Six I-section steel beams had been fabricated and tested to understand the influence of prestressing strand on the load deflection behavior of steel beam. All tested beams are simply supported having the same gross sectional area with clear span (2850) mm, five beams strengthened by two low relaxation seven wire strands, while sixth beam is the reference one. The strengthening beams were subjected jacking stress equal to (1120MPa) and subdivided according to prestressing strand positions (eccentricity). From the experimental tests, it can be noted that, the load deflection curves for strengthened beams are stiffer as compared with reference beam and the percentage of ductility for strengthened beams were decreased when the eccentricity positions change form (0 to 96)mm respectively, on the other hand, the percentage of increasing in maximum applied load for strengthened beams were increased with increasing of strands eccentricity and the maximum applied load reaches to 61.74% as compared with reference, also, the percentage increasing in maximum deflection at middle span for strengthened beams decreases with increasing of strands eccentricity and the minimum percentage of decreasing at middle span of strengthened specimens reaches to 36.31% as compared with the reference beam.

Seven simply supported steel beams were tested to examine the effects, in terms of strengthening, of external prestressing of strands on the shear behaviour of steel beams. All beams had the same steel section and clear span length and strengthening was implemented using two external prestressing strands. The tested beams were divided into two groups based on the presence of the external prestressing strands: the first group thus consisted of just one steel beam, as a reference, while, the second group had six steel beams strengthened by external prestressing strands divided according to the eccentricity location of a prestressing strand with jacking stress fpj = 1120.061 MPa. During the tests, it was found that the shear load strain curves for the tested beams were slightly stiffer than for the reference beams and that the percentage of stiffening increased with any increase in the eccentricity of strand locations from the shear points; the maximum shear load increased by 0.521%, 29.565%, 38.26%, 61.739%, 24.347% and 86.956% with an increases in the eccentricity location from (0 to 165) mm compared to the reference beam. The maximum shear strain increased by 9.664%, 3.553%, 8.121%, 62.436%, 37.563% and 13.451% with an increase s in the eccentricity location from (0 to 165) mm respectively as compared with the reference beam.

Now, using of geogrids as strengthening material are extend used, especially to enhancement of concrete elements as inter layers concrete applications, eight beams were tested to explain the effect of geogrid on the behavior of reinforced concrete beams. Beams tested had equal cross-sectional dimension (100 mm x 200 mm), compressive strength (f’c = 30 MPa), with a simply span length equals 1150 mm, with shear reinforcement (Ф4 @100mm C/C) and subjected to two point load. The tested beams were divided into two groups according to the presence of geogrid layer, with and without geogrid. Each group consists of four specimens, which were sub-divided according to the flexural reinforcement ratio that ranges from (0 to 0.0263). During the tests, it was noted that, the load deflection curve for beams with geogrid layer were stiffer and the percentage of stiffening was increased with increase of the flexural reinforcement ratio. The maximum applied load for beams with geogrid layer were higher than conventional beams without geogrid layer under the same conditions, while, the deflection values for beams with geogrid layer was lower than conventional beams without geogrid layer. The first crack load of beams with geogrid was greater than conventional beams without geogrid layer. So, the geogrids layer offer great enhancements to concrete properties and performance from the first cracking load, load-deflection response, reduce the cracks width and number and ultimate strength of tested in comparison to the conventional beams.

In this research, four steel beams were fabricated and tested to understand the influence of their strengthening (by using carbon fiber) with various span lengths on load deflection, load-strain, and ultimate load responses. All tested beams have the same cross-sectional area, and they are all strengthened by using intermediate stiffeners and cover steel plate at top flange to insure that failure will occur at the bottom flange. The tested steel beams are divided into two groups according to their clear span lengths 1400 and 1900 mm, and each group is subdivided into two beam cases based on whether they are strengthened by carbon fiber or not. From this study, it was found that the load deflection and load-strain curves for the beams strengthened by carbon fiber are stiffer than the original beams (without carbon fiber) with similar clear span lengths (this behavior was more obvious with smaller lengths). Moreover, the load deflection and load-strain responses have shown that beams became stiffer when the effective length is reduced (with and without carbon fiber), and this behavior was more apparent with the beams strengthened by carbon fiber. On the contrary, from the results of ultimate load of the beams, it can be concluded that the percentage of increase in ultimate load for the beam strengthened by carbon fiber is increased with the decrease in its span length. One could also conclude that when the effective length decreases, the ultimate load was increased and the percentage of this increasing is magnified with the presence of carbon fiber.

Seven steel beams were tested under one point load to investigate the effect of existence of external prestressing strands on the behavior of steel beams at yielding stage. All steel beams have the same gross area, overall length and the strengthened specimens are employed by two low relaxation strands. The steel beams are divided into two groups according to existence of prestressing strands, the first group involves one steel beam which is considered as a reference beam, while, the second group involves six specimens divided according to the location of prestressing strand (e) at jacking stress (fpj =1120.061 MPa). During the tests, it can be noted that the load strain response for strengthened steel beams are stiffer and the increasing percent of stiffening increases with the increasing of strand eccentricity and the maximum yielding load of strengthened tested beams increases with the increasing of strand eccentricity and the maximum enhancement in yielding load of strengthened tested beams reaches to 126.331% in the OM234 sample compared with the reference beam. Also, it was found that the yielding strain positions changed from bottom flange to the top region for the strengthened tested beams as compared with the reference steel beam and the increasing percent in the yielding to ultimate load ratio (Py/Pu) for the strengthened tested beams increases with the increasing of strand eccentricity at constant applied jacking stress. The maximum percentage increase in yielding to ultimate load ratio (Py/Pu) of strengthened tested beams reaches to 21.06% in the Straight strand profile as defined by OM234 sample as compared with the reference beam.