Civil Engineering Reference
In-Depth Information
tensile reinforcements, sticking steel plates and prestress retrofit and so on.
These strategies are suitable for varied kinds of members governed by flexural capacity,
such as roof beams, floor beams, crane beams, highway bridge beams, frame beams, roof
slabs and floor slabs.
3.2.1
Cause and Phenomenon of Capacity Insuciency of RC Beams and Slabs
Capacity insuciency of beams and slabs indicates that bearing capacity cannot meet
intended demand or requirement for renovated function; therefore member retrofit must
be implemented to ensure structural safety. In such cases, the phenomena of capacity
insuciency comprise excessive deflection, over-width of cracks, steel corrosion and concrete
crushing in compressed area. In this section, the appearance phenomena and their analyses
of capacity insucient bent members along with damage features of normal sections and
inclined sections are summarized below, which will be of benefit to readers to judge whether
capacity is insucient and whether members need to be retrofitted.
1. Causes of capacity insuciency of RC beams and slabs
The causes result in the capacity insuciency of RC beams and slabs comprise of some
aspects below:
(1) Effect of construction
Insucient reinforcements and error of reinforcement in construction are regarded as
main causes for substandard quality of members. For instance, a garage in Jilin Province,
which had RC framing for the first floor and masonry structure for the second floor, was
found seriously cracked at the surfaces of beams and slabs. Deflection of a slab reached
L
/82 and the crack width was approximately 1 mm, which gave rise to noticeable vibration
even under pedestrian load. Based on the investigation, it was estimated that construction
quality was the main cause. Actually, concrete of grade C10 was adopted instead of design
grade C20 and it only left 763 mm
2
for reinforcement area of the beam rather than design
value of 1251 mm
2
. Consequently, the garage was not applicable for use and had to be
retrofitted. Another cause was malposition of tensile reinforcements, which could result in
cracking of the concrete on tensile side and even rupture of the member, and it occurs more
frequently at the ends of cantilever beams or slabs. A case in point would be an accident
that happened in Hunan Province. As the design thickness of 100 mm remained 80 mm for
balcony slab and the negative moment reinforcements descended by 32 mm, the concrete of
top surface at the end of the slab cracked seriously and ultimately the cantilever slab fell
down.
In addition, incorrect material application in construction will also debase structural qual-
ity and lead to capacity insu
ciency of the building. Some typical cases in practice are
specified as adoption of moistened or stale cement, employment of plain bars instead of de-
formed bars, indiscriminate utilization in concrete mix proportion, and application of sand
or stone with excessive impurity. As a typical case, the concrete explosion of beams and
slabs occurred in several projects after 4 or 5 years of service. Investigation into the cause
revealed that harmful impurity of (MgO), which consisted of alkali aggregates or aggregates,
formed into [Mg(OH)
2
] after absorbing water and accordingly gave rise to concrete explosion
with rapid expansion.
(2) Effect of design
It is generally believed that the discord between loading status and calculation diagrams
and mistakes in calculating loads become the primary design causes for capacity insuciency.
If the secondary beams treated as continuous are calculated as hinge-supported beams to
estimate supporting force, the force at middle bearing will absolutely be underestimated by
over 20% and the capacity of the main beams will be insucient consequently. For instance,
