Repair and reconstruction of civil structures - Assessment and Refurbishment of Steel Structures - page 169

As the reinforced-concrete working platforms at individual floor level are represented by
rigid horizontal stiffeners, for the horizontal loading a thin-walled spatial system was
chosen as the calculation model for the steel structure. In this system, the chords of
vertical lattice stiffeners which for areas of stress concentration connected by diagonal
members, were replaced in their function by full walls of equivalent thickness (Figure
5.41).
A uniform distribution of normal stresses over the column cross-sections and shear
stresses over the cross-section of alternate solid walls was assumed. This means that in
calculating the horizontal loading of the structure, only the columns that were parts of the
stiffeners were considered.
The cross-section of the calculation model varies with the building height, so the
position of the centre of torsion varies too. Therefore the line connecting the torsion
centres of individual cross-sections is not straight but irregular. In the calculation model
of the structure it was assumed for simplicity that the line connecting the torsion centres
(the axis of the thin-walled system) is a straight line. In the calculations for the steel
structure designed a thin-walled spatial system, first-order theory is adequate because the
height of the building is small in comparison to the dimenssions of the cross-section.
The calculations of cross-section characteristics were done on the condition that non-
deformability of individual cross-sections (at the level of floors) was secured by strong
concrete columns according to the theory developed by V.Z.Vlasov. The rigidity of the
columns in the structure was determined from the vertical loading (their overweight, any
temporary weights), the horizontal wind load, and dynamical loadings caused by cranked
and self-balancing sieves and transport conveyers. As well as the bending oscillation
caused by such dynamic loading, the effects of torsion oscillation caused by eccentric
oscillation was also examined with regard to the torsion centres of individual cross-
sections in the calculations model for the structure. The calculations showed that the
cause of the undesirable oscillation of the structure, in the region between column rows G
and K (Figure 5.42) was insufficeint strength of the stiffeners in row K (Figure 5.43).
Figure 5.42 Arrangement of the
original vertical stiffeners in column
row K