Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Distribution of concentrated loads on masonry walls

[Subject Prev][Subject Next][Thread Prev][Thread Next]
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


******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ********