Desk Study – High Rise
Office Building in London
|Buro Happold, 2, Brewery Place, Brewery Wharf,
Leeds LS10 1NE
2042200 Fax: 0870
Buro Happold FEDRA recently completed a desk
study for a proposed high rise building in London. An integrated
approach was adopted that considers the interaction between means
of escape, structural performance in fire and fire fighting operations.
Some detailed studies on evacuation and structural fire performance
were conducted, which is described in the following sections.
Defining Performance Goal
The building is primarily used as an office
space and the tallest block consists of 20 storeys. Having considered
the building occupancy profile, location and its significance
in the city, the design team, together with the client agreed
that the building does not need to perform to a higher standard
than that indicated within the National Building Regulations.
Extensive discussions with Building Control were held prior to
the commencement of the design in order to ensure the correct
checking capability and understanding of the design approach
for the building.
Figure 1: Interaction between Structural Fire Performance
and Fire Strategy ©FEDRA
The main performance goal is to ensure that adequate performance of structural
fire behaviour so that:
- Means of escape is not affected by smoke and/or heat from the fire;
- The structure survives the burnout of a realistic, worst case fire
and significance collapse is prevented;
- The fire is contained within the compartment of its origin; and
- Fire fighting intervention can be carried out safely.
The exits and stairs were sized on the assumption
of a phased evacuation. The number and locations were carefully
considered so that the local evacuation time is sufficiently
low and the travel distances are comparable to prescriptive guidance.
The stair locations are distributed diversely and apart from
each other to prevent all escape routes being compromised by
one incident. In terms of global evacuation, the escape routes
were carefully considered and an evacuation time of 58 minutes
was calculated. This was introduced to the fire time line. (See
Structural Fire Performance
The superstructure would be constructed of
a steel frame with composite floor slab. The building façade
would be glazed with non-fire resistance glazing. Detailed studies
were conducted to investigate the two separate parts of the design:
The prediction of the maximum fire
temperatures and duration using the concept of natural
fire, which considers the behaviour of real fire. It takes
into account the fire load density, ventilation, compartment
geometry and properties of enclosure. When considering
the fire scenarios, an realistic office fire load density
of office was adopted as the base value. Even though an
automatic sprinkler system would be installed, its operation
was not account for when determining the maximum fire size.
It was found that ventilation is the dominating factor
when assessing the natural fire curves. An initial study
on the behaviour of the glazed façade under fire
condition was conducted. It was demonstrated that the glazing
would have failed much earlier than the structural steel
elements and therefore additional ventilation can be introduced
as a result. Further consideration was also given to possible
inner rooms within the office which prevent substantial
ventilation being introduced. The fire temperatures and
duration can be calculated for the entire fire period for
each ventilation case. A typical time temperature history
plot can be found in Fig. 5.
study was also conducted varying parameters such as the
fire load density, wall material and fire compartment geometry.
The adoption of natural fire temperatures is a departure
from conventional design but are considered more realistic
in the performance based design compared to a BS476 Standard
Fire Curve. More importantly, it demonstrates that the
building is being able to survive a full burnout of its
contents without collapse. The real fire temperature curves
form the basis of the time line plot.
The 3-dimensional structural fire performance
was studied using a finite element program Vulcan developed
at the University of Sheffield. It is a non-linear finite
element program which has been validated against results
from large scale fire tests. Initial studies were conducted
on small subframe utilising symmetry and Fig. 2 shows the
deflected shape of the subframe at various heating stages.
For the purpose of this study, the subframe analysis was
heated to a BS476 Standard Fire Curve to enable appropriate
comparison to be made in the sensitivity study conducted
at a later stage.
|Figure 2: Vulcan Model:
Small Subframe Analysis after 30 Minutes (Left) and 120
Minutes (Right) ©FEDRA
The results from one of the analyses are plotted
in Fig. 3 showing the vertical deflections and internal forces
in accordance to BS476 standard fire curve heating. The analysis
assumes all the structural steel elements are fully protected
except the intermediate beams (beams not connected to columns).
Such a protection profile is not new and the Steel Construction
Institute and BRE have published relevant documents on the
concept 1. This is enabled by the enhanced composite slab performance
in fire primarily due to membrane action.
The results of the subframe analysis show that some membrane action formed
in the early stage when the slab is heated and a compression ring formed
around the protected primary beams. Due to the relatively large aspect
ratio of the assumed grid, the slab enhancement delivered the membrane
action is limited and the unprotected intermediate beams are in tension
as early as the 32nd minute. The primary beams remain in compression until
the 90th minute where the majority of the beams are in tension and supported
by catenary action. The catenary forces in the protected primary beams
peak at approximately 120th minute.
