rigid diaphragm 1

[civilungo]

hi:
i'm modeling a 30 floor building in viña del mar (chile), i assigned membrane as a floor section, i have used a spectrum function as require NCh433of96 chilean seismic code. my problem is that i've assigned rigid diaphragm in all floors then i run the program an when y look joint displacement i watch in load case joints moves together but when i watch the joints displacement on any load combination it does not keep the condition of rigid diaphragm.



regards

 

[Diquan]

You have to assing the diaphragm to each level, so at the end you will have 30 diaphragms. Click on the "plan view" and go one by one, the name of the diaphragm have to be different for each floor

Tiene que asignarle el diafragma a cada nivel por separado, al final tendra 30 diafragmas asignados. Le da click a "plan view" y va uno por uno, los nombres de los diafragmas tienen que ser diferentes en cada piso

Diquan

 

[drp181]

The results from a response spectrum analysis represent the maximum values at any time. The results for displacements of the points in the rigid diaphragm will have peak values that occur at different times, meaning that the deflected shape will appear to violate the rigid diaphragm constraint, though it does not.

Before proceeding with the design, be sure that you fully understand the fine points of response spectrum analysis. It is not the same as simple static analysis, and should be treated accordingly. For instance, all signs are lost as a result of the combination of modes, and therefore the code checks must be done assuming the worst combination of different forces (i.e. axial force and moment).

 

[civilungo]

thanks, but i don´t understand well, if i look the modal displacement (each one the different modes) the structure shows the displacement of the joint taking into account the diaphragm rigid condition... all of these movements are represent as the result of the spectrum analysis for each mode... but the question is: what does the displacement of each node that the structure shows represent in the spectrum analysis response (all modes superimposed by CQC) ? is the spectra response displacement case the maximum value of displacement of any node in each mode? i think it should be the superimposed by CQC response from the different mode shapes

Diquan : thanks, but the rigid diaphragm definition can be used the same in all floors because it associates all the joints selected into a master joint in each floor.. but if i want to use two rigid diaphragm in each floor i need to use two diferents rigid diaphragm assignment.

 

[stressed]

The joints on your floors will move/deflect together reacting to in-plane lateral loads with a rigid diaphragm, but will not move together with load combinations if load components of the combination include non lateral loads, such as Dead or Live loads in gravity (-Z) direction. The rigid diaphragm links joints together that are reacting to in-plane forces (membrane area local 1 or 2 direction), and does not link together joints reacting to loads coming from other directions. That is likely why you see joints on the floors moving together reacting to response spectrum lateral loads, but not to load combinations including other load cases.

 

[plopez]

Parece que estas considerando combinaciones de cargas que tienen estados de carga sismicas y el CQC te hace una distorsion de los desplazamientos que pareciera que el diafragma se deforma. Hay que verificar las deformaciones que se producen por cada modo separado.

 

[civilungo]

si plopez, estoy considerando cargas sismicas obtenidas con CQC mas cargas estaticas es cierto que se deforma con ellas y tienes razon, verifique las deformaciones por modo y luego aplico la proporcion con la masa modal de cada uno de los modos.

gracias

thanks everybody

 

[opa123]

well should i get design load from

Mx max and its corresponding My and N max
or
Mx max and My max and N max ?

 

[shdma]

Hi?all

Rigid diagram reduces the degrees of freedom and makes the model run much faster. If you don't include shell element with the rigid diaphragm, you can't obtain bending moments and shears for slab design,and the bending moments and shears for the beams is samller than the real situation, the bending moments and shears for the columns is more than the real situation.

usually I modify the I33 stiffness for the beam to simulate the real situation. According to some CODES amplify 2-3 times.

Good luck
Shdma