Moment Frame Baseplate Design - High Seismic 3

[sticksandtriangles]

I am looking for some guidance on the FEM design of high seismic moment frame baseplates.

A little background:

Design criteria, fixed base column:

Column = W14x730​

Moment = 2200kip*ft, Omega Moment = 6600 kip*ft​

Axial = 140 kips (T) Omega Axial = 420kip (T)​

I have sized up an unstiffened baseplate... 9" thick grade 32 steel baseplate (massive!!). Uses (5) 3 1/4"Ø grade 105 anchor bolts. This is based on a purely rigid BP assumption and based on DG#1 examples.

I want to explore what the thickness of the baseplate needs to be using a stiffened design and do a little FEM modelling to confirm the software rigid BP assumptions.
Using RAM connections stiffened baseplate design module, it looks like a 7 1/2" thick baseplate can get the job done.

Ideally I would like to get into the 5"-6" range of BP thickness with stiffeners as required and have started to dive into a little FEM modelling to see if I can not get into this range.

FEM Model Development:

I have developed a SAP2000 model with compression only elements (gap elements) for the grout bearing, tension only elements (hooks) at the anchor bolt positions and modelled the baseplate as a non-linear shell. This is my first hurrah into heavy non-linear modelling so I am somewhat intimated by trying interrupt my results. I have started with the unstiffened design to get a feel for the more simple approach.

Questions I have:
[ul]
[li]How do i accurately model the stiffness of the grout bed/concrete foundation below?

My thoughts, E of concrete is readily defined, but what do I assume for deflection? What I did in my model was take E*trib area/deflection = stiffness (utilized 8600 kip/in for a 1 square inch finite element)​

[/li]

Utilizing this approach with a linear compression element yields very high concrete stresses. I have stresses in the range of 13ksi. my allowable for 5ksi concrete is around 5.5ksi​

I was thinking maybe I could model some non-linearity into the compression only element to allow for stress resdistrubtion within the bearing area to limit stress to the 5.5ksi range, but I am not sure if that is truly how the grout bed below would behave. I would assume that it locally crushes and fails? My DG#1 original design utilizes a rectangular bearing strength assumption though so I feel ok with this.​

[/ul]
[ul]
[li]Anchor bolt stiffness?[/li]

I did A*E/L for the anchor bolts. Yields a stiffness of 8020 kip/in. I have a long embedment of 60" on these 3 1/4"Ø ABs​

[/ul]

Image of high concrete compressive stress (each square is 1"x1"):

Images of the SAP model for reference under Axial and Moment Load listed above (shows signs of local yielding at edge of flange):

Thanks!




S&T

 

[SocklessJ]

ATSE is spot on. This baseplate is RFI #1 from the contractor. When they bid the job, they probably will assume they can get you to change this detail.

Granted I've never dealt with anything this large, but my starting point would be to cover the column in nelson studs and stick it 10' down in a thick mat foundation. If local crushing is still a problem, you could create a 36" void in the foundation with a pipe, and fill the inside with a high-strength concrete or grout mix.

I'd be interested to see how this is typically handled, as it sounds like sandman21 has some experience in this realm.