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Stiff versus Flexible Tall Buildings

 
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cvrm
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Location: Chennai

PostPosted: Tue Nov 27, 2012 11:40 am    Post subject: Stiff versus Flexible Tall Buildings Reply with quote

Dear Colleagues:

I wish to add to what Professor Swaminathan said about ills of flexible buildings:

Consider the lateral load-deformation curve of a structural member. It can be idealized as an elasto-plastic curve; from here the (idealized) yield deformation is obtained. Similar exercise can be done for ascertaining the lateral load-deformation behavior of a whole building. So, a flexible building has a large yield deformation (displacement relative to the base). And, unless the building yields, it cannot dissipate energy. So, having low stiffness is detrimental to dissipating the input seismic

This is reflected in some codes by setting upper limits on the overall slenderness ratio (H/D) of the building; of course, the upper limit is required also for ensuring psychological and physical comfort of occupants, and for protecting non-structural elements. Usually, this upper limit is ~10 or lesser. Of course, the limits placed on H/D are determined also based considerations of (a) overall lateral stability of buildings under strong lateral shaking, and (b) structural system chosen.

Another issue that arises from the same discussion of lateral stiffness of the building is the matter of overall deformed geometry of the building. The lateral displacement shape will gradually tend towards FLEXURAL cantilever type (as against SHEAR cantilever type) as the building has (1) higher H/D ratios, and (2) smaller axial stiffness of its vertical elements.

From the above points of view, the traditional approach has been to limit the value of H/D to ~10 and lesser.

with warm regards...
C. V. R. Murty
..

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nrk
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PostPosted: Tue Nov 27, 2012 9:59 pm    Post subject: Reply with quote

Dear Prof.C.V.R.Murty:

I wish to add that:

Second order effects and thus the overall lateral stability of the building is the prime concern in the case of a 'rare' earthquake event. The non-structural elements will be severely damaged in such an event. Limiting the damage of structural and non-structural elements is the goal in the case of a 'frequent' earthquake event. Eurocode 8 part 1 takes care of the above issues through a two-tier seismic design. The following are excerpts from the book 'Seismic Design, Assessment and Retrofitting of Concrete Buildings: Based on Eurocode 8', by Prof.Michael Fardis, which explains the Eurocode 8 approach.

Quote:
Part 1 of Eurocode 8 (CEN 2004a) provides for a two-tier seismic design of new buildings, with the following explicit performance levels (“Limit States”):

1. No-(local-)collapse, which is considered as the Ultimate Limit State against which the structure should be designed according to the Eurocode “Basis of Structural Design” (CEN 2002). It entails protection of life under a rare seismic action, through prevention of collapse of any structural member and retention of structural integrity and residual load capacity after the event.

2. Damage limitation, which plays the role of the Serviceability Limit State against which the structure should be designed according to CEN (2002). The aim is mitigation of property loss in frequent earthquakes, through limitation of structural and non-structural damage. After such an earthquake structural elements are supposed to have no permanent deformation, retain their full strength and stiffness and need no repair. Non-structural elements may suffer some damage, which can be easily and economically repaired at a later time.

The no-(local-)collapse performance level is achieved by dimensioning and detailing structural elements for a combination of strength and ductility that provides a safety factor (in the order of 1.5–2) against substantial loss of lateral load resistance.

The damage limitation performance level is achieved by limiting the overall deformations (lateral displacements) of the building to levels acceptable for the integrity of all its parts (including non-structural ones). More specifically, inter-storey drift ratios (defined as the difference between the mean lateral displacements of adjacent storeys divided by the interstorey height) are limited to the following values:

(i) 0.5%, if the storey has brittle non-structural elements attached to the structure (notably, ordinary masonry infills);
(ii) 0.75%, if the storey’s non-structural elements are ductile; or
(iii) 1%; when there are no non-structural elements that follow the deformations of the structural system.

The two explicit performance levels – (local-)collapse prevention and damage limitation – are pursued under two different seismic actions. The seismic action under which (local) collapse should be prevented is the “design seismic action”. The one for which damage limitation is pursued is called the “damage limitation seismic action”. Within the Eurocode philosophy of national competence on issues of safety and economy, the hazard levels for these two seismic actions are left to national determination. For structures of ordinary importance, Part 1 of Eurocode 8
recommends:

1. a “design seismic action” having 10% probability of being exceeded in 50 years (a mean return period of 475 years); and
2. a “damage limitation seismic action” with 10% exceedance probability in 10 years (mean return period: 95 years).

Although not explicit, an additional performance objective in buildings designed to provide earthquake resistance by dissipating energy is to prevent global collapse during a very strong and rare earthquake. This implicit performance objective is pursued via systematic and across-the-board application of capacity design, which imposes a hierarchy of strengths that permits full control of the inelastic response mechanism.

In frame systems and in frame-equivalent dual systems, strong columns are directly promoted, through the capacity design of columns in flexure, so that plastic hinging in columns is prevented. Moreover, in codes that adopt a two-tier seismic design, such as Eurocode 8, strong columns are indirectly promoted by strict interstorey drift limits for the damage limitation seismic action. Unless the columns are large, frame systems cannot easily meet the inter-storey drift limits of Eurocode 8 – especially as the cracked stiffness of concrete members is used in the analysis.

_________________
Regards,
Ravi.
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cvrm
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Joined: 16 Nov 2012
Posts: 12
Location: Chennai

PostPosted: Wed Nov 28, 2012 3:42 am    Post subject: Stiff versus Flexible Tall Buildings Reply with quote

Dear Mr. Ravi:

I like your indirect suggestion/question: Should we do TWO-LEVEL seismic design for Tall Buildings? Hmm... Considering the seriousness and rigour that has to go into a Tall Building, I am inclined to agree with you. It will require all of us to work harder. Let us...

Thank you.

with warm regards...
C.V.R.Murty
..


________________________________________
Dear Prof.C.V.R.Murty:
I wish to add that:
Second order effects and thus the overall lateral stability of the building is the prime concern in the case of a 'rare' earthquake event. The non-structural elements will be severely damaged in such an event. Limiting the damage of structural and non-structural elements is the goal in the case of a 'frequent' earthquake event. Eurocode 8 part 1 takes care of the above issues through a two-tier seismic design. The following are excerpts from the book 'Seismic Design, Assessment and Retrofitting of Concrete Buildings: Based on Eurocode 8', by Prof.Michael Fardis, which explains the Eurocode 8 approach.=

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