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Determining Baseline Performance and the Application to Performance Based
Design
Simon Goodhead, Eddie Farrow
JENSEN HUGHES, Atlanta, Georgia 30305, USA
SUMMARY
Performance based design is a critical tool developed by designers to enhance a buildings’
performance where prescriptive based codes may result in design restraints. This paper serves to
outline instances where a ‘code based design’ has been analyzed in order to set a baseline
performance standard. In each instance this baseline performance standard was then compared
directly to the ‘performance based design’ in order to identify and quantify the performance
benefits.
Comparing results between a ‘code based’ and ‘performance based’ not only determines the
baseline performance criteria, but also provides both the designer and approver with a direct
comparison between both designs using a set series of variables. This method of comparison has
been key in gaining approval for complex designs in Southern USA states, where performance
based design is not widely practiced, or other Fire Protection Engineers are acting as the Approving
Authority.
INTRODUCTION
Within some USA States and other regions where performance based design is not widely
practiced, approaches and negotiations with Approving Authorities are crucial when a building
may not meet the prescriptive local building or fire code requirements. As is recommended in the
SFPE Engineering Guide to Performance-Based Fire Protection Analysis and Design of
Buildings (SFPE Engineering Guide) i, initial discussions with Approving Authorities before
commencing any analysis is vital in identifying concerns and questions which may need to be
answered initially, or addressed as part of overall performance based study, and gaining
agreement of the conceptual approach, and standards for pass / fail criteria. Following the initial
meeting, developing and transmitting the design brief reiterates the discussed topics and
solidifies the pass / fail criteria. The design brief also provides the opportunity for outlining the
applicable code routes that would permit the use of a performance based design in the applicable
jurisdiction. Outlining why and how the approving authority can agree to a performance based
design often ensures, particularly where performance based design is less well known, that the
authority has comfort in their ultimate decision to allow the process to move forward.
Following the completion of the agreed performance based analysis method, clear presentation
of all results and identifying variation from the previously agreed document is fundamental in
presenting results to the Approving Authorities. As the design develops through the iterative
process of analysis, the initial assumptions and provisions may need modification – highlighting
these changes and outlining the reasons for the change helps to ensure the designer’s integrity is
maintained.
DETERMINING BASELINE PERFORMANCE CRITERIA
The baseline performance criteria that the code achieves has to be determined in a numerical
fashion. Were the determination to be qualitative, demonstrating an equivalent or higher level of
safety would be subjective. Through determining the baseline code performance criteria
numerically, the designer and approver gain an understanding of what results a code complaint
design would yield. The code compliant results are subsequently recorded, and directly
compared with the performance based design method. The methods of determining the
performance must therefore be based on the applicable measurement; time, temperature, heat
flux, etc. The following case studies outline how the baseline code performance was established
and set as the standard by which the performance design would be judged.
CASE STUDY #1 – ALABAMA, USA – UNIVERSITY CLASSROOM BUILDING
An existing library building in Alabama, USA, proposed to install a new atrium classroom
addition to the rear of the existing library building. As the existing building was unsprinklered,
and the new addition was to be sprinklered, two fire areas would be created, requiring a 2-hour
fire barrier separation. Given the existing condition of the library façade, the preference was to
leave the existing building untouched. To determine whether the new addition would require
construction changes, a comparative analysis was undertaken to study the impact of a fire on the
existing (previously exterior) non-fire rated wall located between the existing library space, and
the proposed new atrium classroom addition. The physical construction of the existing wall
would provide fire resistance rating between fire areas; however the existing glazed elements
were unrated. The local code required the fire area separation wall between each of the spaces to
be fire rated, with opening protective within the wall being not greater than 25% of the wall. The
existing wall openings were exceeding 25%, and the opening protective requirement was not
being met. A comparative CFD analysis was undertaken to determine the differences between
code requirements of limiting the glazing to opening protective for 25% of the wall area, and the
proposed conditions of leaving the existing conditions “as-is”.
A clear intent was identified and agreed with the Approving Authorities in Alabama as part of
the performance design brief:
1. Means of Egress is not adversely affected by untenable fire and smoke conditions within
the atrium addition.
2. Openings in the external wall between the existing library and the new atrium addition
will not compromise the means of egress.
3. The smoke exhaust system in the new atrium addition would allow for fire from the
library to not impact the new structure (primarily for temperature) beyond what may
occur from the new addition.
4. The peak received radiant heat flux and peak temperature by the new structure with the
glazing failure as non-rated does not exceed the code compliant approach.
Figure 1 - Geometry of the building as simulated within the CFD Model
Following the completion of both scenarios, the results were presented within a final report to the
key stakeholders including Approving Authorities where a model duration of 2-hours was
presented.
Figure 2 - example of final results presentation
Following the agreement of the performance based design approach, and the presentation of the
final results to the AHJ, the alternative method was approved resulting in a design which was
cost-effective, fitted with original architectural design aspiration, and was shown to be an
improvement against the performance of the code.
Location of wall with
unprotected openings.
