Page 231 - Design of Reinforced Masonry Structures

4.94 CHAPTER FOUR

Height of masonry above the lintel to the top of wall = 20 – 4 – 4 – 0.67 = 11.33 ft >
3.67 ft.
Hence, the arching action can be assumed to be present, and the lintel may be
designed to carry the dead load due to the mass of masonry contained in the 45° triangle
(with base equal to the effective span of lintel) and its own self-weight. Roof loads are
assumed dispersed through arching action in the wall, and not transmitted to the lintel.
2
Unit weight of masonry wall = 78 lb/ft
Load due to the triangular mass of masonry wall,
W = ½(6) (6/2) (78) = 702 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(702) = 983 lb
Design moment due to the triangular mass of masonry,
)
(
u
M = WL e = ( 983 6) = 983 lb-ft
u1
6 6
Design shear due to the triangular mass of masonry, V = W u = 983 = 492 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 6) 2 = 491 4 . lb-ft
u
u2 8 8

. )(
V = wL 3 = ( 109 2 6) = 327 6.lb
u
u2
2 2 2
Design forces:
M = M + M = 983 + 491 = 1474 lb-ft
u
u1
u2
V = V + V = 492 + 328 = 820 lb
u
u1
u2
c. Right side opening (Fig. E4.26D)
Clear span (width of opening),
L = 8 ft
Length of bearings on each side of opening = 8 in.
Effective span = Center-to-center of bearings,
L = 8 + 8/12 = 8.67 ft
e
Height of masonry wall = 20 ft
Height of opening = 12 ft