Civil Engineering/Floor Diaphragms



"Assigning a Diaphragm to an area object provides a Diaphragm constraint to all of the corner points of the area object"
When we modelling a building we can select all area objects in particular plane and assign Diaphragm.

1, What happen if we don't apply above Diaphragm .
2. The point where all of the lines of the Diaphragm intersect is the centre of mass. what is this point....
3. What happen when auto lateral loads apply without assigning Diaphragm.

ANSWER: Dear Priyantha

Usually slabs on the floor constrain all nodes on a floor and as a matter of fact all nodes behave in relationto each other. Then diaphragms develop such a constrain. This performance is related to the stiffness of the slab or diaphragm.

1- if you dont apply a diaphragm each mass on the floor behaves independently which is not an efficient solution considering the stiffness of the diaphragm.

2- The mass center of the diaphragm is the location of applied seismic forces.

3- see 1


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1. When determining mass center is it consider mass of slab only what about mass of beams.

2. "When modelling a structure which is not subjected to earth quake loads, it is not required to assign diaphragms to slabs" is this statement correct...

3.say there is a symmetric structure. we can calculate total wind load of particular floor and apply at mid point (say column beam joint). Now this has to distribute according to stiffness of each shear elements(columns and walls).

Is it require diaphragm to do this or automatically will it do?

ANSWER: Dear Pryantha

1- A mass center must include all sources of mass.

2- Diaphragm concept is not related to earthquake loading . It is a general concept regarding the integrity of floor diaphragm.

3- A Diaphragm distribute the external loading according to the mamber stiffness. For wind loading you should apply forces along the facade of the building according to the geometry of your building.


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When we see "set building view,diaphragm extent" it shows lines connecting at a particular point denoted as D1. Is this the centre of mass.

When we delete a beam or two in particular column that specific point will not change what is the reason..

Dear pryantha

That is the geometric center of the diaphragm. It is assumed this would me the mass center. For a better modelleing you can model the roof as a slab with actual thickness and neglect the diaphragm identification. The cenetr of mass will be more accurate in this model.


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Babak Esmailzadeh Hakimi


I can answer questions in Structural Engineering, Earthquake Engineering, Project Management and Project Control topics!


I am experienced in the area of earthquake engineering in electrical power industry, i.e. Lifeline Earthquake Engineering. My focus has been on structural seismic design of power installations. Recently I have got involved in project management in the category of constructional engineering.

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Bs degree in Civil Engineering from Ferdowsi University ,1988, Mashhad Iran, Ms degree in Earthquake Engineering, Sharif University of Technology, 1993, Tehran, Iran

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