Construction Warehouse

Author

Ben Johnson, California Polytechnic State University - San Luis ObispoFollow

Date

6-2014

Degree Name

MS in Fire Protection Engineering

College

College of Engineering

Advisor

Frederick Mowrer and Christopher Pascual

Abstract

The purpose of this report is to provide a comprehensive evaluation of the fire protection and life safety design of a warehouse building that serves a large construction project. The results and conclusions from this evaluation are documented herein and supported by analysis when necessary. The gross floor area per story of the construction warehouse is 39,000 ft2. This building is used as a warehousing facility in support of a large construction project. Rack storage of Group A plastics is permitted in this facility to a height of 20 ft. beneath a 24 ft. ceiling on the main floor. Shelf and solid pile storage of Group A plastics is permitted on the second floor of this facility to a height of 10 ft. It is also used for office space in large open and enclosed areas for the operations and maintenance personnel. It is classified as a mixed occupancy used for moderate storage (S-1) and business (B) purposes. The occupant load calculated for this building is 466 occupants. A walled in mezzanine area, used for business (office) purposes, above the main floor office area is present in this building. The gross floor area of the main floor office and mezzanine office areas is 4,100 ft2 each. The structure was constructed in 2002 in accordance with the Uniform Building Code, 1997 Edition, and is a Type II-N structure. The building is evaluated as a Type II-B structure according to the International Building Code, 2009 Edition. The building is protected throughout by an automatic fire sprinkler system per NFPA 13, 1999 Edition, and a fire alarm system per NFPA 72, 2002 Edition. Life safety for this building is provided according to NFPA 101, 1997 Edition. The installed fire protection and life safety (egress) systems in this facility are evaluated to NFPA 13, 2010 Edition , NFPA 72, 2010 Edition, and NFPA 101, 2012 Edition. Main storage (rack storage) floor is protected by an automatic, supervised, Early Suppression Fast Response (ESFR) fire sprinkler system. The second floor is protected by a fire sprinkler system for protection of storage using Control Mode Density / Area (CMDA) design methods. The balance of protected areas, including office areas, loading dock, and tool room areas, are protected as an Ordinary Hazard Group 2 classification. The overall assessment concludes that the automatic fire sprinkler systems installed in this building meet NFPA standards for the hazard that requires protection. Any design differences noted between the 1999 Edition and 2010 Edition of NFPA 13 will be discussed with the building owner. The entire facility is monitored and protected by an automatic fire alarm system that serves as a protected premises fire alarm system and a building fire alarm system. For performance based analysis purposes, notification of building occupants is assumed to occur upon audible or visual signals from the building fire alarm system. Initiating devices include manual pull stations at each exit. Smoke detectors (for elevator recall) are located near each elevator door, in the elevator machinery room and at the top of the elevator hoist way. Duct smoke detectors are placed to shutdown air handling units upon detection of smoke in air supply plenums. One fixed temperature detector is installed at the top of the elevator hoist way, and another is installed in the elevator machinery room, to support the elevator shunt trip function. The fire alarm system automatically reports fire alarm, supervisory, and system trouble signals to the fire department via a high frequency radio transmitter. Overall, the construction warehouse building is in compliance with NFPA 72, 2010 Edition. Any minor deficiencies noted will be discussed with the building owner. Four (4) exits are provided on the main floor of the warehouse that discharge directly outdoors. Additionally, four (4) exterior stairwells, each accessible from both the main and second floor of this facility, are provided. The west stairwell provides direct access to an exit from the walled in mezzanine office level. Each stairwell is separated from the building by 2 hour fire-resistive construction and each stairwell discharges outdoors at grade level. Storage areas within this building are characterized as large, open areas. The maximum travel distance to any exit (or protected exit stairwell enclosure) is less than 300 ft. There is one dead end corridor in the warehouse that extends 49 ft. Common path of travel is limited to the distance required to traverse a typical office and no common path of travel exceeds the 100 ft. limitation for an S-1 or B classified occupancy. The construction warehouse meets prescriptive NFPA 101, 2012 Edition, requirements for egress. This evaluation documents a performance based life safety analysis. Two (2) fire scenarios are considered: · 1) A rack storage fire on the main floor involving unexpanded polystyrene cups contained in cardboard cartons, placed in a 4 by 4 by 4 fuel array (stacked four (4) tiers high). · 2) An office workstation fire on the mezzanine level involving a typical office cubicle Criteria for the performance based analyses was taken from NFPA 101, 2012 Edition, Section 5.2.2: Performance Criterion, "Any occupant who is not intimate with ignition shall not be exposed to instantaneous or cumulative untenable conditions." This performance criterion is met when the calculated available safe egress time (ASET) exceeds the calculated required safe egress time (RSET) (i.e., ASET > RSET). The calculation of RSET assumes that: · 1) Detection time is dependent on the specific fire scenario, but that detection occurs upon first sprinkler or smoke detector actuation (detection by human senses is not assumed) · 2) Notification time is 30 seconds based on fire alarm system electronic processing time · 3) Pre-movement time is assumed to be 60 seconds for some occupants · 4) Evacuation time is determined based on Pathfinder egress modeling software in STEERING mode which is shown to be comparable to independent hand calculations for personnel on the second floor to enter the protective enclosure provided by the west stairwell (the west stairwell is the most limiting egress path) – is calculated to be 158 seconds. Tenability limits are established according to external references. For this analysis, tenability