sap2000 time history analysis

[jiao]

Hi, There:

What should I do first to perform a TH analysis of a model subject to initial displacement and velocity using SAP2000? Since SAP only allows me to define initials as the end state of a previous TH

 

Hi Jiao,

I think you're mistaken that SAP2000 requires you to start from the end state of a previous time history analysis.

What's a little confusing about SAP is that you define your dynamic forces/moments or dynamic imposed displacement by first defining a "static" load case and then select and assign your dynamic load(s) or imposed displacement(s) under this new static load case.

Next, under define menu, define your time history function. SAp enables you to manually type the function or import a function from a file (it's often convenient to create a sine, cosine, etc function using excel and then saving as txt file and import into SAP). Remember, you can apply this TH function to a force or to an imposed displacement which you assigned under your newly created static load case.

Next, define a time history case, options for periodic, linear, or nonlinear time history analyses. At bottom of dialogue, you associate your newly created dynamic case with a time history function. Although you have the option, you do not have to start from previous time history. I think the default is NOT to start from a previous time history.

It's usually best to contact a software vendor's technical support for help

I gotta get back to work so I can buy shoes

 

[jiao]

Thanks Barefoot.
For initial displacement, it's ok to firstly define it as a static load case and then apply TH to it.
But how can I make it for initial velocity?

Thanks in advance.

 

Since velocity is a vector quantity whose magnitude is expressed as displacement as a function of time, you could do something for example, like assign a displacement value of 1 under your dynamic load case and apply a TH function to that load case equal to time vs. displacement to account for initial velocity. As you probably know, you can select and assign displacements to whichever areas and/or joints you wish on a load case by load case basis.

You can combine the results of that dynamic load case with whatever other load cases you want to combine it with.

If you are looking to first assign an initial displacement and then apply additional velocity loading you have a couple of options: 1. assign your initial displacement under one of your static load cases, then apply additional velocity as described above 2. or if initial displacement is a dynamic load case, create a load case and apply a TH function with time vs. displacement equal to initial displacement, then apply velocity

If you're doing a nonlinear analysis (you didn't say), you could start from a previous TH case to apply additional velocity load after initial displacement, or, I think you may be able to also use Define >Static pushover case, and define a load pattern with static and dynamic load cases..dynamic on a mode by mode basis (not completely sure how it works though), even if your nonlinearities have nothing to do with plastic hinge pushover analysis..Under analysis menu, you would run pushover. Probably ought to contact CSI for more info on use of nonlinear pushover options for non-pushover application

I heard CSI is coming out with a new version of SAP in a couple of months which will enable us to run any nonlinearity as a static or TH case. Right now, although it's accurate, it's a bit inconvenient to "fake" a nonlinear TH case to equal a nonlinear static load case by having to use .99 damping and flat line TH functions.

Sorry for such a long reply...this moonshine makes my head spin

 

[jiao]

I got lost, Barefoot.
My initial problem was how to perform a TH analysis for a model subjected to a set of initial displacement and velocity at certain DOF? not at restraint joints.
Can you put your post in another easy-to-follow words, please?
thanks

 

Can you give me a better description of your problem? Velocity by itself or velocity vs. period? is it linear or nonlinear analysis? more info. needed.

What specifically are you trying to analyze (earthquake, vibrating machinery, etc)?

I never said you had to appy loads at "restraint" joints.

 

[jiao]

To make it simple:
Analysis the linear TH of a 1-dof system m=1, k=1 subjected to a initial displacement of 1" and a initial velocity of 1"/s.

 

Last try -

You did not say whether you wanted to apply initial displacement as static load and combine with other TH loads, or whether you want to run everything TH. You can always assign disp. to Deadweight load case as static load

If you want initial displacement and velocity all defined as TH loads: Create static load case called "TH1". Select joint(s) and assign initial displacement value of 1" in one direction only (you said 1 DOF) under TH1 load case. Next, define TH function called "DISP", with Time values and function:

.9 1
1 1
1.1 1

For initial velocity, define another TH function called "VEL" for time vs function values of:

0 0
.5 .5
1 1

Define TH case with Load TH1 matched with function DISP and another load assignment with load TH1 matched with function VEL. Run as periodic analysis type and make sure # time steps X time step size = 1

For additional loads after initial displacement and velocity, use the same procedure as with initial displacement but define load as Force instead of disp.

Please see vendor if you're still unclear.