The Optimized Calculation Of Anchorage Length Of Anchor Bar For Rock Bolted Crane Girder

As a kind of advanced structural pattern, rock bolt crane girder has been widely used in underground house of hydraulic power station. Anchor bar plays an important role in the structural stability of rock bolt crane girder. Connect the reinforced concrete crane girder with the rock tightly by anchor bar to form a stable system of anchorage so as to attain the goal of safe work.People do not have a deep understanding to the anchorage principle of anchor bar because of the complex mechanical characters of the rock mass, therefore the calculation method of the rock bolt crane girder remains to be improved. At present, the rigid equilibrium method is very common which has a clear thread and an easy bearing capacity model, so it is convenient to be used in project, but this method can not consider the coordinate deformation between the crane girder and rock mass and the effect to the structure caused by the excavation of cavern, so this method needs improving. Carrying out the numerical calculation to the rock bolt crane girder can consider the effect to the anchor bar caused by both the excavation of cavern and the load of crane, and also simulate the loading situation of structure accurately, which makes it possible to realize the purpose of optimized calculation.This article analyzes the anchorage length of the tension anchor bar upside of upriver wall rock bolt crane girder in main cavern of Xia Ban Di project underground house in Xin Jiang. Come to a conclusion that the anchorage length of anchor bar is 9.0m by the traditional rigid equilibrium method first, and carry out the numerical simulation to the structure making use of limit element analysis software, establish the limit element calculation model under the condition of different anchorage length, the anchorage lengths are 9.0m,8.5m,8.0m,7.5m,7.0m,6.5m,6.0m and 5.5m. Consider the effect to the structure caused by the excavation of cavern and the load of crane when the anchorage length is different, and get the variety law of the stress along the anchor bar,the displacement at the top and bottom of the crane girder and situation of stress distribution on the joint plane. Draw the main conclusions as follow:1. The law of stress distribution along the anchor bar is basically same in the condition of different anchorage lengths, the maximum stress appears nearby the joint plane, and anchor bar elements with the maximum stress tend to come close to the joint plane when the anchorage length is decreasing, and the stress decreases and tends to be stable along the anchor bar gradually.2. The maximum stress of anchor bar elements,the displacement at the top and bottom of the crane girder and the tension stress and compressive stress decrease when the anchorage length turns short, this tendency is inconspicuous when the anchorage length is increasing.3. The structure is influenced by both the excavation of cavern and the load of crane, the stress of anchor bar elements have a evident change before and after the load of crane, but the displacement of the crane girder and the stress on the joint plane changes little, the influence to the structure caused by the load of crane tends to be obvious when the anchorage length is getting shorter.4. When the anchorage length is 5.5m, the stress of the element on the top of the joint plane reaches to the tension intensity and it is possible for the joint plane to be destroyed. When the anchorage length is 6.0m, the whole structure is stable and the maximum stress and deformation is less than allowed value. So the result of optimized calculation is 6.0m.On the base of the conclusions above, compare and analyze the results calculated by different methods, the results are different because there are some defects in considering the excavation of cavern and the coordinate deformation and the adhesion stress between the crane girder and rock mass for rigid equilibrium method, and the ascertaining of safety factor is always oversize which result in the oversize anchorage length of anchor bar.Hence ascertain the parameters of the anchor bar of rock bolt crane girder by rigid equilibrium method, according to which carry out the limit element simulate calculation to verify and modify the result, make the contrastive calculation by two methods assisted by in-site construction measurements to ensure the calculation results are more accurate and reasonable.