Page 238 - Design of Reinforced Masonry Structures

DESIGN OF REINFORCED MASONRY BEAMS 4.101

reinforcement in the lintel assuming d = 12 – 4 = 8 in. Alternatively, try one No. 5 Grade
2
60 bar (typical), A = 0.31 in. , and calculate the moment strength of the lintel. From
s
Eq. (4.9),
Af (. ) (
031 60)
a = sy = = 15.22in.
08 fb ′ 08 20 7625)
.
(
.
)
.(
.
m
⎛ a ⎞ ⎛ 152 ⎞
.
φM = φA f d − = 09 031 60 8.( . )( ) −
n s y ⎝ ⎠ 2 ⎝ 2 ⎠
= 1121 2. k-in. = 10 100 lb-ft > 4144 lb-ft
,
The moment strength of the lintel is 15,683 lb-ft > 4144 lb-ft (design moment).
Commentary:
a. The design moment in the above example is relatively small compared to the
moment strength of the lintel. This is because the width of the opening is rather
small, which qualifies for the arching action in the supported masonry. The design
shears and moments would be significantly large in the case of wider openings as
illustrated in Example 4.26.
b. In the above example, one No. 5 bar has been used for tensile reinforcement.
It would have been just as satisfactory to use two No. 4 Grade 60 bars (A =
s
2
0.4 in. ).
4.14 MASONRY WALL BEAMS
(DEEP WALL BEAMS)
4.14.1 Wall Beams Defined
Section 4.13 presented a simplified approach to analysis and design of masonry lintels that
are parts of masonry wall panels spanning openings. The qualifier “simplified” is used here
to emphasize the fact that the masonry lintel is not an isolated flexural member; rather it
is a part of the entire wall panel spanning the opening. The simplified approach assumed
the existence of a lintel of limited depth within the wall panel, which carried loads from
the supported wall panel (with or without arching effect). Wall panels that span openings
instead of acting as bearing walls with continuous line support along the bottom (Fig. 4.28)
are often referred to as “wall beams”[4.16]. “Deep beams” and “deep wall beams” are other
terms used in the literature to describe wall beams. The term “wall beams” would be used
throughout the following discussion.

There are no design provisions or guidance for analysis and design of wall beams in
the MSJC Code [4.3], presumably because of the lack of test data on the behavior of wall
beams (or deep masonry beams), unlike reinforced concrete, for which guidance for analy-
sis and design (based on strut-and-tie model) is provided in App. A of the ACI Code (ACI
318-05, Ref. 4.2).

4.14.2 Concept of Deep Beams

ACI 318-05 (Section 10.7, Ref. 4.2) defines deep beams as “members loaded on one face
and supported on the opposite face so that compression struts can develop between the load
and the supports, and have either: