SAP2000- incorrect period

[rajisace]

I am trying to construct a 6-storey concentrically steel framed model on SAP2000 v16. The frame is 18m high and made from S275 steel. From the formula in EC8, I know that T is supposed to be around 0.5s. The model my supervisor has provided me with confirms this with his at 5 storeys.

Yet when I do mine it comes out at 0.1s. I have even tried copying his model, yet I still come out with a different answer. I have gone through what I think are all the parameters and double checked my model with his and also redone the model around 10 times now.

Please can somebody help me, what am I missing??

 

[JoshPlumSE]

How did you define your mass? Did you multiply by the acceleration of gravity? Specifying mass in SAP (if I remember correctly) needs to be done in true mass units. Meaning that the program won't merely convert your applied forces into mass.

You're off by a factor of approximately 5 which is suspiciously close to sqrt (32.2 ft/sec^2).

 

[rajisace]

I am defining the mass source with the mass source as you said, but it doesn't seem to be making a difference, do I need to assign the mass to members?

 

[rajisace]

Also curious with how you would go about displaying a hysteretic response diagram from a conventional braced system. I can only find out how using links

 

[jhardy1]

If the period is too short (i.e. the frequency is too high), it suggests you have either got too much stiffness, or too little mass.

Does the model give "sensible" deflections for normal static load cases? If so, this would suggest your stiffness model is "reasonable".

If the stiffness is OK, you are looking for "missing" mass. The most common mistake I have seen people make is that they have set up a model for static analysis, with a variety of dead load and live load cases, where the dead and live loads are applied as forces and pressures (kN point loads, kN/m distributed loads on beams, kPa loads on slabs, etc). You need to convert all such applied-force load cases to non-structural mass cases, otherwise, your model will only have the structural frame "self-weight mass", but none of the masses from the applied load cases.

http://julianh72.blogspot.com

 

[rajisace]

Thanks for the advice. The loss of stiffness and change in period was because of the lateral loads applied to the system