When it comes to the design of the basement slab, there is something the
structural engineers can make the big mistake. With the same loading as with the other
floor or a little bit higher due to the usage as the parking in some cases, the designer
may design the live-load only for the load applied upon the slab. The slab can be design
as thin as 12 to 15 cm.(5-6 inches). But the elevation of -3.5 to -5 m.(15 feet) underground
with the hugh uplift pressure that can be as high as 5 tons/sq.m. the slab weight and
the live-load of,says, 400 kg/sq.m. seems to be negligible. In the city of Bangkok
with the flooding during the monsoon season, the underground water has a big effect
to the life of concret. The corrosion of concrete and steel can eat out the structure with
thin coverings of the concrete over the reinforcement. The covering designed as 4 cm.
is not enough. Also the accuracy of the workmanship have to be taken into account.
Another errors frequently occur is the misevaluation of the output from the computer.
For example, generally, the sign convention of the stress vary from one software to another.
The designers who use variation ones may find themselve confused whether should the compression
be positive or negative among the software they use to analyze when they are designing the
steel structures. If they interpret the compression as the tension, the problem may occur since
most of the members can resist high tensile force but very low when come into compressive force
and can buckle if the designers do not check the slenderness ratio of the members they are
investigating.
The basement slab.
One of the most common error done by the contractor during the construction
of the cantilever slab is the incorrect step of removal of the temporary
strut. We, the designer, usually design the cantilever slab only for the
negative moment that will occur after the concrete is harden and seldom
provide the bottom reinforcement for the slab since there is no need for this
kind of structure to resist any positive moment. The formwork required the
struts to bear the weight of the slab selfweight. After the slab is harden, some
constructors, without the analysis knowledge or careless may remove the
strut from inside out. When come to the last one outside, the slab already acts
as a prop-cantilever slab with the struts as another supports which have
the positive moment between the span length. If the designer didn't provide
some temperature steel in the bottom, the positive moment capacity can be
lower than the moment occurs and the failure may happen eventually. Notice also
that most of the time, the workers like to stock the garbage or broken bricks in
this area for the convenience of the removal, worsen the situation since
the increasing of the loading.
The building owners or the residents, without the conscious or lack of the knowledge, can be
the persons who have to take responsibility in the collapse of the structures. Wanting to
have more spaces or carry more load during the occupancy are the common cases of the situation.
For example, again, with the cantilever slab designed to use as a canopy or shading for the
floor below, the designed live-load can be only 150 kg/sq.m. or less since it's not designed
to resist any other load except some drainage water that remain after the rain. The occupants
may want more spaces and as a result, remove the wall outward and put the heavy weight such as
air-conditioner units or some machines on this area. The live-load can go beyond 300 kg/sq.m.
With the factor of safety of (1.7 for uncertainty of live-load and 0.9 of the material)
1.7/0.9=1.9, lower than the load increased (2 times of the design one).