limits for visibility, temperature, and carbon monoxide dose are established and documented by this report. A 10 m visibility limit is established for large open areas within this building such as storage areas. For office areas, where occupants are familiar with the location of exits and large travel distances are not required, the visibility limit is established at 3 m. A 100°C temperature limit is established and carbon monoxide dose, measured in terms of Fractional Effective Dose (FED) of 0.3 is also established. The visibility and temperature limits are evaluated at an elevation of 6 ft. (1.82 m) above walking surfaces. The FED is calculated and increases with time and concentration of carbon monoxide by the fire. The concentration of carbon monoxide is determined based on a ratio of the mass of carbon monoxide released into the affected building volume divided by the mass of all gases and products of combustion in the same volume. Appropriate factors must be applied in this calculation to ensure that the parts per million of carbon monoxide is correctly input into the FED equation. Soot and carbon monoxide yields are input into Fire Dynamics Simulator (FDS) and are calculated in proportion to the mass of each component that makes up the fuel array. Both fire scenarios are modeled in FDS with the assumption that fire severity is limited to the heat release rate at the time of sprinkler activation. Sprinkler activation time is determined using detector activation (DETACT) analysis. Growth of the main floor rack storage fire is assumed to follow a “t-cubed” fire growth curve which has been observed in actual fire tests for the initial growth period of this type of fire. This growth is modeled using a ramp function and applying appropriate factors at appropriate times, within FDS. Sixteen (16) burners are modeled representing each carton in the fuel storage array. The burner for each successive tier of cartons turned “on” at 0, 30, 60 and 90 seconds, respectively, to simulate the spread of fire within the rack fuel array. DETACT analysis predicts ESFR sprinkler actuation for the rack storage fire scenario 60 seconds following ignition. In this scenario, the fire is controlled at 2,592 kW, and the top tier of cartons does not ignite. The fire continues to burn at this rate deep within the fuel array. The performance based aspects of the main storage floor rack storage fire, with detection time of 60 seconds, provides an RSET value of 308 seconds. FDS output shows that the time to reach the 10 m visibility limit is 400 seconds. For the rack storage fire scenario, ASET (400 seconds) > RSET (308 seconds), and this report has determined that the building would meet performance based life safety design criteria for this fire scenario. Additionally, this report concludes that the radiant heat from this fire does not, itself, cause the spread of fire to adjacent racks of storage located 8 ft. away, and that maximum ceiling temperature of does not exceed 180°C (356°F). The case where sprinklers fail to actuate is also evaluated. In this case the fire is allowed to grow to 36,500 kW in 150 seconds. As a result, this uncontrolled fire is predicted to cause extreme danger to building occupants due to hampered egress, spread to adjacent racks and commodities, and cause significant structural damage to the second floor (maximum temperature at the ceiling is predicted by FDS to reach 877°C (1,611°F)). Sprinklers in the warehouse are deemed to be essential for safe performance based egress and protection of property. The mezzanine workstation fire scenario follows a “t-squared” fire growth curve. If left unchecked this fire would grow to 6,730 kW in 530 seconds. Sprinkler actuation, predicted to occur 170 seconds following ignition is assumed to control the fire at 694 kW. The smoke detector located on the ceiling near the elevator door is predicted to detect this fire in 54 seconds by hand calculations. For comparison, FDS predicts heat detector activation 62 seconds into the fire scenario. Detection time is assumed to be 54 seconds. The calculated value of RSET for the workstation fire is 302 seconds. FDS output shows that the time to reach the 3 m visibility limit on the mezzanine is 130 seconds. In this case, ASET (130 seconds) < RSET (302 seconds), leaving a deficit of 172 seconds. This report has determined that the building does not meet performance based life safety design criteria for a fire involving a workstation on the mezzanine office level. This report evaluates several alternatives for improving performance based egress design: ·1) Replace standard response sprinklers with quick response sprinklers in the mezzanine area. The corresponding RSET reduction is 30 seconds. This alternative does make up for the 172 deficit between ASET and RSET. ·2) Remove the suspended ceiling, relocate and replace standard response sprinklers with quick response sprinklers. This alternative provides an 80 second increase in ASET and has no effect on RSET. This alternative does make up for the 172 deficit between ASET and RSET. · 3)Add an additional protected exterior stairwell on the north side of the mezzanine. This alternative improves the time for all building occupants to exit the building and reduces RSET by 42 seconds. This alternative does make up for the 172 deficit between ASET and RSET. ·4) Install a mechanical smoke control system. The revised FDS model shows that the value of ASET may be increased indefinitely. This is a costly alternative. · 5)Remove the mezzanine walls so that this area will be open to the main storage floor below. The rack storage fire scenario would become the bounding scenario but would need to be revised to account for exposure to personnel on the mezzanine. The additional 65 mezzanine occupants would be almost immediately susceptible to tenability limits. This alternative, which would solve the mezzanine workstation fire performance based life safety design issues, is expected to fail the main floor rack storage fire scenario performance based design criteria. ·6)Do nothing, control the number of occupants on the mezzanine floor, or control combustible loading. The mezzanine currently meets prescriptive egress criteria of NFPA 101, 2012 Edition. This alternative is subject to approval of the Authority Having Jurisdiction. Controlling the number of mezzanine occupants is expected to improve egress time but may not make up the 172 second deficit. Control of combustible (workstation) material is not feasible.

DOI

10.15368/fperpt.2014.5