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Re: Distribution of concentrated loads on masonry walls

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Cliff,

I don't recall any responses to your question, so I will take a stab.

You raise a rather good point.  I have always thought that the
concentrated load provision that you are referring to was for checking the
local bearing effects on the masonry and that the load would continue to
spread at a 45 degree angle for overall design of the wall (assuming
running bond).  In otherwords, it seems to me that the provision that you
cite is for local failure-type design of the wall, not the "global"
design.

But, as you point out, it is not clear.  It certainly can be interpreted
that the provision is for the global design of the wall as well.  This
appears to be another question to pose to the MSJC committee or someone
involved in the committee.

I will see what I can come up with.

HTH,

Scott
Ypsilanti, MI


On Sat, 14 Sep 2002, Cliff Schwinger wrote:

>
> I do not have a lot of experience designing multi-story (7 or more
> floors) masonry bearing wall and precast plank structures. I have
> designed only two such structures - the last one being 20 years ago when
> I was young and knew everything.  (My experience has been primarily with
> structural steel and cast-in-place concrete construction.)
>
> The wise old engineer who watched over me and taught how to design such
> structures instructed me to take the concentrated loads that occur at
> the ends of the walls (at corridors) at each floor and distribute them
> out on a 2 vertical to 1 horizontal spread as each concentrated load
> "traveled" down the building.  This 2:1 distribution pattern resulted in
> concentrated loads applied at the ends of a 30 foot long wall to be
> distributed over the full 30 foot length after the load traveled down 60
> feet of wall height.  This seemed to make perfect sense and in fact that
> old guy gave me a copy of an NCMA publication dated 1968 that basically
> confirmed what he told me.  When I designed my bearing walls, I had
> uneven loads along the length of most walls and it was the higher loads
> (kips / foot) at the ends that most often dictated the required f"m of
> the masonry.  Additional axial loads (and shear forces) due to the walls
> acting as shear walls resisting lateral wind forces were also taken into
> account.
>
> Fast forward 20 years.
>
> I am now a stupid old guy.
>
> Recently I was asked what effective width of wall could be used for
> support of concentrated loads at ends of walls from lintels supporting
> plank over corridors.  Incredibly I somehow remembered what I was told
> 20 years ago and I said that the loads could be distributed gradually
> across the width of the wall on a 2 vertical to 1 horizontal pattern. I
> was then shown that ACI 530 has for many years limited the width of load
> distribution in a running bond wall to a dimension equal to the bearing
> width + 4 x wall thickness.  I assume that when loads are applied at the
> end of a wall this dimension would equal the bearing width + 2 x wall
> thickness.
>
> This code provision seems to be somewhat overly conservative.  I don't
> think there are a huge number of 10 to 15 story masonry bearing wall
> structures being built anymore (at least not in my neck of the woods)
> and so maybe this conservativism hasn't been much of an issue.  Does
> anyone know why there is such rigid limit the effective width of load
> distribution of concentrated loads on masonry walls?  It would seem to
> me that if you had a 10 story masonry bearing wall structure, your
> design would get clobbered by this provision.  It seems crazy to have to
> assume that a concentrated corridor load applied to a wall 10 floors
> above the foundation level (about 88 feet up) would remain distributed
> over a constant 24" width (8" bearing + 2 x 8" wall thickness) as it
> traveled down the full height of the building.
>
> Since there are one of these concentrated loads dumped onto the end of
> the wall at each floor, the axial load in the wall would pile up very
> fast as you went down the building due to your inability to spread the
> concentrated loads out over more than 24".
>
> It seems unrealistic for the Code to say that an 8 kip corridor load
> applied at the end of a wall 10 floors up would have the same effect on
> the first course over above the foundation (4 kips / ft) as the 8 kip
> corridor load applied at the second floor.
>
> I would not have a problem with this provision for one or two story high
> walls, but I would think you should be able to spread the load out much
> further for higher masonry walls.
>
> Does anyone else out there have an explanation for why the concentrated
> loads are not permitted to be spread out over wider wall widths?
>
> Thanks for any advice anyone can shed on this topic.
>
> Clifford Schwinger
>
>
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