The calculation method of shear force correction for bulk carrier is clearly required in CSR code. The calculation process of shear force of hull girder seems to be impeccable. However, why do we need to correct the shear force result?
As a hull girder, the shear force at the cross section of the hull is equal to the difference between the gravity and buoyancy of the ship on one side. We can calculate the corresponding gravity and buoyancy accurately, and then get the shear force on the corresponding cross section. The calculation process is in accordance with the theory of structural mechanics. However, according to the requirements of the code, this result can not be directly used for shear check, and the shear calculation results need to be corrected. The shear correction changes the shear distribution of the hull girder, so where does this part of the shear change come from or how can it be reduced out of nothing?
The calculation of shear force is to regard the hull as a beam floating on the water surface. The difference between the weight distribution and the buoyancy distribution can form a curve, which is bounded by the calculation section. By integrating one side of the curve, the shear force of the hull beam at a certain cross section can be obtained. The shear force is distributed in the form of shear flow in the cross section here. In the previous CSR code, the shear flow distribution is calculated directly by the shear flow formula of rectangular section beam. In the new version of CSR, the concepts of static shear and non static shear are introduced, and more reasonable results can be obtained by using this method. Although the two calculation methods are different, the two calculation ideas are basically the same. They are based on the calculation method of beam system to get the shear, and according to the calculation method of shear flow of rectangular section beam to calculate the shear flow of cross section. The structure near the neutral axis bears larger shear stress, and the farther away from the neutral axis, the smaller the shear stress. At the deck and bottom, the minimum shear stress is borne.
As can be seen from the above figure, the shear force on the bottom longitudinal girder is very small, which is inconsistent with the actual situation. Through careful analysis of the load transfer in the hull structure, we find that the vertical force on the hull is not perfectly transferred to the hull girder. The bottom load of bulk carrier is the combined force of internal cargo and external water pressure. The bottom load is transmitted to the longitudinal bone through the inner and outer bottom plates, to the strong beam through the longitudinal bone, and then to the bottom longitudinal truss and side outer plate through the strong beam. When the rigidity of the strong beam is large enough, most of the load will be transferred to the side shell plate of the hull, and the distribution of the shear flow in the cross section of the hull is consistent with the distribution of the shear flow in the hull structure calculated by the rectangular beam. But in fact, the rigidity of the strong beam is not large. Some of the loads borne by the bottom of the ship are transmitted to the side shell of the ship, and some of the loads are directly borne by the bottom longitudinal truss. Therefore, the distribution of shear flow in the cross-section does not meet the general rule of the rectangular section beam. The method of solving the shear flow by using the rectangular beam directly can not get the shear force of the bottom longitudinal truss, and the results of the shear flow of the side shell plate are not accurate enough.
As we know, the shear force of cross section is the integration of shear flow along the vertical axis. Then, the shear force of longitudinal girder at the bottom of the ship can be removed from the shear force of cross section of the ship. The remaining shear force is used as the shear force of this cross section and calculated according to the calculation method of shear flow of rectangular beam, so as to obtain the shear flow of other components except the bottom longitudinal girder of the cross section This is the basic idea of shear correction. Based on the above analysis, it can be seen that the cross section shear force calculated according to the theory of hull girder is the actual shear force of this section, which is neither increased nor decreased. But this shear force can not be used to calculate the shear flow of the corresponding cross section. The so-called shear correction is to correct the shear force on the bottom longitudinal truss from the shear force on the cross section, and then calculate the shear flow of other hull members except the bottom longitudinal truss according to the calculation method of rectangular section beam with the modified shear force. However, the shear force of the bottom truss should be calculated directly by the finite element method. In other words, the revised shear diagram represents not the shear distribution of the whole hull girder, but the shear distribution of the hull structure except the bottom longitudinal truss.
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