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Re: Masonry wall with epoxy anchors

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Dave,

See comments below:


On Thu, 8 Sep 2005, Nuttall, Davin J. wrote:

> List,
>
>
> Due to contractor error, I had to externally reinforce a 12" CMU stack
> bonded cavity wall after-the-fact.

If I may ask, why the stacked bond.  If the wall will be furred out, then
it appears that appearance is not the driving factor, so running bond
should have been the better choice.  The exception could be that only one
side will be furred and the exterior a face of the wall will be exposed.
If such is that case, then I have to wonder what the type of load the wall
must carry is, how was it deemed insufficient as constructed, and what are
the full details of the repair.  It sounds as if it could be flexure due
to wind pressure and repair could be external steel plates as flexural
reinforcement (all speculation...could be something else).  If so, then
reinforcing one side would not seem to solve the potential problem as the
wind in such as situation could be either positive pressure (resulting in
tension stresses on the inside face) or negative (resulting in tension
stresses on the exterior face).  Just curious.

>
> My detail included continuous steel plates with epoxy anchors drilled
> into the center of existing solid grouted cores.  The wall will be
> furred out, so the repair will not be visible after the project is
> finished.
>
> I now find out that the contractor ignored portions of the submitted
> fix, and in some cases, located the anchors right in a mortar joint.
>
> Questions:
>
> 1.  Because the wall is grouted solid, would you be concerned with edge
> distance of the bolt relative to the edge of the individual block?

Generally, I would say no.  It will in general be the edge distance of the
bolt to the edge of the masonry assembly not the unit.  This would be
largely true even if ungrouted.  This is assuming you are looking at it
from a global point of view.  Where edge distance to a individual block
could be an issue is for local effects that could further weakness in the
"weak points" (i.e. say you use an expansion style anchor in a mortar
joint...could cause the joint to crack/fail due to the expansive force,
which in an ungrouted situation could cause the generally considered
weakest point - the mortar joint - to be even weaker).

>
> 2.  Are the blocks considered as individual units with respect to the
> new bolts and their capacities, or is the entire cmu wall acting as 1
> monolithic unit?

Generally considered acting as 1 monolithic unit.  But again, it depends
on what "thing" (i.e. failure mode, behaviour, etc) you are looking at.
Masonry is a "non uniform" material, so some failure modes can be governed
by the individual components more readily than the overall assembly.  So,
it will require knowledge of how masonry behaves and what the bolts are
"doing" (i.e. how are the bolts loading the masonry) to really make such a
determination.  And I don't have enough information of the specific
situation to make such an assessment in this situation.

>
> 3.  How much of a strength reduction would you take because the bolt is
> located in or right next to a mortar joint, and not the published
> distance away from a free edge?

Again, it depends on what the bolt is "doing" and the configuration of the
bolts as well as the configuration of the wall as is and what kind of
loads it must carry.  More than likely if the bolts are engaging the grout
in the cores, it will likely not matter if the bolt is in a mortar joint
or not.

>
> The worst case scenario would be to remove and relocate the plate and
> bolts to the specified location(s).  As always, time is a major concern,
> and they would like to move on with minimal rework.
>
> Thanks
>
> Dave Nuttall, P.E.
> Green Bay, WI
>


HTH,

Scott
Adrian, MI

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