34 Damage to concrete structures
the tricells had to resist a substantial hydrostatic pressure. The loss of the
structure was attributed to the failure of the wall of one of these tricells”.
Reading this conclusion, one might think that the designers made a substantial error. Hydrostatic pressure is a well-known loading case, and the
design of a structural element withstanding this pressure should have been
straightforward. Or not? One interesting consideration related to the specific type of concrete structure considered here, is formulated by Collins and
Vecchio as follows: “However, unlike the situation for a typical land-based
structure, the designer does not have the option of greatly increasing the wall
thickness to ensure a very conservative design. If the walls are too thick, the
structure will not float, or will not be hydrostatically stable during the tow
to the field. These severe constraints mean that for these weight-sensitive
structures rather low factors of safety are employed. As a consequence,
great care is required in all aspects of design and construction”.
This last sentence might really point to the main issue concerning the
design of the Sleipner A platform. As previously mentioned in this chapter, additional control can be very helpful to avoid many human errors in
structural design. Finite element simulations with commercially available
software were performed in order to calculate internal forces and stresses
within the cell walls, including the tricells. As mentioned by Collins and
Vecchio, “The tricell wall that failed did not contain stirrups because the
global finite element analysis performed as part of the design seriously
underestimated the magnitude of the shear at the ends of the walls, while
the sectional design procedures used seriously overestimated the beneficial
effects of axial compression on the shear strength of the wall”.
It is always easy to give comments afterwards, once the failure has occurred.
Nevertheless, Collins and Vecchio are very clear when legitimately formulating one of their general recommendations, as one of the lessons learnt from
this remarkable failure: “… no matter how complex the structure or how
sophisticated the computer software, it is always possible to obtain most
of the important design parameters by relatively simple hand calculations.
Such calculations should always be done, both to check the computer results
and to improve the engineers’ understanding of the critical design issues”.
It is worthwhile mentioning that afterwards, refined finite element analysis has been capable of predicting the water depth at which failure of the
Sleipner A platform occurred. This kind of sophisticated computer model
is very valuable indeed, but common engineering sense is always needed to
verify orders of magnitude.
2.2.3 collapse of a 13-storey apartment
building in shanghai, china 2009
On June 27, 2009, a nearly finished 13-storey apartment building in Shanghai
just tumbled over, killing one worker (see Figure 2.5). As in many collapses,
the tricells had to resist a substantial hydrostatic pressure. The loss of the
structure was attributed to the failure of the wall of one of these tricells”.
Reading this conclusion, one might think that the designers made a substantial error. Hydrostatic pressure is a well-known loading case, and the
design of a structural element withstanding this pressure should have been
straightforward. Or not? One interesting consideration related to the specific type of concrete structure considered here, is formulated by Collins and
Vecchio as follows: “However, unlike the situation for a typical land-based
structure, the designer does not have the option of greatly increasing the wall
thickness to ensure a very conservative design. If the walls are too thick, the
structure will not float, or will not be hydrostatically stable during the tow
to the field. These severe constraints mean that for these weight-sensitive
structures rather low factors of safety are employed. As a consequence,
great care is required in all aspects of design and construction”.
This last sentence might really point to the main issue concerning the
design of the Sleipner A platform. As previously mentioned in this chapter, additional control can be very helpful to avoid many human errors in
structural design. Finite element simulations with commercially available
software were performed in order to calculate internal forces and stresses
within the cell walls, including the tricells. As mentioned by Collins and
Vecchio, “The tricell wall that failed did not contain stirrups because the
global finite element analysis performed as part of the design seriously
underestimated the magnitude of the shear at the ends of the walls, while
the sectional design procedures used seriously overestimated the beneficial
effects of axial compression on the shear strength of the wall”.
It is always easy to give comments afterwards, once the failure has occurred.
Nevertheless, Collins and Vecchio are very clear when legitimately formulating one of their general recommendations, as one of the lessons learnt from
this remarkable failure: “… no matter how complex the structure or how
sophisticated the computer software, it is always possible to obtain most
of the important design parameters by relatively simple hand calculations.
Such calculations should always be done, both to check the computer results
and to improve the engineers’ understanding of the critical design issues”.
It is worthwhile mentioning that afterwards, refined finite element analysis has been capable of predicting the water depth at which failure of the
Sleipner A platform occurred. This kind of sophisticated computer model
is very valuable indeed, but common engineering sense is always needed to
verify orders of magnitude.
2.2.3 collapse of a 13-storey apartment
building in shanghai, china 2009
On June 27, 2009, a nearly finished 13-storey apartment building in Shanghai
just tumbled over, killing one worker (see Figure 2.5). As in many collapses,