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

i Chapter 6, Defining Stakeholder and Design Objectives, Performance-Based Analysis and Design, SFPE

Engineering Guide to Performance-Based Fire Protection, Analysis and Design of Buildings, National Fire

Protection Association, 2000, ISBN: 0-87765-422-0