Principle Directions in UBC 97

[dawn836]

Hi all
Here is some questions related to Response spectrum analysis on UBC97

1- what the term principle direction refer to and how can i determine principle directions of a building?

2- Does structures have two diffrent fundamental periods in each direction? and if so should we use diffrent scale factor in each direction to account for scaling dynamic base shear to static base shear?

3 The factor for scaling dynamic base shear will be applied to the 3d model by mutiplying it by gravity accelration and then using this as a scale factor for Design response spectrum(Fig 16-3)

 

[mrMikee]

dawn836,

I haven't worked with the UBC so I can't specifically answer your question. However, I assume that when they are referring to the principle directions (or maybe principle axes) of a building they are talking about a structure in plan view where you could have an x and y axis. In a normal building the principle directions would be perpendicular the faces of the building but this depends on symmetry and other details, geometry, etc. An orthogonal condition is when seismic forces are not on either of the principle axes and instead is a combination of both with either the 100%/30% rule or with SRSS.

Concerning your second question there are in fact many vibration modes (one for each mass) depending on the sophistication of the model. The two of interest can be identified by looking at the animated modes of vibration and picking out the ones with motion in primarily the x and y directions. The simplified design methods are based on the fundamental modes.

I hope this helps.

Regards,
-Mike

 

[dawn836]

Thanks Mike
so should i determine first the prinicpal axis of the building and then apply the seismic force once on each direction of both?

Does Fundamental mode differ in both directions?(i.e. there is 2 fundamental modes and the are determined from the participation factor they have in each drection)?

 

[mrMikee]

It's been a long time since I've done a design with anything other than the simplified procedure for seismic in the IBC except for a machine design job I did last year. This job was done per a specialized spec written by the contractor based on some of the nuclear seismic spec. It was a simple model for quoting only.

I don't know if this will help or not but here's what I did. My model was a 4 column structure with some framing that supported two masses. I was interested in translation of the masses only so the model had six degrees of freedom and six natural frequencies with associated mode shapes. To me the fundamental mode is the mode with the lowest frequency but there are modes that move in predominantly the x,y and z directions. These can be picked out by looking at animated modes in a computer model.

I had to evaluate 24 load cases but for a building I would use the load combinations defined in the IBC or UBC spec. You would design for each x and y directions independently and possibly orthogonal loads depending on the zone or IBC Seismc Design Category.

-Mike

 

[chichuck]

dawn836,

The fundamental mode of vibration is the one with the lowest natural frequency, it is unique (there is only one) and it may be in either of the principal directions. I am not certain if it can be in some other skewed direction as well.

If you have a 3D structural analysis software available, eg STAAD or RISA, etc, they should allow you to model and analyze your structure to determine the lowest modes, you can select the lowest 3, 4 or 10 or whatever. You can examine the output and see which has the lowest frequency and examining the mode shape will tell you which direction that mode is in.

Hope this helps.

Regards,

chichuck

 

[NewImhotep]

dawn836,

u can get the principle axis using software like section builder or prokon, but it is useless and difficult specially with irregular structures.
the alternative solution as mrMikee said to ,take SRSS combination or 100% 30% for each X & Y axis if u in the static analysis

the scale factor for spectrum X and Y can be difference about the building translation in X & Y directions .

it is better when u calculate the static seismic force for scaling to take the fundamental period from the dynamic output and u have to select the mode with the mass participation in the case of study axis as the maximum ( mostly u will get diffrent T for each axis and So different base shear for each direction), T should be limited to 1.3 or 1.4 of T from method A as section 1630.2.2,if u are using Etabs the program automatic doing these things by applying auto seimic load.

Hope this helps

Ahmed

 

[dawn836]

well does the fundamental mode is the mode with highest mass partcipation factor?