CASE STUDY #2 – SOUTH CAROLINA, USA – ASSEMBLY BUILDING
A building owner in South Carolina proposed a two-story addition (Figure 4) to an existing
facility which was primarily classified as an A-3 (Assembly) occupancy group. This existing
facility was originally constructed as a Type IIB (unprotected, non-combustible construction
type), one-story, unlimited area building of Group A-3 occupancy in accordance with the local
fire code. Based upon the requirements, the local code did not permit a two story building to
remain a Type IIB unlimited area building of Group A-3 occupancy. This was problematic for
the client, as a tight building schedule and arduous upgrades to the existing building would result
in costs exceeding the clients initial build budget.
In order to perform a comparative analysis between a one-story, prescriptive building and a two-
story, performance based design building, an equivalent design was considered. The second floor
of the assembly building was appended to the first floor to create a one-story, unlimited area,
Group A-3 occupancy version of the building. This single story version (Figure 3). of the
building would be permitted as a Type IIB unlimited area buildings.
Consideration was then given to what parameters in the South Carolina Building Code
prescribed the fire resistance rating for assembly buildings. The parties agreed that occupant
egress and fire department access were the two primary life safety risks considered when the
unlimited are provisions were outlined in the code.
The single story comparison design approach was therefore used to determine the baseline
performance criteria for egress that would be afforded by a code based design, compared to that
afforded by a two story building with the same gross area and the same occupant loading.
Both building layouts were studied to include an egress analysis and Fire Service access
provisions with the aim to demonstrate that the two story building configuration operated to a
better standard that the single story. Cost effective life safety enhancements would be offered
within the two story configuration where these provided significant benefits to egress times, and
Fire Service access, without upgrading the buildings’ structural fire resistance.
A timed egress analysis was therefore conducted through the use of egress modelling software to
gain an understanding of egress times associated each layout. The occupant characteristics for
the population using the building were then gathered through field observations. The occupant
characteristics provide calibration data for the egress models, and when compared to the default
settings for the selected egress model, yielded a 60% increase in movement time – thereby
demonstrating the importance for calibration studies when using egress models in a comparative
analysis.
Figure 3 - Code compliant assembly building layout (single level approach)
Figure 4 - Level 1 and Level 2, assembly building (performance based design layout)
The results of both scenarios were analyzed for multiple occupancy configurations to determine
both the baseline egress time, and a comparing time for the performance based design layout.
Where differences were observed between the two, enhancements were made to the performance
based design layout in order to improve exit widths and detection times in-order to reduce egress
times. The performance based design model was subsequently altered to include these
enhancements, then simulated a second time in-order to compare results – the performance based
design method subsequently resulted in improved egress times in comparison to the code based
design approach.
The design enhancements were primarily focused upon improving the egress times associated
with the 2-storey facility, and improving Fire Department operations for responding personnel.
The enhancements included: incipient smoke detection within selected areas, Class 1 Standpipe
connections, and full perimeter access for fire department vehicles.
Upon applying egress improvements to the performance based approach, the results were
compared against the code compliant scenario:
Figure 5 - Comparative analysis, results – RSET column stating egress times.
In completing the analysis, compensatory improvements were applied to the performance based
design approach which were focused upon means of egress and firefighting access
improvements, saving the client major expenses on improving the structure of the existing
building. Approval was gained from the State Engineers Office and the owner.
CASE STUDY #3 – GEORGIA, USA – INCREASED OCCUPANCY OFFICE
Ever increasingly, existing office building tenant improvements aim to increase the number of
occupants within their leased space from individual enclosed offices to open-plan spaces. This
offers the leaseholders reduced costs, and increased collaboration between their workforces.
However, this approach also often creates a means of egress concern, where the increased
population may not be accommodated by the existing stair infrastructure.
In one such instance a tenant in an existing 24-story building with two stair enclosures wanted to
increase the number of occupants on Level 17 to 344 people. Subsequently, it was determined
that the stairs were the limiting factor, regulating the occupant load to 292 occupants on Level
17.
A comparative analysis was therefore conducted to compare two egress scenarios within the
building. The first egress scenario examined the time taken for 292 occupants to egress from
Level 17 using the minimum code required fire safety features and alarm initiation (specifically
the fire alarm water flow switch). The second egress scenario studied the time taken for 344
occupants to egress from Level 17 when egress is initiated by spot-type smoke detectors.
The first scenario (baseline code performance) assumed initiation of the building fire alarm
based upon receipt of alarm from the Level 17 waterflow switch following sprinkler activation
(manual fire alarm ignition was omitted). Occupants were considered to be equally distributed,
including traveling from the furthest point on the floor plate, at a constant speed. The time from
fire initiation, activation of the alarm, movement to the Exit and flow rate into the Exit
determined the baseline code minimum performance for the Level 17 egress time.
Scenario 2 (performance based design) assumed initiation of the building fire alarm system
based upon the receipt of an alarm from a spot type smoke detector within the same floor. The
spot type smoke detection method exceeds the minimum prescriptive ‘code based design’
requirements for the building.
