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Civil Engineering/ETABS beam ends releases & MRF


QUESTION: Dear sir

In case of braced structures columns can be designed only for axial loads. lateral stability is provided by other means (shear wall etc)

But as you earlier said when lateral load applied to a structure it will distribute according to the stiffness of the members. by detailing outer beam column joints as pin it will only make the outer columns less stiffer but inner columns get their due portion according ti their stiffness.

So then is it reasonable to design columns only for axial loads
in braced structures

ANSWER: Hi Priyantha,

You need to design all columns for axial loads plus bi-axial moments that you get from the analysis. You can simulate reduced column/beams joint stiffness by modifying the beam end connectivity releases and then reinforce the beams accordingly.

Please note that unless you design/detail all beams as more or less simply supported, and unless the bracing/shear walls are far stiffer than the MRF, the beams will definitely attract additional moments from lateral loading, which you will need to design for. So it is not as simple as some statement make it out to be.

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"You can simulate reduced column/beams joint stiffness by modifying the beam end connectivity releases and then reinforce the beams accordinly"

in that case to achieve reduced column/beams joint stiffness in whole structure is it enough to detail beam ends only. Normally column r/f are continue through the beams and internal columns / beam stiffness cannot reduced. is there any way of detailing internal columns also

ANSWER: Hi Priyantha,

Yes, becuause it is the beam ends that provide the degree of fixity to the columns and hence affect the overall lateral stiffness of the MRF. So, if you want to ensure that most lateral load goes to shear walls/bracing, just design the beams as simply supported, but with nominal rebar at the top to obviate cracking and to ensure that you comply with ties requirements for robustness.

However, if lateral loading is high and gravity loading is low (e.g. for short-span beams), lateral loading can cause tension at the bottom of beams at supports, so there may still be some lateral loading resistance from the frames. You analysis outputs will demonstrate that.

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1."You need to design all columns for axial loads plus biaxial moments that you get from the analysis"

To ensure most lateral loads goes to shear walls/ bracing beams are to be designed as simply supported. when we analyse beams as simply supported how can the moments transfer to columns

2. most buildings has balconies and beams cantilever from the column ends.
in those structures how can we achieve simply supported condition

Hi Priyantha,

As I have explained, in reality for RC structures you can never have a pure hinge at supports, so there will always be a degree of moment connection. I suggest the maximum moment release you apply at the beam ends in ETABS is 70-75%, which coupled with stiffness modifiers for the beams (say 0.35) will minimise moment transfer to columns, if that is the desired outcome.

You cannot have any support release for cantilevers, so they will transmit the full moments into the columns, or shared between the columns and any back-spanning beams.

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Arshad Khan


I can answer any questions to do with civil and structural engineering consultancy and construction industry in East Africa and the Middle East, and specifically with the analysis and design of reinforced concrete structures. My particular expertise is in the aseismic design and optimisation of tall buildings.


Employment history: 40 years in Construction and consultancy in the UK, Kenya, Tanzania, Uganda, Ethiopia, Eritrea, South Africa, Somalia, Zambia, Austria, Bahrain, Kuwait, Doha and the U.A.E.

.Fellow of Institution of Structural Engineers (UK) .Fellow of Institution of Civil Engineers(UK) .Member of the Institution of Engineers, Kenya .Registered Engineer, ERB, Kenya .Member of the Architectural Association of Kenya (Engineers Chapter) .Chartered Engineer (UK)

•1984: International Conference on the Art and Practice of Structural Design, London •1994: 3rd Int. Kerensky Conference in Structural Engineering, Singapore •2008: International Conference on High-Rise Towers, Abu Dhabi •2013: IEK International Conference, Kisumu, Kenya

BSc, 1st Class Hons, in Building Engineering, University of Bath, UK MSc in Concrete Structures and Technology, University of London. Diploma of Imperial College, UK.

Awards and Honors
•Science Congress Special Award (for 2-seater Hovercraft - 1968) •Institution of Civil Engineers Award for outstanding performance at Bath University (1975) •Concrete Society Postgraduate study Bursary Award (1976) •Consular Representative for British High Commission, Nairobi. (1995 to 1998) •Examiner for Institution of Civil Engineers Professional Interviews, Nairobi. (1997 to 1998) •Branch Representative in Vienna for PI assessment for Inst. of Struct. Engrs. (1999 to 2010)

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