seismic analysis using robot software

8/22/2019 Seismic Analysis Using Robot Software

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SEISMIC ANALYSIS USING ROBOT SOFTWARE

The following report contains direction guidelines about how to performseismic analysis (modal response spectrum analysis) using ROBOT software.

A simple 3-D structure was used containing only 30x30cm RC columns and

300mm thick RC slabs. C40 concrete was used for all the elements.

This process is indicative and can be generalised in order to carry out seismic

analysis for any kind of structures.

It can also be used as a general guideline for seismic analysis irrelevant to the

software used, just by using the same commands on the desired software.


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MODAL ANALYSIS


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Select "New" to create a

new Analysis Case


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Select "Modal" as type


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Select the desired number of modes (depending on

the type and the complexity of the structure).

Select the mass matrix type. In principle, consistent

mass matrix gives more accurate results and should

be used. When lumped mass matrix is used, a

portion of the structure's mass is allocated to thesupports and therefore not taken into account for

seismic calculations since the nodes at the supports

have no seismic degrees of freedom (see following

pages for illustration).

Select the desired active mass directions.

In principle all directions are active mass directions but sometimes it may be

desired to check specific directions only. For instance:

-When modelling a strip (eg. 1m wide) of a structure we are not interested in the

out of plane mass direction and it should not be selected.

-The vertical (Z) mass direction can generally be neglected, unless there is a

specific interest in looking into effects such as uplift of pre-cast slabs. When the

vertical mass direction is active it will give vertical modes which can be disturbing,

especially when slabs are not held by columns for long distances and so they are

free to oscilate vertically.

However these modes have almost zero impact on the results (assuming that the

vertical translation is not of interest) since their effective modal mass is only activeon the vertical direction.

See next page for advanced parameters.


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By selecting "Advanced parameters" there are a couple of more

options available.

Probably the most useful is the option of setting an analysis limit

based on the percent of mass participation. Most codes only require

90% mass participation (participating mass/total mass).

This option can be useful, or save some calculation time when the

number of modes can not be foreseen. ROBOT will only use as

many modes required to get 90% participating modal mass for each

active mass direction.

It should be noted that ROBOT will only use up to the number of

modes defined by the user. It may be required to increase thisnumber in order to get 90% participating modal mass in all active

mass directions.

The analyses hereafter were run with

this limit set as Inactive


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After running the analysis, select Modal analysis relults.


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It can be observed that because the stiffness of this structure is

symmetrical (square columns and equal spans at both directions)

we get flexural modes with equal period/frequency.

Eg, mode 1 (flexural, x direction) has a period of 0.37sec, same as

mode 2 (flexural, y direction).

See following pages for modes illustration.

Note the difference between the total mass here and on the

following page. In this analysis half of the ground floor columns

mass is lumped at the supports, therefore being inactive seismically.

Analysis run with

Lumped mass matrix


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Analysis run with

Consistent mass

matrix

Note that the total mass is equal to the total weight of the structure

(see next page) divided by the gravity acceleration.

Cumulative Participating modal mass for each direction.Modes with zero

participating translational

modal mass are torsional

modes.


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Total weight


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Select display-maps-deformation-active to

display the modes.


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Mode 1 - flexural - x direction.


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Mode 2 - flexural - y direction.


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Mode 3 - torsional.


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Mode 4 - flexural - x direction.


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Mode 7 - vertical mode.


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MODAL ANALYSIS USING ADDITIONAL LOADS


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Define a new load case for

the load that should be used

as additional seismic mass

(for instance backfill).


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Go to Analysis - Analysis Type and select

the tab "load to mass conversion"


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Select the desired case to convert and assign any

coefficient (for instance 50% of the load).

Assign mass direction with the same principle as

when defining the modal case.

Click add.


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Modal analysis results.

Note the total mass is now the sum of DL plus the

roof load.


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MODAL RESPONSE SPECTRUM ANALYSIS (ASCE7-10)


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Select tools-job preferences


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Go to design codes and select"more codes"


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Under "seismic loads" choose

the desired code and add it (forthis case IBC 2006)


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Go to analysis types-new and

select "seismic" and the newly

defined code.


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Define the parameters according to the desing code.

Here the analysis is according to ASCE7-10, using site class

C, S1=0.2g, Ss=0.47g, TL=8sec, R=4 and I=1.25 (note: be

careful between S1-Sd1 and Ss-Sds when using ASCE).

See next page for direction.


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By default ROBOT will apply the earthquake to all 3

directions (not simultaneously).

If one direction (eg. Z) is set to zero then ROBOT will not

apply the earthquake to this direction.


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Automatically 2 components of the earthquake analysis

were created (no component for Z since it was set to 0 in

the previous step).


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The tab combination sign allows for definition of the

positive sign of each seismic case (if desired to define

one), as well as the combination rule used for the

directions of the motion (if excitation is applied at different

direction during the same case or combination)

seismic analysis using robot software

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