LI Verification for damage limitation

Clauses The damage limitation requirement for buildings is simply an upper limit on the interstorey, drift ratio demand under the frequent (serviceability) seismic action. The limit on the interstorey drift ratio is set equal to:

(1) 0.5%, if there are brittle non-structural elements attached to the structure so that they are forced to follow structural deformations (normally partitions)

(2) 0.75%, if non-structural elements (partitions) attached to the structure as above are ductile

(3) 1%, if no non-structural elements are attached to the structure.

The interstorey drift ratio demand for storey i is determined at the most adverse relevant point in plan, as the ratio of the difference of the lateral displacements at the top and bottom of the vertical element there, di and d¡_v Ad¡, divided by the height, ht, of storey i. The most adverse relevant point is the one where the interstorey drift ratio attains its maximum value over the part of the plan where the same limit value applies (e.g. if there are no partitions over the part of the plan where interstorey drifts are at a maximum, taking into account the effects of natural and accidental torsion, but ductile partitions are attached to the structure over the rest of the plan, then the maximum interstorey drifts over these two parts of the plan should be separately checked against the corresponding limits).

The interstorey drift ratio demand should be determined under the frequent (serviceability) seismic action, which is defined by multiplying the entire elastic response spectrum of the design seismic action for 5% damping by the same factor v that reflects the effect of the mean return periods of these two seismic actions. If the analysis for the design seismic action is linear-elastic based on the design response spectrum (i.e. the elastic spectrum with 5% damping divided by the behaviour factor q), then the values of the displacements to be used for d¡ and di_l are those from that analysis multiplied by the behaviour factor q. If the analysis is non-linear, the interstorey drift ratio should be determined for a seismic action (acceleration time-history for time-history analysis, acceleration-displacement composite spectrum for pushover analysis) derived from the elastic spectrum (with 5% damping) of the design seismic action times v. The rules of Section 4.9 should be applied to take into account the effect of the two simultaneous horizontal components of the seismic action on drifts. Interstorey drift demands to be checked against drift limits 1 or 2 listed at the beginning of the present section should be computed within the plane of the relevant partitions attached to the structure. Interstorey drift demands to be checked against drift limit 3 should be computed within the plane of lateral-force-resisting systems, which are normally parallel to the directions of the horizontal components considered.

If there are (brittle or ductile) non-structural elements attached to the structure, and unless most of the lateral force resistance is provided by - concrete, composite or masonry -walls or heavy (steel) concentric bracings, member sizes will be controlled by the limit on interstorey drift ratio. For this reason, compliance with the damage limitation requirement should be established, before proceeding with dimensioning and detailing of members to satisfy the no-collapse requirement. Given the criticality of the damage limitation requirement for member sizing, there is a strong incentive for the designer of concrete buildings to use the default (high) stiffness of 50% of the uncracked gross section stiffness, instead of pursuing more accurate and representative alternatives that may be less conservative for the force-based dimensioning of members to satisfy the no-collapse requirement, but may make the damage limitation requirement more difficult to meet.

Given the large global stiffness necessary to meet interstorey drift limits, the limits on the sensitivity coefficient 9 for P-A effects (an upper limit of 0.3, and geometrically non-linear analysis required if 9 > 0.2) are normally not critical for buildings. In fact, equation (D4.20) shows that the value of 9 is equal to the interstorey drift ratio at the storey centre of mass divided by the storey shear coefficient (ratio of storey shear to weight of overlying storeys), both under the design seismic action. So, P-A effects may be important at the base (where the storey shear coefficient is minimum), but mainly in moderate-seismicity regions, where the seismic action is relatively low, but not low enough for a iow-dissipative' design to be used with a low value of q.

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  • asmait
    What is a limitation of an earthquake?
    8 years ago

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