Step 0. Considerations before carrying out a probabilistic time equivalence assessment

Step 0. Considerations before carrying out a probabilistic time-equivalence assessment

Reasons of time-equivalent probability reliability assessment

Intention

time equivalence probabilistic reliability assessment (TEQPRA or known as PRA to many) is one of the theoretical fundations to structural fire resistance (B3) recommendations in many design guidences in the UK (e.g. Approved Document B [1] and BS 9999 [2]). The fire resistance requirements that are given in the guidence are based on results of a generallised compartment and associated (also generallised) fire model parameters. This generalisation resulted in a simple-to-use fire resistance table which indicates a 'required fire resistance rating' for given building height and occupancy characteristics. Inavoidably, the base model that used for the study [3] that underpins the recommendations in the guidence imposes certain design limitations (i.e. building height, compartment size etc) which are not directly obvious to end users.

As such, additional fire engineering assessment is necessary for buildings with higher consequence class (greater than X as defined in [ref X]).

This note is intended to assist one to identify the value/benefit for carrying out a TEQPRA assessment for a specific project or building.

Process overview

The process to determine whether a TEQPRA assessment is necessary/benefitial brokes into the following three steps:

1. Check if additional fire safety engineering provisions are required for B3 other than following prescriptive guildence? This is normally defined in the guidence.

   Section 0.3 in BS 9999:2017 noted the following:

   "However, the increased design demands on structural integrity, services, fire safety systems, means of fire-fighting and evacuation generated by buildings in excess of 50 m high might mean that specific evaluation of all fire safety provisions is needed using a qualitative design review in accordance with BS 7974."

   PD 7974-6 [pd7974-6]

2. Is time equivalence hand calculation method adequate to address the above recommendation? A hand calculation method is available in PD 6688-1-2 [pd6688] (originated from EC 1991-1-2). Similar to the fire resistance table, this hand calculation method is derived based on analysis of generallised fire and compartment parameters. Therefore, limitations detailed in PD 6688 and the parametric fire model should be followed.

3. If hand calculation method is not adequate (i.e. given building is outside of the limitations) then a detailed time equivalence assessment is necesary.

Considerations

• 1.1 Building height.
  • is the building greater than 50 m? BS 9999, ADB and other guidances that share similar theoretical basis are not designed for high rise buildings over 50 m. These buildings will need to be engineered to make sure fire safety is appropriately applied. In this instance, PRA TEQ takes care of B3 to ensure appropriate level of structural stability (that related to fire) is maintained.
• 1.2 Building use and active fire suppression systems.
  • Residential use. Is a BS 9251 sprinkler system provided in residential areas?
  • Commerical use. Is a BS 12845 sprinkler system provided in commercial areas?
• 2.1 Compartment geometry.
  • is the biggest compartment over 500 m²? This is the upper limit of compartment size defined in PD 1991-1-2. Note BS 6688-1-2 explicitly removed this limitation without justification.
  • is the opening factor, $O$, within range 0.01 to 0.2 m$^{0.5}$?
    • $O=\frac{A_v\cdot \sqrt{h_v}}{A_t}$, where:
      • $A_v$ is the total ventilation opening area.
      • $h_v$ is the averaged height of all ventilation opening area.
      • $A_t$ is the total surface area of the compartment, inclusive of opening areas.
  • is the opening coefficient, $\alpha_v$ within 0.25 and 0.025? $\alpha_v=\frac{A_v}{A_t}$.

[1] adb [2] bs9999 [3] kirby [4] consequence class [pd7974-6] [6688]

todos

• what is in regularion 2?