Page 219 - Design of Reinforced Masonry Structures

DESIGN OF REINFORCED MASONRY BEAMS 4.83

3. Wall height above the arch height (i.e., above the apex of 45° triangle) is at least 8 in.
4. Minimum end bearing, typically 4 in., is maintained
5. Control joints are not located adjacent to the lintel
6. Sufficient masonry exists on each side of the opening to resist lateral thrust from arching
action

Some of the aforestated criteria, which are based on sound engineering judgment, need
clarification. It is assumed that load disperses in the wall at an angle of 45° from the verti-
cal. For proper dispersion of load through the wall, the masonry units in one course should
overlap the units in the course below, hence the requirement for wall to be in running bond.
In a wall laid in stack bond, masonry units are stacked on top of each other, without any
overlapping of units. Such an arrangement of units makes it impossible for the load to dis-
∗
perse in masonry units of the bottom courses . For item 3, the arch height is measured from
the top of the lintel. This means that masonry should extend half the effective span plus 8
in. above the top of lintel for arching action to exist. Minimum end bearing of 4 in. is a code
requirement (MSJC-08 Section 2.3.3.3, Ref. 4.3). Control joints by their very nature create
discontinuity in a masonry structure. If control joints are provided adjacent to the lintel,
load dispersion cannot occur across them, and arching action cannot be assumed to exist.
And finally, sufficient masonry should be present on each side of the opening and above the
lintel to absorb the horizontal thrust created by the arching action in the masonry.
It is very important for a designer to clearly understand the ramifications of assuming
the presence of arching action over an opening in a wall because this assumption reduces
considerably the design load for the lintel. Where uncertainty exists, it would be wise to
ignore the arching action and design the lintel for the full load supported over it.
4.13.2.3 Design Loads for Lintels Design loads for lintels comprise the following:

1. Self-weight (dead weight) of lintel
2. Dead load of wall above the opening
3. Dead load and live load transferred from the floor or the roof supported by the wall over
the opening
Items 1 and 2 are shown schematically by load P in Fig. 4.22c, Item 3 by load P . These
2
1
loads are determined as follows:
1. Self-weight of lintel
All masonry beams and lintels must be solid grouted. The self-weight of a lintel can be
determined if its cross-sectional dimensions are known.
In a design situation, the cross-sectional dimensions are assumed for preliminary design.

Because a masonry lintel forms an integral part of a wall, its width is the same as the
wall width; therefore, only the depth of a lintel is required to be assumed. For prelimi-
nary design, the overall depth (h) of a lintel may be assumed as ¾ in. per liner foot of
span, rounded off to a multiple of 8 in. (for typical nominal 8-in. concrete masonry
units), subject to a minimum of h = 8 in. for beams as required by the code. The self-
weight of a masonry lintel can be determined based type of masonry units (lightweight,
3
3
medium weight, or normal weight) and the unit weight of grout (105 lb/ft or 140 lb/ft )
used for the wall. Refer to Tables A.19 and A.20 for pertinent information. Alternatively,
estimated dead weights of lintels having specific sizes (width and depths) as suggested
by NCMA [4.14] and shown in Table 4.8 may be used without any appreciable error.
∗Refer to Chap. 6 for more discussion on this topic.