Page 218 - Design of Reinforced Masonry Structures

4.82 CHAPTER FOUR

FIGURE 4.23 Minimum embedment for lintel reinforcement.

portion of the wall load to lintel. This assumed arching action amounts to a tacit assump-
tion that if a lintel fails or is removed, only a triangular portion of the wall immediately
above the opening would collapse because the masonry will form an arch over the opening,
which will support the load above [4.14, 4.15]. Because a lintel is a horizontal member, it is
sometimes referred to as flat arch. In the absence of the arching action, the load distribution
as shown in Fig. 4.22 (similar to load distribution to a conventional beam) is assumed, that
is, load from the entire wall height above and the contributory service loads (superimposed
loads) are to be supported by the lintel. These loads are shown in Fig. 4.22b; all loads
applied below the level of point C must be carried by the lintel.
It is important to clearly understand the conditions under which arching action may be
assumed to exist over an opening in a wall. The development of arching action in a wall
over an opening is contingent upon the mass of masonry on either side of the opening
providing sufficient restraint to resist the horizontal thrust developed by arching. In areas
where limited mass of masonry is available, for example, near corners or between the
adjacent openings, it may be necessary to check the resistance of the wall to the horizontal
thrusts. If the adjacent masonry is found to be incapable of resisting this horizontal thrust,
tension ties should be provided to resist this force, for the lintel or the beam reinforcement
cannot be counted upon to provide the required horizontal resistance [4.14]. The required
tension ties can be provided by sizing the lintel reinforcement to resist both the beam stress
and to provide the tie for the arching action over the opening. For the tensile reinforcement
to also serve as tension tie to resist the arch thrust, it must be adequately anchored within
the masonry piers on both sides of the openings by providing minimum embedment lengths
equal to the required development lengths (discussed in Section 4.18) as shown in Fig. 4.23.

Arching action also requires that adequate depth of masonry be present above point C in
Fig. 4.22 to carry the horizontal compressive forces from the arching thrusts.
Uncertainty in the exact nature of load distribution over openings in a wall makes the
load to be carried by a lintel an indeterminate quantity. There is also a great degree of
uncertainty as to the ratio of the depth of masonry above the lintel to its span for the arch
action to occur. It is common practice to assume existence of arching action if the depth
of masonry above the top of the lintel is at least half its effective span lintel plus 8 in.,
where effective span is equal to clear span plus 8 in. (assuming 8 in. end bearings). This
recommended practice assumes arching action to be contingent on meeting the following
criteria [4.14–4.15]:
1. Masonry is laid in running bond
2. Sufficient wall height above the lintel exists to form a 45° triangle