over the world. Due to earthquakes, great disasters in cities with collapsed structures and human losses have been caused. More specific, old buildings that have been built based on old regulations and design building codes do not fulfil anymore the new criteria of seismic designing.In this study, an old building has been evaluated for the seismic load in order to decide if there is a need for strengthening it using retrofitting methods. The seismic evaluation is based on Eurocode 8 and after the application of retrofitting techniques the building fulfilled its seismic design criteria. The existing building is a four-storey, concrete structure that has been built in 1970 and is located in Athens (the capital city of Greece). The seismic evaluation is conducted by using the software Seismostruct.Two analyses are performed in order to evaluate the seismic behavior of the building. First, an eigenvalue analysis is conducted before and after retrofitting. By using this analysis the torsional sensitivity of the building has been checked. Then, using pushover analysis, the comparison of the target displacement (expected displacement of the building for the design seismic action) for each limit state and the displacement of the building when the first member of the building reached the corresponding limit state, is presented. Target displacement must not be greater than this displacement in order to ensure the safety of the building. If the comparison shows that target displacement is greater, the weak links in the facility should be identified and the proper retrofitting method should be applied for the improvement of the seismic behavior of the building. Pushover analysis is conducted before and after the application of retrofitting methods.After performing the eigenvalue and pushover analysis of the existing building it was found that the building was torsional sensitive and shear failures occurred in many beams of the structure. Regarding the bending failures, the target displacement was not greater than the displacement of the building when the first member of the building reached any of the corresponding limit states. Therefore the building was safe against bending failures. With the application of X-shaped steel braces in selected frames, the building had higher stiffness and it was not torsional sensitive but shear failures occurred again in many beams. Furthermore, compressive failures occurred in columns that were connected with the steel braces. Finally, with the application of fibre reinforced plastic jacketing in the members that failed in the previous pushover analysis there were no shear or compressive failures. Finally the structure was safe against seismic actions.The application of retrofitting methods improved the seismic behavior of the building and the structure fulfilled the updated regulations of Eurocode 8 regarding seismic design. This project thesis may give rise to further studies and researches concerning seismic retrofitting and seismic damage prevention.