Page 223 - Design of Reinforced Masonry Structures

DESIGN OF REINFORCED MASONRY BEAMS 4.87

For design of lintels, effects of concentrated loads on a wall may have to be checked
at various locations, for example, under a bond beam and at midheight. As shown in
Fig. 4.25b, the effective length of wall subjected to concentrated load dispersion can be
expressed by Eq. (4.122):
Effective length = h/2 + b (4.122)
pl
where h = height of wall measured from the footing to the load point
b = width of bearing plate
pl
When two concentrated loads are placed adjacently so that the distance between them
is smaller than half the wall height, then the effective length of the wall under each load is
given by Eq. (4.123):
Effective length = 2(a + b ) (4.123)
pl
where a = distance between the inside edges of bearing plates
The contribution of the concentrated load on the lintel is equal to the uniform load, w ,
P
which is calculated based on the effective lengths. For the arbitrary position of the load in
Fig. 4.25, only partial length of the lintel (segment DB) carries this uniform load.
According to another approach, the concentrated load is first distributed over a certain
limited length of the wall as specified in codes. When a wall is laid up in running bond, this
specified length is taken as the smaller of (1) the width of the bearing area plus 4 times the
wall thickness or (2) the center-to-center distance between the concentrated loads (MSJC-
05 Section 2.1.9.1, Ref. 4.3). A concentrated load (e.g., reaction from a glued-laminated or a
steel beam) is typically supported on a bearing plate placed on the top of a wall (Fig. 4.26),
and is assumed to be dispersed in the wall at 30° angles (to vertical) from the edges of the
bearing plate. The resulting uniform load, w (Fig. 4.26), can be obtained from Eqs. (4.124)
and (4.125) (choose the larger value of w):
w = Concentrated load (4.124)
Width of bearing + t 4
w = Concentrated load (4.125)
r
Distance between concentrated loads
where t = actual thickness of wall. The larger of the above two values should be used for
design purposes. This load is then transferred to the lintel as uniform load w. Concepts of
load distribution in lintels are illustrated in Examples 4.25 and 4.26.

4.13.2.4 Lintel Depth Considerations Where there is a considerable height of wall above
the opening, it is difficult to define the portion of the wall height that exactly constitutes the
depth of lintel. Because the lintel is a part of the wall, the width of the lintel is the same as
the width of the wall. It has been suggested that for wall heights up to 3 ft above the soffit
of the beam, the full height could be considered as the height of a lintel. For greater wall
heights above the soffit of the beam, the lintel depth d could be arbitrarily assumed.
A common engineering practice is to establish a depth of lintel which can resist the
entire shear in the lintel without shear reinforcement, and which can be determined as dis-
cussed in Section 4.10. The nominal shear strength of a transversely loaded beam without
any shear reinforcement, V , can be determined from Eq. (4.96):
m
.
V = 225 A f ′ (4.96 repeated)
m n m