Page 232 - Design of Reinforced Masonry Structures

DESIGN OF REINFORCED MASONRY BEAMS 4.95

Top of wall

2'-8" Roof line
Apex of 45° Δ

5'-4"
4'-4"
45° 45°

L = 8'
Le = 8'-8"

FIGURE E4.26D

Assume the overall depth of lintel = 8 in. = 0.67 ft (least permitted depth)
The minimum depth of masonry above the lintel required for arching action to
exist is

L e + 8 = . 867 + 067 = 5 ft
.
2 12 2
Height of masonry above the opening to the top of wall = 20 – 12 – 0.67 = 7.33 ft.
This is greater than 5 ft height of wall required to develop arching action. Hence, the
arching action is assumed to exist, and lintel should be designed to carry the dead load
due to the mass of masonry contained in the 45° triangle (with a base equal to the effec-
tive span of the lintel) and its own self-weight.
Unit weight of masonry wall = 78 lb/ft
2
Load due to the triangular mass of masonry wall,
W = ½(8.67) (8.67/2)(78) = 1466 lb
Self-weight of 8-in. deep lintel = 78 lb/ft
Load combinations:
The lintel does not carry any live load from the roof. Thus, the factored load due to
the triangular mass of the masonry is
U = 1.4D = 1.4(1466) = 2052 lb
Design moment due to the triangular mass of masonry,

.
)
(
M = WL e = ( 2052 8 66) = 2962 lb-ft
u
u1
6 6