The main objective of the analysis was to
investigate the optimum fire protection profile to achieve
the performance criteria set out. Sensitivity studies were
conducted using the small subframes, varying the protection
thickness, heating profile and sizes of structural elements
(reinforcement mesh, concrete grade, beam sizes etc.). Once
the preferred solutions were identified, a full-frame model
was created comprising a quarter of the entire floor slab.
The deflected shape of the model produced is shown in Fig.
4. The displacements and forces were studied carefully to ensure
the performance criteria were achieved. For example, the vertical
displacements of the beam and slab were checked against their
limit particularly where vertical compartmentation exists.
Using this approach ensures the overall structural stability
including the interactions between beams, columns and their
connections. For instance the additional catenary action experienced
by the primary beams in fire was checked against the column
strength the connection design so that there is sufficient
strength and ductility for energy absorption thereby reducing
the risk of progressive collapse.
Figuer 4: Vulcan Model – (top) widespread fire across
the entire floor, (bottom) small compartment fire ©FEDRA
As a result of the studies, a best value solution was reached
by increasing the performance of the slab in exchange for the
omission of fire protection for the intermediate beams. Where
passive fire protection is required, the material and thicknesses
were carefully selected to give the required performance in
fire, with sufficient robustness over time in service.
The solution increases the inherent fire resistance of the superstructure,
relies less on the passive fire protection, and therefore is less susceptible
to damage or failure (due to less maintenance requirement). In other words,
it is a more robust solution. The additional benefit is the saving generated
from the omission of passive fire protection and the associated time savings
in construction stage.
Fig. 5 shows one of the integrated plots that considers the
elements discussed above. Three key events are identified that
represent three different scenarios and their consequences.
Figuer 5: Performance based approach using time line ©FEDRA
The first scenarios
assumes the automatic sprinklers are fully operational
and the fire size is successfully controlled. As a result,
flashover does not take place and structural deformation
is not incurred.
This scenario assumes the automatic
sprinkler system fails and a fully developed fire takes
place. The fire temperature curve shown in Fig. 5 was
calculated based on a typical ventilation condition.
The maximum fire temperature achieved is approximately
1300°C and the heating phase lasts for 50 minutes.
The fire will eventually burnout and the vertical displacement
for the beams at the time would be approximately 350mm.
These deflections are not shown in the plot.
Further analyses were also conducted
using the BS476 Standard Fire Curve. This was to provide
additional comfort in the design and so that comparisons
could be drawn with a solution that would be derived
using a more traditional prescriptive method. The predicted
beam displacements shown in Fig. 5 demonstrate a fire
resistance period of 130 minutes. This can also be seen
as a factor of safety against the unlikely event where
the heating phase continues
Future Design Improvement
Although the design approach adopted for
the building comprises a considerable amount of engineering
assessment utlising some of the latest technology and research
knowledge, there is room for future improvement. Some thoughts
are discussed below.
- Combined hazard – In tall buildings, it may
be appropriate to account for combined hazards such as
the occurrence of fire after explosion.
- Evacuation - the modelling of evacuation assumes occupants
behave rationally in the event of fire and follow the
evacuation procedure designed for the building. A better
understanding of human behaviour would benefit the model.
The current strategy also disregards the use of elevators
for means of escape.
- Risk based approach - Introduction of the risk based
approach in parallel with structural fire engineering
may enable of engineers to produce better value solutions.
For example, the effect of automatic sprinkler systems
may be included in the structural fire assessment in
- Material properties at extreme temperatures – From
the calculation of fire temperatures, maximum atmosphere
temperature can reach up to 1300°C. There may be
a need to investigate the performance of material at
the extreme temperatures.
Conclusion and Important
Fire strategies for large buildings
cannot be derived in isolation. Instead, the design for
fire should be an integral part of the design process
and the fire strategy, structural, and environmental
designs should be considered simultaneously. This is
a more robust approach to ensure adequate building performance
The only way to ensure a fully integrated
solution for fire is to measure the building performance
Redundancy and/or diversity are vital
in the design of large buildings. This includes active
and passive fire protection systems, which should be
designed to withstand the exposure conditions to which
they may be subjected.
Research and testing have brought
performance-based structural fire engineering to the
The assumptions behind any calculation
procedure should be understood, justified and checked
and any computer programs should be well validated.