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RE: Masonry lintel design

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William,

What you are describing is a "deep girder".  A deep girder does not have a
stress distribution that is like that of a shallow masonry beam.  The bottom
of the lintel, and the adjacent tension-reinforcing will be in tension, but
a compression block conforming with the usual beam theory will not develop
unless the total depth of the beam is considerable less than the lintel
span.  [Think about where the compression block of a conventional beam
stress pattern might develop if it were to develop well below the top of a
wall that passes over an opening -- such a stress pattern can't develop
unless the load were to be applied before the upper part of the wall is
built.]

PCA publications ST66 & ST79 provide a procedure for designing deep girders.
ST66 is based on research done in the 1940's and 1950's.  The procedure is
in a reference to ST79 in ACI 318-95, but that procedure seems to be no
longer recognized by ACT 318-02; newer procedures are required for concrete
design in accordance with Appendix A of 318-02 -- I have not used the newer
procedures.  The ACI principles should be applicable to masonry as well as
to concrete.

However, ACI 318-02 still includes reference to ST79, as well as other
references.

Unless there is a very heavy load applied to the top of the wall over the
opening, you may be able to satisfy yourself that two #5 are adequate
without a complicated analysis.  Some assume that the load carried by a
lintel is the weight of a triangular mass of masonry bounded by the bottom
of the lintel and 45-degree lines rising from the jambs, with the weight of
masonry above that triangular mass being carried by "arch-action" of the
wall above. However, "arch-action" is not included in the provisions of the
Code.    

Nels Roselund, SE
South San Gabriel, CA
njineer(--nospam--at)att.net

-----Original Message-----
From: Sherman, William [mailto:ShermanWC(--nospam--at)cdm.com] 
Sent: Monday, May 08, 2006 4:32 PM
To: seaint(--nospam--at)seaint.org
Subject: Masonry lintel design

I am designing a single course masonry lintel using two #5 bars near the
bottom of a masonry lintel block - what is a practical "d" distance to
use for vertical bending?  I have consulted numerous references on
masonry design but none of them give any guidance.  The lintel design
examples show an assumed "d" distance - with each reference using
different assumptions.  I need to get as much capacity out of the lintel
that I can, but I also need to make reasonably conservative assumptions
about the constructed condition. 

(FYI: The masonry supplier has not been selected yet, so I do not have
accurate information on the dimensions of the lintel block.) 

(RANT: This is the most frustrating aspect of masonry design to me - the
design formulas are well defined but the construction details are not
well defined.  It is very difficult to apply engineered design to
masonry, when the industry does not define such simple "details".) 


William C. Sherman, PE 
(Bill Sherman) 
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com

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