This comparative analysis determined that earlier detection of the fire via spot type smoke
detectors in the incipient stage will result in faster detection as compared to activation based on
sprinklers at the maximum spacing, and therefore provides additional time for occupants to
egress. The additional time available for occupants to egress therefore permits more occupants to
egress in the same time period as would otherwise be available in a code compliant occupant
load (Scenario 1).
An alarm time sequence comparing both scenarios was conducted, and presented in a tabular
format within a final report to key stakeholders include the Approving Authorities:
Figure 6 - Alarm initiation comparison
A table comparing both times egress results was presented in a tabular format within a final
report to key stakeholders include the Approving Authorities:
Figure 7 - Timed egress results comparing a 'code' and 'performance' based design.
The final results detailed an improved condition (8.08 vs. 10.21 minutes) where spot detectors
were installed throughout Level 17 as a compensatory feature improving means of egress times.
CASE STUDY #4 – BAKU, AZERBAIJAN – PARTIALLY CONSTRUCTED BUSINESS
BUILDING
An owner of a partially constructed building located in Azerbaijan requested assistance on a code
deficiency. Exit stairs within this office building were not adequately separated as per the local
fire code requirements. This deficiency was even more problematic to the client where the building
was partially constructed with major structural elements already cast in concrete.
An approach to this problem was developed to study the effect of the ‘as-built – performance based
design’ compared with a ‘code compliant’ design which should have been built.
Each scenario consisted of two fire models with each model simulating an identical fire. The
focus of the study was the configuration of the exits, and in particular the deficient distance
separation between the exits. The ‘as-built’ condition (Figure 9, Analysis A) represented a layout
where the exits were not separated by the required distance factor, and the ‘code-compliant’
condition (Figure 10, Analysis B) represented a scenario where the exits were separated by the
required distance factor.
Figure 8 - As built scenario where stairs are not adequately separated in line with the adopted fire code requirements.
Figure 9 - Code compliant scenario where stairs are separated adequately in line with the adopted code requirements.
Following the completion of the analysis, the results studying tenability criteria (temperature,
visibility, radiated heat flux and carbon monoxide) within each of the office floor scenarios were
gathered and presented clearly within a results table within the final report:
Figure 10 – Comparative Analysis Final Results as presented within the final report.
It was observed that all code compliant scenarios had failures occurring at a time prior to their
as-built conditions. The aim of the analysis was not to determine whether failure criteria were
reached, but rather which scenario failed first. The as-built design maintained tenable conditions
longer than a prescriptive layout would have achieved within the occupied spaces (due to the
stair and lift core being of a smaller size and therefore taking up less volume within the space).
In addition, the as-built condition was shown to be better than the same core arrangement with a
smaller occupied space. The single point of failure of the elevator lobby was also studied, and in
this instance, the approving body was a peer Fire Protection Engineering Firm who worked with
the engineers throughout the analysis period as advocated by this paper.
DISCUSSION AND CONCLUSION
Performance based methods can offer significant benefits, primarily in the instance where an
existing building is undergoing significant alternations, and retrofitting prescriptive requirements
may be inefficient from a constructability or financial position.
Setting an end goal and expectations with the key stakeholders is also very important. Initial
discussions with the design team and AHJ to understand their initial concerns, and suggesting a
route forward with possible compensatory features can help to guide the process from an early
stage setting expectations later on in the timeline.
The next key step is the development of the fire protection engineering design brief. This brief
should reflect the prior discussions with all key stakeholders. In order for the performance design
analysis to be successful, all stakeholders should have prior buy-in and agreement with the
written brief. Further, the benchmarked performance should be numerical to avoid a subjective
argument of betterment.
A pragmatic data based approach, coupled with clear and continuous communication from the
outset, and using standards based agreements for pass / fail criteria achieves a successful
approach using performance based design. When utilizing these approaches, regions where
performance based designs had not previously been approved were able to accept the concept
and approve advanced design methods. Discussing the methodology, allowing input, and
presenting results clearly will allow all stakeholders to understand the method of analysis, and
clearly observe the design improvement over the prescriptive approach.
Total Egress Time (s) 166 166 196 196 758 758 178 178
Sustained visibility failure at time (s) 10m. 270 255 240 230 80 75 82 70 to 75
Tenability Failure at time (s) 270 255 240 230 80 75 82 70
Pass / FailPass - better
conditions than 1B
Pass - better
conditions than 2B
Pass - better
conditions than 3B
Pass - better
conditions than 4B
Scenario 1A 'as-
built'
Scenario 1B 'code-
compliant'
Scenario 2A 'as-
built'
Scenario 2B 'code-
compliant'
Scenario 3A 'as-
built'
Sceanrio 3B
'code-compliant'
Scenario 4A 'as-
built'
Scenario 4B 'code-
compliant'
Sustained temperature failure at time (s) 60°C.does not occur 1200 540 510 155 135
145 130
Sustained radiation failure at time (s) 2.5kW/m2. does not occur does not occur does not occur does not occur does not occur
Intermittent CO failure (s) 800ppm does not occur does not occur does not occur does not occur does not occur does not occur does not occur does not occur
does not occur does not occur does not occur
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