Fire
Protection and Fire Response of Structures
Fire protection of
structures is becoming an increasingly important aspect
of modern life. The public quite rightly expect public buildings and their place of work
to be designed to allow effective evacuation in the event of fire. Certain industrial
structures have wider fire protection requirements, oil and gas production and processing,
nuclear related product storage and processing, chemical process and storage and
transportation routes for product for example.
Fire protection
provision can be a emotional subject. From the point of view
of the safety and licensing authorities, the structure must be capable of safe evacuation
in the event of fire. From the point of view of the purchaser of the structure, the fire
protection is often an expensive statutory feature. From the point of view of the
operator, fire protection must be maintained to preserve the safety margins declared in
the safety documentation. On some structures, weight is critical, and fire protection
measures are often heavy. There is, therefore, a need to balance these conflicting
requirements when specifying fire protection.
Fire protection falls into two main categories, prevention and protection. Preventative measures will include control of flammable
inventories, control of ignition sources, monitoring of environmental conditions leading
to initiation of alarms and automatic process control, and fire detection systems designed
to extinguish fires immediately on detection.
Protection measures fall into two categories, passive and active. Passive
measures include fire barriers, fire resistant enclosures, fire doors, fire
retardant coatings and fire protective coatings. Active measures
include water and chemical sprays or deluges, foam dispersion and inert gas dispersal.
Fire protection can be provided as an all encompassing scheme, or it can be
functionally designed to optimise on cost, weight and maintenance. Work carried out
by CREA has shown that pressure vessels subjected to fire, but which are
successfully blown-down can resist the fire without rupture, without the application of
fire protection. Evidently the resulting vessel is not usable following the fire,
but there is no guarantee that a fire protected vessel could be re-used.
Engineers at CREA
Consultants have experience in the analysis of structures subjected to fire to
devise fire protection schemes. Using well defined techniques, the analysis takes a
definition of postulated fires and carries out a time domain thermal analysis of the
structure. This model will include the basic thermal transmission phenomena, radiation,
conduction and convection, the material temperature dependent properties of materials and
the location and nature of fire protection measures. The resulting thermal histories are
then applied to the structure to predict time to collapse, or to demonstrate the degree of
collapse. From this, the structure can be economically protected to meet the safety
requirements.
|
Explosion
Response of Structures
The assessment of the response of
structures to explosion is an increasingly important factor in design, particularly where
the storage and processing of explosive materials is concerned. Many structures are
required to be "blast resistant" to protect personnel and neighbouring plant,
and to reduce the possibility of escalation of events. These structures are
therefore designed to contain the effects of explosion or to act as a significant barrier.
Prediction of blast response is not a
new science, many hand and computational techniques have been developed with the aim of
predicting how a structure will respond.
Explosion
prediction is the task of defining the form of the
explosion to be resisted. There are many methods, for instance:
Cubbage and Marshall, a methodology
for predicting overpressures due to methane explosions;
Computational Fluid Dynamics based
approaches, programs such as FLACS, EXSIM and BRILLIANT can be used to calculate the
probable development of an explosion, leading to blast overpressures, durations and gas
velocities;
Phenomenological models, such as
CHAOS, developed by BG (British Gas) to study hydrocarbon explosions.
CREA Consultants
(CREA) provide services for
the prediction of explosion prediction.
Structural
Response to Explosion, this is the prediction of how a
structure will respond when subjected to loading from an explosion. Techniques for
the assessment of structures subjected to explosion overpressures have been developed, and
are being developed further. CREA are currently involved in research projects to further the
explosion response technologies.
Pseudo-dynamic or pseudo-static
methods. These are the simplest to apply since they take the explosion overpressure
as a blanket loading, and are usually combined with dynamic amplification factors.
The methods can assess both elastic and inelastic responses and can be applied to complex
structures.
Single Degree of Freedom Method,
this is a dynamic analysis technique, which predicts the response of a structure by
reducing the structure to a simplified spring/mass system. The method is effective
for simple structures that behave in a manner analogous to a spring/mass system. The
method can assess both linear and non-linear responses and with care can be applied to
more complex structures.
Finite Element Analysis. FEA
can be used effectively to solve explosion response problems, taking account of geometric
and material non-linearities. The method has to be applied with care, since there is
a fine line between a model which is detailed enough to predict the response, and coarse
enough not to run for impracticable lengths of time. Evidently, the increase in the
speed and capacity of computers is reducing the resource problems. The model detail
is a matter to be addressed by the analysts carrying out the work, as is the choice of
solution method. The two most common time domain solutions, implicit integration and
explicit integration can both be used, each having their own pros and cons.
CREA can provide FEA based solutions using both ANSYS and DYNA-3D.
We have experience in the assessment of structures subjected to explosion loading
using both hand and FEA based methods. Combining the structural response analysis
with the explosion prediction analysis it is possible to optimise structural resistance.
Research work by CREA has shown
that blast walls designed elastically can be demonstrated to resist 2.5 to 3 times their
design load when assessed in a non-linear dynamic fashion.
|
