Research On The Crack Developing Law And Reinforcement Properties Of Hybrid Steel Concrete Beams

Cracks have a vital influence on the performances of concrete structures.In this paper, the experimental beams are hybrid ones reinforced by both ordinary and high strength steel bars, which were designed according to a new structure design idea. Through theoretical and experimental research, the author studied the crack development law and the performances of the hybrid beams before and after reinforcement by filling under a sustained load, and then the test results were verified by the finite element analysis. The main conclusions of this paper are as follows:①The hybrid beams do not crack until its load reaching the sustained load when reinforcing. The loads when the hybrid beams re-crack are larger than the sustained load. The tests indicate that the microscopic cracking load of a hybrid beam before reinforcement is 3.93 T and its macroscopic cracking load is 8.45 T. The hybrid beam was reinforced by filling under the sustained loads when the moments of the beam at its pure bending section were 50%, 70% and 100% of Mu respectively, and the corresponding loads are 15.62 T, 25.72 T and 38.13 T. After reinforcement, the cracking load of the beam when reloading were 16 T, 27 T and 40.5T respectively. Compared with those before reinforcement, the cracking loads of the beam after reinforcement were improved by 2.43%, 4.98% and 6.22%.②The method of reinforcement by filling under a sustained load can solve cracking problems at not only the normal section but also the oblique section of the hybrid beams. Experiment shows that the hybrid beam did not crack when it was re-loaded to 50% Mu again after being reinforced at 50% Mu by trisection point loading. After re-loading, the loading points were changed to some new positions which cause more unfavorable load bearing status at the oblique section. In this case, the hybrid beam still did not crack until its load reaching the sustained load. After reinforcement by filling at the sustained load of 70% Mu at the new loading points, the hybrid beam did not crack when it is re-loaded to 70% Mu again.③The method of reinforcement by filling under a sustained load can effectively control the emergence, width and development of cracks. The hybrid beam reinforced do not crack until the load it bears exceeds the sustained load when reinforcing. After cracking of the beam again, the development of the crack width can be effectively controlled. Experiment shows when the hybrid beam was reinforced at the sustained load of 50% Mu, its maximum crack width decreases significantly when the beam is reload to 70% Mu, which is just 22.22% of the crack width of the beam when being reinforced to the load of 70% Mu by one time.④The method of reinforcement by filling under a sustained load has little influence on the development of crack height of the hybrid beam at the ultimate load. Experiment shows that cracks develop fast under the ultimate load. However, the crack stopping height of hybrid beams is often between 47 cm and 50 cm before and after reinforcement.⑤After the reinforcement by filling under a sustained load, the final distribution form of cracks of the hybrid beam turns from "wide" and "close" to "narrow" and "thin". Experiment shows that the final crack spacing of the hybrid beam before reinforcement is at average 9.08cm; that of the hybrid beam reinforced at the sustained load of 70% Mu is 12.97cm; and that of hybrid beam reinforced at the sustained load of 100% Mu is average 28.34 cm.