Page 471 - Forensic Structural Engineering Handbook

MASONRY STRUCTURES 13.17

the secret of its 10,000-year success. Masonry is the most forgiving of building materials.
Performance failure may result from a combination of errors in design, construction, manu-
facture, and maintenance. For example, a masonry durability failure may result from improper
flashing design, unfilled mortar joints, under-fired brick, and deteriorated sealant joints.

Structural Failures
Quality Control in Reinforced Masonry. Poor workmanship, characterized by unfilled
mortar joints, reduces the compressive strength of air-dry masonry by about one-third. The
wet-to-dry compressive strength ratio for clay brick masonry is about 0.83. In concrete
masonry, saturated strength loss is about 25 percent. Poor workmanship and a soaking rain
can cut the strength of masonry in half. Freeze-thaw cycles produce a further reduction.
2
The average compressive strength of dry clay brick is 11,300 lb/in . Building codes
2
assume a compressive strength of 3790 lb/in for inspected masonry built with that average
brick laid in ASTM C 270 type S mortar. The average tested compressive strength of such
2
masonry is 5370 lb/in . If tested strength is cut in half by poor workmanship and a soaking
rain, the masonry strength is less than three-quarters of the assumed strength.
46
The Masonry Designers’ Guide is an authoritative treatise of structural masonry. One
of the example problems in that text can illustrate the effect of poorly built wet masonry. If
the reinforced clay masonry wall in example 11.1-7 were built with poor workmanship and
became wet, analysis indicated that the masonry would be overstressed 40 percent. If that
5
1
wall became wet and the bars were misplaced the / 2 in (13 mm) permitted by the code, the
masonry would be overstressed by 118 percent and steel would be overstressed by 17 per-
cent. If, when no one was looking, no. 5 bars were substituted for no. 6 bars but properly
placed, steel in an air-dry wall would be overstressed by 40 percent. Spliced bars lapped
less than 48 in (1219 mm) are deficient.
The average compressive strength of hollow concrete masonry units (CMUs) is about
2
5
2
3000 lb/in on net area. Building codes assume a compressive strength of 2121 lb/in for
dry masonry built with that average CMU laid in ASTM C 270 type S mortar. The average
2
tested compressive strength of such masonry is about 2000 lb/in . If tested strength is cut
in half by poor workmanship and a soaking rain, the concrete masonry strength is less than
one-half of the assumed strength.
Another example from the Masonry Designers’ Guide 46 can illustrate the effect of
workmanship on wet concrete masonry. If the reinforced wall in example 11.1-2 were built
with poor workmanship and became wet, analysis indicated that the concrete masonry
1
would be overstressed by nearly 50 percent. If the bars were misplaced the / 2 in (13 mm)
5
permitted by the code, the masonry would be overstressed by 62 percent and steel over-
stressed by 7 percent. If, when no one was looking, no. 5 bars were substituted for no. 6 bars
and misplaced / 2 in, steel would be overstressed by more than 40 percent.
1
Cracks. Rogmephobia is the fear of cracks, especially if they are haphazard. A contrac-
tor applied epoxy paint over sealant control joints in a concrete masonry wall. As the con-
crete masonry shrank, the paint cracked, whereupon the owner sued the architect and the
contractor for fear that the building was about to collapse.
Although most masonry cracks are benign, all masonry structural failures begin with a
crack. Applied force or restraint of volume change may induce excessive stress. Stiffness
attracts load. Masonry is stiff but weak in flexure. Placing rigid masonry veneer over flex-
ible steel studs results in cracked masonry. 21,23,24 Cracks beget leaks, corrosion, stains, and
spalls. Cracks precede collapse. Clay masonry has irreversible net expansion, and concrete
masonry has irreversible net contraction. Unaccommodated differential movement
between juxtaposed materials may cause cracks. Table 13.11 provides data on volume
change in masonry materials.