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Re: MBS Base Plate Grouting

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> From: "Mark D. Anderson PE" <mark(--nospam--at)alaskaengineer.com>
> 
> I am designing a foundation for a MBS with 100 foot clear span and =
> column bases that are deeper than what I am accustomed to seeing - 8" x =
> 2'-3, with (2) rows of 5 anchor bolts.  I seldom see grouting beneath =
> the base plates of ordinary MBS, but then I have seldom seen base plates =
> this deep with so many anchor bolts, which are nevertheless considered =
> to be "pinned" per the design report.

Deep bases are usually semi-rigid connections. As you note, the
structure is typically designed assuming pinned bases. As a steel
designer, I would make a couple checks to confirm the actual effects.
The objective is to "bracket" the effects to the foundation to confirm
that the pinned base assumption is adequately valid for foundation
design (e.g. small base rotations are effectively, close enough to
pinned).

>From a foundation perspective, you could check the moment capacity of
the base relative to the rotation required to achieve the theoretical
eave deflection (the manufacturer/designer probably can't give you the
base rotations for the design load cases). You could even assume that
the base is loaded at the inner/outer flanges to provide some
conservative load eccentricity using the reactions that have been
provided.

I have seen erection failures where the "pinned" base has rotated the
footing out of the ground. However, the pinned condition would have been
a good estimate for fully constructed service.   

> I am considering specifying a 1/4" steel plate as a stay-in-place =
> template at the surface of the pilaster, but can't decide about the need =
> or lack of need of grout on top of the template.  Before I impose the =
> cost of grouting the column bases on the project, I would appreciate =
> some other opinions.  I have even considered the use of rubber bearing =
> pads to accommodate imperfect alignment if no grout is used.

The cover sheet or general spec sheet from the manufacturer will
probably state something to the effect that, "the base must be finished
level." Unfortunately, this is the steel drawing without consideration
of the ACI placement tolerances and is not given to the concrete
contractor. So, on the concrete drawings/specs make an adequate
levelness spec for the base or use grout.


> From: "Dave Handy" <dhandy(--nospam--at)trg.ca>

> The anchor bolts are designed for tension and shear only (I believe).  =

This may not be true if the grout is spec'd in advance of building
design, sometimes, maybe, ...

> If a grout space is added then there is additional bending taking place =
> between the two plates. We normally avoid making a gap and leave it to =

In theory this is true. There are redundancies such as friction beween
base/grout and grout/foundation. I haven't seen much evidence to suggest
that this is a big problem.

Is anybody aware of a metal building failure caused by bolt failure on a
grouted base due to bending of the bolts?

-- 
R. Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ado26(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>

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