Base plate type

[allimuthug]

Hi ,

I HAVE A BASE PLATE SUPPORTING HE300A COLUMN. THE ANCHOR BOLTS ARE PLACED OUTSIDE THE FLANGE PLEASE REFER THE ATTACHMENT .
THIS BASE PLATE HAS TO BE DESIGNED AS FIXED OR PINNED ?

 

[WiSEiwish]

What is the result of your structural analysis? Did you consider the base connections pinned or fixed? You should have base reactions.

 

[Brad805]

I believe the concern you have is the unintended moment that occurs after installation. We can assume whatever we like, but the structure will behave according to the actual field conditions. I have a project on my desk that I have the same problem, so I decided to complete a more detailed analysis to check the degree of fixity using the actual base plate and frame geometry I intend to use. The analysis is not as exact as it could be, but it was done within a reasonable time frame. I thought I would post a couple of screenshots in case you are interested.

Some stats:
- Base plate is 1 1/4" x 14" x 20" (will reduce thickness now)
- Four 1 1/4" diameter anchor bolts (intend to re-size)
- Mf if assumed 100% fixity = 96 kip*ft from typical frame analysis
- W18x76 Column
- View magnification = 150 in attached images.
- Anchor rods assumed to be 100% fixed.
- Modeled compression block to be 3" wide. This is wider than it should, but I found the base plate deformation did not appear correct with a narrower zone. I have not determined a good method to model a compression only support in this software package.
- Modeled anchor head with a 0.15" gap between nut and plate. This was done to ensure there was no bonding between the nut and the base plate that would cause a compression force on the anchor rod.

From this I found the maximum tensile force on the two anchor rods totaled 23,500lb. This equated to a base moment equal to 30kip*ft or approximately 27% fixity. With a thinner base plate and tighter anchor bolt spacing that value will reduce to the point where my initial pinned assumption would be very reasonable.

 

[rb1957]

my 2c ...
analyze as pinned,
give yourself a healthy margin;
if moments develop, well, first there's the margin that allows you to accept some additional loading (over and above the worst code loading, with minimum material property);
and second, the worst i see happening is some local plasticity that'll help relieve the moment.

i think there's more to designing the preload than worrying about moments.

another day in paradise, or is paradise one day closer ?

 

[jike]

It is really not very difficult to calculate how much fixity a particular base plate/foundation can carry.

The equations for rotation are already worked out in the PCI Manual based upon:

1) Elongation of the anchor bolts
2) Bending of the base plate
3) Rotation of the foundation

I don't have the reference anymore (since I retired) otherwise I would scan it and post it. You can easily write a spreadsheet if you want to look quickly at the fixity of various arrangements.

 

[Brad805]

jike is correct. PG 4-46 of the PCI manual does outline the steps. I would post the forumla's if not for the copyright.

 

[allimuthug]

Can you specify the PCI manuval detailed name .

 

[Brad805]

I have the 7th revision, but it was in the old one as well. PCI Design Handbook, MNL-120-10 (

https://www.pci.org/bookstore/storefront.aspx).

 

[allimuthug]

Hi can you give me the screen shot of front page of pci design hand book , I couldnt find from the link give above.

 

[rb1957]

"Hi can you give me the screen shot of front page of pci design hand book , I couldnt find from the link give above."

i found it on the site, search for "MNL-120"

another day in paradise, or is paradise one day closer ?