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April 3, 2023
Density and selectivity are two of the most important factors to consider when designing a warehouse. Density refers to the amount of storage capacity in a given area and is typically measured in terms of cubic feet per square foot. On the other hand, selectivity is a warehouse's ability to store different types of items or products in different areas and efficiently access those products.
A higher warehouse density means you can store more items in a smaller area, which is important for maximizing the use of available space. Typical methods to optimize inventory to increase storage capacity include extending racking higher vertically, providing a mezzanine level, reducing aisle widths, and reconsidering storage medium. Each of these methods has fire code implications that must be carefully considered in addition to impacting selectivity.
Selectivity is equally important when it comes to warehouse design. Having higher selectivity allows the warehouse to store items in a way that is optimized for fulfillment. This ensures that items can be found quickly with high inventory accuracy, which helps to reduce fulfillment time and improve overall throughput. It can also help to improve efficiencies associated with stock replenishment and order accuracy.
But if high density and selectivity are so beneficial to warehouses, why aren’t both implemented? The answer is prescriptive code.
Building and fire codes set requirements for construction type, smoke and heat venting, aisle widths, travel distances, fire sprinkler systems, fire alarm systems, fire-rated separations, setbacks, etc.
Strictly following the prescriptive code can hinder warehouse density, selectivity, and overall utility. Think of density and selectivity as a spectrum. As warehouse design leans towards one of these objectives, the other tends to suffer. If both density and selectivity are equally prioritized, then construction costs become prohibitive at the hand of prescriptive code requirements.
And, unfortunately, following prescriptive codes doesn’t always mean a warehouse is safe. Just google “warehouse fire 2023,” and you will find countless stories of warehouses damaged or destroyed by fires. And that’s just for the first few months of the year.
One major factor is the evolution of modern warehouses. The eCommerce boom has led to the demand for near-immediate access to goods, forcing greater demand for warehouses located closer to city centers. Automation has also changed the way warehouses are designed, from automated storage and retrieval systems (AS/RS) to autonomous mobile robots or collaborative robots. Trends in automation tend to outpace the development and adoption of building codes further exacerbating the problem.
So, while prescriptive code means well, it can lead to constraints that ultimately hurt warehouses. Let’s explore them now.
Most warehouses are Type II-B construction, meaning they’re non-combustible and not required to provide a fire-resistance rating of building elements. Larger scale or “mega-warehouses” that exceed 1 million square feet leverage the unlimited area provision contained within the International Building Code (IBC) so long as the building does not exceed two stories in height.
However, multistory warehouses greater than two stories are generating more demand due to land limitations near city centers. The building code requires Type II-A or Type I construction, depending on the desired number of stories. These construction types, however, require that building elements are provided with fire-resistance ratings.
There are a few methods to provide fire-resistance ratings for structural elements, including using concrete of a minimum thickness, encasement, intumescent painting, and sprayed fire-resistive materials (SFRMs). All of these methods are usually cost-prohibitive.
Pick modules (pick mods) can also create a structural fire-resistance rating challenge. Pick modules are walkable, multilevel platforms/catwalks supported by a modular racking system where employees can pull and “pick” products for shipping. Some jurisdictions consider these as additional stories, thus requiring a fire-resistance rating. Providing such protection is not feasible for many reasons.
Mezzanines are often utilized to increase the leasable floor area within warehouses while not exceeding the maximum allowable number of stories which are limited based on the construction type of the building.
While mezzanines are permitted per code, the maximum allowable floor area is limited to only 1/3 of the building area. If this area is exceeded, then the mezzanine must be classified as an additional story, which could stipulate the fire-resistance rating of building elements.
Smoke and heat vents, in accordance with Section 910 of the International Building Code, are generally required in warehouses used for high-piled storage. They are not intended to maintain tenability for egressing occupants.
Historically, smoke and heat vents have aided in firefighter response activities. They are openings in the roof that activate in response to heat accumulation. However, the effectiveness of smoke and heat vents has long been questioned, most prominently the interaction between smoke vents and fire sprinklers.
The performance of smoke vents suffers greatly under certain environmental conditions, such as wind and warmer temperatures, which can cause a reverse stack effect. The reverse stack effect happens when the inside air is cooler than the outside air and a buoyancy-driven top-to-bottom airflow occurs.
Smoke vents rely on the natural buoyancy of hot smoke to push itself through the openings in the roof. However, sprinklers are known to have a significant cooling effect on smoke, reducing the buoyancy and thus the upward movement of smoke.
In addition to the overall ineffectiveness of smoke vents, they are generally undesirable due to upfront costs, ongoing maintenance requirements, decreased cooling efficiency, reduced sustainability and potential for water intrusion.
Specialty fire sprinkler approaches such as Early Suppression Fast Response (ESFR) or Control Mode Specific Application (CMSA) can be utilized as an exception to providing smoke and heat vents. However, these design schemes are extremely demanding of the water supply and may not be possible without significant investment into fire water supply infrastructure – and even then, they may be cost-prohibitive.
The building code sets a maximum travel distance of 400 feet from any point in the building to an exit for storage occupancies. However, this can be difficult to maintain in large or mega warehouses, especially for points that are located centrally in the building.
In addition, meeting the requirements of the prescriptive code can be made more difficult by the inclusion of mezzanines, lengthy rows of shelving, and other elements.
Through fire and egress modeling design approaches, it’s possible to demonstrate alternative means of code compliance that do not strictly adhere to prescriptive building code requirements. This is accomplished through a process known as performance-based fire safety design.
During this process, fire safety objectives are established, such as maintaining tenable conditions for egress, preventing structural collapse, limiting smoke spread, and maintaining conditions for firefighter response. Then, correlating performance criteria are quantified, such as maximum carbon monoxide concentrations, minimum visibility distance, failure temperatures of building elements, etc.
The objective of performance-based fire safety design is to evaluate how the building performs during a likely fire scenario through various computer modeling simulations. Fire scenarios are determined based on the fire characteristics of the stored commodities.
The modeling process is iterative in nature, simulating how various features of fire protection and life safety perform and are able to provide an equivalent or greater level of fire protection and life safety compared to prescriptive code compliance. This allows design priorities of increased storage density and selectivity to be achieved while also optimizing the provided level of fire protection and life safety.
Performance-based fire safety design solutions are possible for each of the prescriptive code challenges we mentioned earlier: structural fire-resistance ratings, smoke and heat venting and travel distance. These solutions allow for a more efficient and tailored fire safety system design for each warehouse compared to the prescriptive code requirements.
Warehouses today are looking for ways to increase operational efficiency and reduce costs through increased density and selectivity. Unfortunately, prescriptive codes can be a major hindrance. Performance-based fire safety design is an alternative solution that can help warehouses achieve greater storage density and selectivity with an equal or greater level of fire protection and life safety than what is required by prescriptive codes.
Performance Based Fire Protection Engineering provides state-of-the-art fire and egress modeling for large warehouses. Contact us to learn more.
