Design against the adverse effects of heightwise irregular infills

A soft and weak storey may develop wherever the infills are reduced relative to the other Clause storeys (notably the overlying storey). The consequences for the global seismic performance are most critical in buildings with an (almost) open ground storey, which, unfortunately, seems to be the most common case of infill irregularity in elevation.

A reduction of the infills in a storey relative to adjacent storeys increases the inelastic deformation demands on the columns of the storey with the reduced infills, owing to:

8 the concentration of the global lateral drift demands to that particular storey (soft/ weak-storey effect)

• the near-fixity conditions of the columns of that storey at floor levels, due to the restraint of drift in the neighbouring storeys by the infill panels.

Unlike columns, floor beams above and below that storey are protected from excessive damage owing to the low magnitude of their chord rotation demands. Moreover, the columns of storeys with reduced infills cannot be effectively protected from plastic hinging through application of equation (D4.23). The reason is as follows.62 As the storeys above and below that with the reduced infills develop low interstorey drift ratio(s), the chord rotations at the ends of the columns of these storeys will also be very low. In fact, if the infills of these storeys are very stiff and strong, column chord rotations there may have a sign opposite that of the beams, so that their algebraic sum indeed gives a low interstorey drift ratio. As the magnitude of moments at column ends is directly related to that of chord rotations there, the end sections of columns in the storeys with reduced infills will get very little aid from the other column section across the joint in resisting the sum of beam flexural capacities, Y.MRh, around the joint, without yielding.59'62 The end result is that, despite fulfilment of equation (D4.23) at the joints of the frame, plastic hinges may develop at both the top and bottom of the columns of the storey with the reduced infills; moreover, chord rotation demands at these plastic hinges may be large enough to exhaust the corresponding capacities. The outcome may be storey collapse.

Clauses To safeguard against the possibility that the columns of a storey where infills are reduced, relative to the overlying storey will develop pre-emptive plastic hinging that may lead to, failure, EN 1998-1 calls for these columns to be designed to remain elastic until the infills in the storey above attain their ultimate force resistance. To achieve this, the deficit in infill shear strength in a storey should be compensated for by an increase in resistance of the frame (vertical) members there. More specifically, the seismic internal forces in the columns (bending moments, axial forces, shear forces) calculated from the analysis for the design seismic action are multiplied by the factor 77:

2-tVEd where A VRvi is the total reduction of the resistance of masonry walls in the storey concerned, compared with the storey above, and Hvm is the sum of seismic shear forces on all vertical primary seismic members of the storey (storey design shear force). If the value of the factor 77 turns out to be lower than 1.1, the magnification of seismic action effects may be omitted.


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