Seismic Story Drift Combination

[Mistieats]

Hi EngTip folks,

I'm trying to design a structural connection between two buildings in a seismic zone. The inelastic story drifts in NS and EW are 10" and 6" respectively. So for the designed structural connection, I need to consider the combined story drift in NS and EW directions. Is there a combination factor for the seismic story drift in two directions? I cannot seem to find it in ASCE 7. Or do I simply just use SRSS?

Thanks!

 

[driftLimiter]

ASCE 7-16 Section 12.12.3.
This section provides the minimum separation allowed.

 

[swearingen]

SRSS is definitely the way to go, but your question raises other questions:

- Are you trying to design a connection that will let the two buildings move independently of each other?
- Will you tie them together and prevent relative motion in just one direction, NS or EW?
- Will you tie them together and prevent relative motion in both directions?

There are a host of issues with any of the above 3 choices. Also, I hope those aren't story drifts as you have stated, but total drifts.

-5^2 = -25 ;-)

 

[Mistieats]

Hi Driftlimiter, thanks for the reply. Section 12.12.3 talks about the required design distance with a reduction factor for structural opening to account for the inelastic movement of both buildings in the same direction. I was more wondering what if I consider inelastic seismic movement in orthogonal directions, is there a reduction factor I can use for dy and dx? Would it be Sqrt(dx^2+dy^2)? Or would it be dx + a*dy, where "a" is a reduction factor

Thanks!

 

[Mistieats]

Hi swearingen,

Thank you for your reply and follow up questions. To give a bit context, I'm designing a skylight fixture between two buildings, and it's fixed to one building and only vertically supported by the other building. On the simply supported side, I need to leave some space for the skylight to slide both in EW and in NS directions. So I am trying to figure out the worst position for the skylight to be at, possibly building movement of NS of 10" + EW of 6". Or can I use a reduction factor of NS of 10" + EW of a*6", where a is a reduction factor.

Thanks!

 

[andrews2]

You seem to be missing some information. The separation needs to be sized for relative movement between the structures, so you need the seismic drift of both structures. You need to use the SRSS as stipulated in ASCE 7 12.12.3 like driftLimiter stated, but you should be using the drift of the two buildings and doing the check in each direction, not doing 1 check with the drift in each direction of just 1 structure.

So,
SQRT(dx1^2 + dx2^2) = Separation required in the X direction
and
SQRT(dy1^2 + dy2^2) = Separation required in the Y direction

 

[Deker]

For an element that has gravity support on two seismically independent structures, Section 12.12.4 applies. This requires not only using the absolute sum of displacements in lieu of SRSS, but also amplifying those displacements by 1.5R/Cd to get them up to MCE level amplitude. Diaphragm displacements should also be included. Basically you don’t mess around with something that could lose gravity support and fall several stories if it slides off its bearing.

Edit: You’ll need to sum displacements for both directions. The total magnitude of the displacement is SRSS of both directions, but that’s not really relevant to your design since the ability to accommodate both x and y drifts independently will automatically accommodate the SRSS value. Suggest sketching out the displacements in plan view if it’s not clear.

 

[HTURKAK]

Copy and paste of the relevant clause , Members Spanning between Structures 12.12.4,
......
Members Spanning between Structures. Gravity connections or supports for members spanning between structures or seismically separate portions of structures shall be designed for the maximum anticipated relative displacements. These displacements shall be calculated as follows:
1. Using the deflection calculated at the locations of support,per Eq. (12.8-15) multiplied by 1.5R∕Cd,
2. Considering additional deflection caused by diaphragm rotation including the torsional amplification factor calculated er Section 12.8.4.3 where either structure is torsionally irregular,
3. Considering diaphragm deformations, and
4. Assuming that the two structures are moving in opposite directions and using the absolute sum of the displacements.

.....

So , you need to provide flexibility ( say support length ) at least absolute sum of the displacements.



He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48