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Strong column-weak beam principle

 
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nrk
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PostPosted: Mon Nov 26, 2012 12:11 pm    Post subject: Strong column-weak beam principle Reply with quote

Clause 7.2.1 of IITK-GSDMA-EQ11-V4.0 (available on NICEE website) states: "At a joint in a frame resisting earthquake forces, the sum of the moment of resistance of the columns shall be at least 1.1 times the sum of the moment of resistance of the beams along each principal plane of the joint (Fig. 7). The moment of resistance of the column shall be calculated considering the factored axial forces on the column. The moment of resistance shall be summed such that the column moments oppose the beam moments. This requirement shall satisfy for beam moments acting in both directions in the principal plane of the joint considered. Columns not satisfying this requirement shall have special confining reinforcement over their full height instead of the critical end regions only."

Satisfying the above requirement is not practical for the edge and interior roof columns of a multi-storey building. I believe that the above requirement should be waived off at the roof level of a multi-storeyed building. Eurocode 8 part 1 waives off this requirement for the joints of the top floor. An excerpt of the book 'Seismic Design, Assessment and Retrofitting of Concrete Buildings: Based on Eurocode 8', by Prof.Michael Fardis, which explains the above, is quoted hereunder.

Quote:
Eurocode 8 does not require meeting Eq. (1.4) in the following cases of columns of frame systems or of frame-equivalent dual systems:
Around the joints of the top floor. As a matter of fact, it does not make any difference for the plastic mechanism whether the plastic hinge forms at the top of the top storey column or at the ends of the top floor beams. Moreover, columns of the top floor have low axial load, hence good ductility, and are less critical for the stability of the whole than the columns of lower floors. After all, it is difficult to satisfy Eq. (1.4) there, as only one column contributes to the left-hand-side.
In two-storey buildings, provided that in none of the ground storey columns the axial load in any of the combinations of the design seismic action with the simultaneous gravity loads exceeds 30% of the cross-sectional area times the design value of the concrete compressive strength, fcd. Columns with such a low axial load ratio have good ductility and develop low 2nd-order (P-Δ) effects. So, if a soft-storey mechanism develops at the ground storey of a two-storey building
these columns can withstand a displacement ductility demand of about twice the displacement ductility factor, μδ, corresponding to the value of q used in design.
One-out-of-four columns of plane frames with columns of similar size, the designer may choose to skip fulfilment of Eq. (1.4) at an interior column rather than at an exterior one, as only one beam frames into exterior joints and it is easier to satisfy Eq. (1.4) there.


While applying the capacity design equation of clause 7.2.1 to an edge or interior column, one of the beams meeting at the joint has hogging moment. As per the above book by Prof.Fardis, there is ample experimental and field evidence that, when the beam is driven past flexural yielding in negative bending and into strain hardening, the reinforcing bars in the slab parallel to the beam are fully activated as tension reinforcement of the beam, even when they are at a significant distance from the web. So, the moment capacity of that beam has to account for the contribution of slab bars parallel to that beam. Does IS 13920 have any provision to take into account the contribution of slab reinforcing bars for the calculation of moment capacity of the beam in negative bending?

I invite the opinions of everyone, particularly the moderators and the resource persons on these issues and request them to throw some light on the relevant provisions of IS codes that deal with these issues.

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Ravi.
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knsheth123
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PostPosted: Thu Nov 29, 2012 5:01 am    Post subject: Strong column-weak beam principle Reply with quote

Dear Engineers,

1. The post by Er. Ravi is very informative for incorporating Soft Storey especially for low rise buildings, where we have Initial 2-3 floors for general purpose and upper floors for Auditorium.

If the upper floor is a soft storey and the column loads are small < 0.1 fck, the detailing will be as a flexure dominant member instead as Column using capacity design concept. For column loads < 0.3 fck conventional ductile detailing can be adopted. Special soft storey requirements to be imposed for large comp. stress in columns.

2. IS 13920 guidelines for confining reinforcement is based considering complete conc. section mobilized to max. comp. stress in concrete. It has to reduced by Pu, actual/ Pu,capacity for the column.

3. The penalty for having lower strey as soft storey is imposed on the soft floors. But If we have upper storey as soft storey the ductility demand for the storey below the soft one might require special provision.

With a request Prof. C. V. R. Murthy to discuss the issues.

K. N. Sheth
Dharmsinh Desai University,
Nadiad.



On Mon, Nov 26, 2012 at 5:41 PM, nrk <forum@sefindia.org (forum@sefindia.org)> wrote:
[quote]            Clause 7.2.1 of IITK-GSDMA-EQ11-V4.0 (available on NICEE website) states: "At a joint in a frame resisting earthquake forces, the sum of the moment of resistance of the columns shall be at least 1.1 times the sum of the moment of resistance of the beams along each principal plane of the joint (Fig. 7). The moment of resistance of the column shall be calculated considering the factored axial forces on the column. The moment of resistance shall be summed such that the column moments oppose the beam moments. This requirement shall satisfy for beam moments acting in both directions in the principal plane of the joint considered. Columns not satisfying this requirement shall have special confining reinforcement over their full height instead of the critical end regions only."

Satisfying the above requirement is not practical for the edge and interior roof columns of a multi-storey building. I believe that the above requirement should be waived off at the roof level of a multi-storeyed building. Eurocode 8 part 1 waives off this requirement for the joints of the top floor. An excerpt of the book 'Seismic Design, Assessment and Retrofitting of Concrete Buildings: Based on Eurocode 8', by Prof.Michael Fardis, which explains the above, is quoted hereunder.

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