Page 233 - Design of Reinforced Masonry Structures

4.96 CHAPTER FOUR

Design shear due to the triangular mass of masonry,

V = W u = 2052 = 1026 lb
u1
2 2
In addition to the above loads, the lintel must carry its own weight.
Dead load due to nominal 8-in. deep lintel,
w = 78 lb/ft length
Factored dead load,
w = 1.4w = 1.4(78) = 109.2 lb/ft
u

Factored moments and shear due to the dead load of lintel are
.
)
.
(
M = wL 2 e = ( 109 2 8 67) 2 = 1026 lb-ft
u
u2
8 8

.
L
)
(
.
V = w L 3 = ( 109 2 8 67) = 473 lb
u
u2
2 2
Design forces:
M = M + M = 2962 + 1026 = 3988 lb-ft ≈ 4 k/ft
u1
u
u2
V = V + V = 1026 + 473 = 1499 lb ≈ 1.5 k/ft
u2
u
u1
Design loads are summarized in the following table:
Left opening Middle opening Right opening
Design forces L e = 20.67 ft L e = 6 ft L e = 8.67 ft
64.78 k-ft 1.47 k-ft 4 k-ft
Moment, M u
12.54 kips 0.82 kip 1.5 kips
Shear, V u
Example 4.27 Design of a lintel: simplified approach.
Figure E4.27a shows details of a concrete masonry wall having a 5 ft 4 in. wide
opening. The wall is 8 in. wide (nominal) and grouted solid (medium weight units, grout
3
weight 140 lb/ft ). The reinforced masonry over the opening is to be utilized as lintel. The
wall supports two superimposed concentrated loads 12 kips each (30 percent dead load
and 70 percent live load), transferred to it as reactions from roof trusses spaced at 8 ft

o.c. The parapet extends 2 ft 8 in. above the roof level as shown in the figure. Determine
2
the tensile reinforcement (Grade 60) required for the lintel if ′ =f m 2000 lb/in. .
Solution
Commentary: In this problem, the masonry above the opening is to be used as lintel.
The wall is solid grouted. Because there is ample depth (7 ft 6 in.) of masonry above
the opening, we will assume that sufficient depth of masonry lintel is available so that
the lintel can carry shear without any transverse reinforcement. Alternatively, we can
determine the smallest depth of masonry required for flexure, and provide the neces-
sary reinforcement to carry shear. This example uses the first of the two suggested
alternatives because of its simplicity. The problem is solved in two parts: (a) analysis
of loads and (b) strength design of lintel.