Civil Engineering Reference
In-Depth Information
pouring and curing, another reason is the adoption of non-factory certification cement. In
a number of projects, columns burst in multiple places four or five years after completion.
This is due to alkali aggregate reaction or periclase in aggregate, the volume of which will
expand when producing brucite after absorbing water.
c. Low level of competence and lack of responsibility. These issues may cause the shortage
of steel blanking length, overlapping length and anchorage length, wrong numbering of
reinforcing bars and insu
cient reinforcement. For example, a teaching building is a ten-
story frame shear wall structure, 59.4 m long and 15.6 m wide. The column section size and
reinforcement of the sixth story were applied to those of the fourth and fifth story. For the
fourth and fifth story, the area of reinforcement of inner column decreased 4453 mm
2
(66%
of design area) and the area of external column decreased 1315 mm
2
(39% of design area),
which caused serious accident. Detailed retrofitting calculation of this example is shown in
Example 3.8.
d. Poor management of construction site. At construction site, these circumstances often
occur such as the rebars are bent and offset, or the form boards are bumped obliquely, or
pouring concrete directly without adjustment. Take a factory for example, which is a five-
story cast-in-place reinforced concrete frame structure. Lifting large components induced the
second floor to tilt seriously. For another cast-in-place reinforced concrete frame structure,
column forms were inclined when pouring concrete by reason of loose form, which made
the longitudinal bars in columns inaccurate. When frame beams had been finished, the
longitudinal bars in columns were uncovered. To ensure the thickness of column covering
layer, construction workers bent the longitudinal bars in the shape of “ ” by error. Without
timely retrofitting and strengthening the building is subject to structural danger.
e. Differential settlement. Differential settlement causes subsidiary stress in columns,
which makes columns crack or lose bearing capacity. A factory building built on soft soil
ground in Nanjing is 44 m long and 21 m wide. The superstructure is a monolayer bent
structure composed of reinforced concrete columns and roof trusses, while the foundation is
reinforced concrete isolated footing. Many years after completion, with the increase of output
and stacking the amount of differential settlement had achieved 216
422 mm, columns tilted
in different directions. Column corbel cannot bear the extra horizontal force and cracked
seriously. Ultimately, the factory had to suspend production and retrofit because it cannot
be used anymore.
f. Usage of commercial concrete. Generally, commercial concrete quality is better than
self-mixed concrete. However, there are also some quality problems:
a) Concrete has set before arriving at construction site. Constructors re-mix this concrete
with water and resume using, despite decline in concrete strength. Such circumstance oc-
curred in an 18-story building in west Shanghai. Later, they had to retrofit these deficient
columns.
b) Commercial concrete is always pumped up and lubricating mortar in conduit pipe is
left in place. Sometimes, builders pour this lubricating mortar into a column, making a
“mortar column”. For instance, in a 26-story oce building in Shanghai, such an accident
occurred during building of the fifth mechanical floor, and had to be retrofitted.
Other factors can make the column bearing capacity insucient. High-temperature fire
will burn through the concrete, decreasing the strength of concrete and reinforcement. Ve-
hicle impact will severely damage the column. Retrofitting upper structure or functional
changes will increase the ability of a column to withstand loads.
Appropriate retrofitting method should be chosen and then applied in time according
to column appearance, checking calculation and site condition after understanding failure
feature.
There are many column retrofitting methods including section-enlarging method, replacing
∼
