Stability of Frame-Rod Structural Systems on Subsiding Soils

Purpose of research.   To meet the requirements of Federal Law No. 384 “Technical Regulation on the Safety of Buildings and Structures”, a deeper study of the stability of frame-rod structural systems in the event of an emergency associated with subsidence of the base of one of the supports, and the development of a methodology for assessing the resource of resistance of building elements located in difficult engineering and geological conditions.

Methods. As a stability criterion, the sign of the work of the end moments and transverse forces is adopted. A negative value of this work is a sign of "active" loss of stability of the rod, a positive - "passive". In the event of an emergency related to the subsidence of one of the frame supports, the work of the longitudinal force ceases to contribute to the bifurcation of the rod. Therefore, at a certain strain value Ssl, a separate rack can go from active loss of stability to passive, changing the critical stability parameters of the entire system as a whole. The formula for determining the critical drawdown S_cr is given.

Results. A two-span frame is considered, in which the central strut is loaded with the force P_kr, the extreme - with the forces αP_kr. The type of rod bifurcation is determined. Using the above equations, critical parameters and forms of buckling of the structural system under consideration were found before and after the emergency in question. An assessment is given of the influence of the draft of the base of the supports of the frame racks on the character of the bifurcation of the rods of the structural system as a whole. The analysis of the dependence of the critical drawdown value S_{sl, cr}. on the loading scheme of the frame under consideration is performed. It is shown that when the coefficient of load application is α = 0.6, at the first stage, when the system in question is stable, the first rack loses stability passively, the rest actively. When the base of the second pillar sags by 21 mm or more, a change in the type of its bifurcation occurs, while the critical stability parameters of the remaining elements do not change significantly. In the event of a similar emergency with the base of the right frame pillar, the critical drawdown value is 140 mm. Moreover, a change in the type of bifurcation of the third pillar leads to the transition of the first pillar to active bifurcation, as a result of which the critical parameters of the entire system as a whole change significantly.

Conclusion. An analysis of the calculation results showed that with a certain drawdown value, the emergency in question will lead to a change in the nature of the bifurcation of both individual elements and the entire structural system  as  a  whole.  The  proposed  stability  criterion  for  frame-rod  structural  systems,  the  basis  of  which  are subsidence soils, makes it possible to relatively easily identify elements with low resistance to buckling in difficult engineering and geological conditions.

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