Expert: Babak Esmailzadeh Hakimi - 1/13/2012

Question
QUESTION: What is the purpose of applying diaphragms to slabs . Why this facility is not available in SAP

ANSWER: Dear Priyantha

The essential decision to be made when modelling a structure is wether the diphragm in  each story must be regarded as rigid or flexible. For example an industrial gable roof must be regarded as flexible while in conventional multi story building with reinforced concrete slab , each level's story must regarded as rigid. This assumption influences the loading distribution between structural elements. In conventional building you must assume the diagram being rigid in 2 horizontal directions and twisting around z axis .

Etabs is a building structural analysis and design software while Sap is a general structural design one. The diaphragm option is available in both  sap2000 and Etabs. In sap you can choose constraint option and then select ADD Diaphragm , ......

Don't hesitate to ask me further

regards
Babak E Hakimi



---------- FOLLOW-UP ----------

QUESTION: Dear Sir

When a diaphragm is assigned to a slab

1)How it decides center of diaphragm.(say a building with columns of different x'section, no shear wall)is it shear center of the building

2) In case of auto lateral load we can see lateral load applied at the center of diaphragm at each floor. how can we see the amount of lateral load applied to each column

3) in case of auto lateral load applied to a RC structure ,SHOULD the infill brick walls to be modelled

ANSWER: Dear Priyantha

1- Gravity center of your story will be regarded as your diaphragm center, because seimic loads apply on mass center. If you dont apply any mass to your slab the program will use generalized mass calculated from frames mass.

2- The option "show loads" on view will show you every load which is applied on each node.

3- You have to model your structure carefully. If the infill walls are significant they have to be modelled. But I want to tell you a secret , infill walls loose their strength and stiffness very quickly in earthquakes then in my opinion you must consider their negative effect on columns or girders in design and detailing process. Sometimes such a walls bahave like a murdrer for a column in earthquakes.


Hope been  satisfactory
Babak E Hakimi

---------- FOLLOW-UP ----------

QUESTION: Dear Sir

1)In case of RC frame  structure (without shear wall) if you apllied lateral loads(loads applied as point loads to nodes in external face) without applying diaphragm / applying diaphragm what sort of difference will they make

2)"In particular floor total lateral load applied to centre of diaphragm and they distributed among the lateral load resisting elements(COLUMNS) propotionate to their fleXural rigidity (EI)"
PL. explain whether this statement is correct or wrong

Answer
Dear Priyantha

1- In the case you dont use a solid diaphragm, loading on each node will be absorbed by the structural element in the right vicinity of loading.If you have only a shear wall in your structure , the loads applied directly on shear wall will be absorbed by shear wall and the other loads will make other non resisting elements showing intensive deformation. The solid diaphragm would transfer loadings far from shear wall to the shear wall.

2- When you use a rigid diaphragm , the displacement of all strucural element will be equal in each direction ( trans x, trans y , twist z) , then the lateral loading will be distributed proportional to the stiffness of structural elements. Then your statment is true.

Please dont hesitate to ask me further.

Regards
Babak E Hakimi