Last week I discussed across a common misconception with porte-cochere sprinkler requirements and how code addresses sprinkler protection for these structures.
This week I’m diving a little deeper with some estimates of how a porte-cochere fire would actually affect a main building, based on distance from the building.
It’s important to note that this exercise is largely academic: with the calculations below I’m making some gross assumptions that overly simplify the situation. This has not been vetted with Ph.D. experts nor gone through full scale fire testing. I’m just running some basic numbers with big assumptions to illustrate a point.
From what science gives us - heat is transferred by three methods. Conduction, convection, and radiation.
Conduction is the transfer of heat by objects touching each other. The direction of transfer is dictated by hot-to-cooler materials in direct contact.
Convection is the transfer of heat caused by the movement of gas (or a fluid). The direction of transfer is largely dictated by overall movement of the fluid, and for smoke tends to be vertical.
Radiation is the transfer of heat from the emission of electromagnetic waves. The direction of transfer is in all directions, but can reflect and re-emit from other surfaces.
Heat Transfer for a Flame
For a flame, depending on the fuel, most of the heat will be transferred away from the flame source primarily by convection. The chemical reaction (oxidation) of a flame will cause gases to heat. The heated gas’ molecules will become more active and less dense. With less-dense gas than surrounding cooler air, the warm gas will rise up and away from the flame source and carry solid particles forming hot smoke.
Radiation will typically comprise 20-35% of the overall heat release rate for a fire. Radiation transfers heat from the source in all available directions until it contacts another surface. Once in contact with other surfaces, radiation can be absorbed or re-emitted from the surface, depending on the surface material.
Conduction is the least important mode of heat transfer in an open fire. Radiation near a flame’s origin, for example, often emits and heats up adjacent surfaces with more impact than conduction. For wall assemblies, conduction of heat through penetrations becomes important, but for flames in open environments conduction plays only a small role.
Three Porte Cochere Scenarios
Now imagine a porte-cochere that is 100 feet (30 m) from the face of a larger main building to the center of the porte-cochere. If the porte-cochere is completely inflamed, how would it transfer heat to the main building?
It would transfer heat only by radiation; and in very small amounts. Assumptions include a 5 megawatt (MW) fire from a wood-built porte-cochere, a 100-foot (30 m) center distance from the main building, an atmospheric transmissivity of 0.95, and a 30% of the overall heat loss as radiation.
Using the Lawson and Quintiere Point-Source Method, the incident radiant flux (a measure of the heat energy per area) is 0.13 kW/sqm.
This radiant flux is about 10% of the flux for a 1st degree burn on unprotected skin.
Now move the porte-cochere to be 30 feet from the face of the building. Radiation will again transfer heat to the face of the building, but in a much larger amount. Because radiant flux is related to the inverse square of the distance between the targets, this 30-foot distance will actually have a radiant heat flux 10 times greater than a porte-cochere fire 100 feet away. For the same size fire as before but at 30-feet, this could be about enough heat for a 1st degree burn.
At the 30-foot distance, however, heat transfer to the main building is still primarily by radiation. The hot, buoyant smoke is still primarily driven upward from the porte-cochere and would likely not reach the main building unless strong winds directed the hot gases.
Now imagine this same porte-cochere, but this time centered only 10 feet (3 m) from the main building. Radiation heat transfer is now 10 times greater than the 30-foot distance, and 100 times greater than the 100-foot distance.
At only 10 feet from a 5 MW fire, the heat flux is enough easily cause 2nd degree burns for unprotected skin.
Additionally, this heat flux is now approaching the critical heat flux for ignition of some building materials. The critical heat flux is the minimum amount of heat, per area, required to cause ignition. There's several factors that contribute to ignition including exposure time, material thermal properties, surface temperatures, and the actual heat flux versus critical heat flux - but for our purposes I'm only showing this critical heat flux for a couple siding materials.
Wood, for instance, has been tested to have a critical heat flux of approximately 10 kW/sqm. Vinyl siding has a critical heat flux of approximately 15 kW/sqm (both values from SFPE Handbook of Fire Protection Engineering, Table A.35, 5th Volume).
When we look at the heat flux already produced by a fire of this size at 10 feet we can see that we're already approaching the critical heat flux for both wood and vinyl.
Now let's speak in practicality. Porte-cocheres are built to allow visitors to enter and leave cars without exposure to rain or sun. Is a 30-foot or 100-foot separated porte-cochere provide any value to a building? No, of course not. This exercise just shows that with reasonable assumptions, a 10-foot physical separation assuming a 5 MW fire begins to approach the critical flux needed to ignite a nearby building.
Would the actual fire be 5 MW? It's difficult to predict and will vary widely by the materials used and the shape it conforms. A point-source approximation is a large oversimplification given that a wooden canopy would burn in a very different configuration than a condensed pile of wood pallets, for instance.
What about convection? Up to now we've still only discussed heat transfer by radiation. If a porte-cochere is close enough to a building, convective heat transfer from the hot smoke will begin to contact the main building and heat surfaces along the face of the main building. This could also be aided by wind conditions as well.
As I explored a little last week, a porte-cochere that is only separated inches or a couple feet from a building is hardly any different than a porte-cochere that's attached to the building. That's largely because of convective and radiative heat transfer. The further away the porte-cochere is, the less convective heat transfer plays a role and the lower amount of radiative heat will be transferred.
What if we create a firewall or fire barrier? Both would slow the spread of fire and help prevent the main building from burning. The International Building Code relaxes the physical separation with fire-resistive construction, and for good reason. Heat flux becomes much less important when the exterior is of non-combustible construction.
It can be easy to get lost in code minutiae and live by the black and white lines of what code reads. I find that it's important to remind myself about context about each building and where good engineering judgement plays a role in protecting buildings from fire.
This overly-simplified series of calculations just shows the tiers of radiative heat transfer and how much it is affected by the separation distance. The further away a building is from another, the less convective heat transfer plays a role (if any) and the less radiative heat transfer occurs.
If you found this interesting, let me know by leaving a comment here. Always happy to hear other opinions. If you don't already follow the weekly blog, consider subscribing here. Thanks for reading!
Unless you're tuned in as an AHJ yourself, you've likely made a few "code calls" to a code authority and asked a litany of questions to make sure your project's design meets the local requirements.
I'm not even sure if the term "code call" is a common term, but I've heard it enough that I suspect you already know what I'm talking about regardless of where you call home.
I enjoy this process now, but I didn't always. Fresh out of school I'm pretty sure I was visibly shaking the time I first made a code call. I was sure that within seconds my cover would be blown and it would be all too obvious that I had no idea what I was talking about. Despite my awkwardness (I make a good engineer, right??) nothing went sour and since then I've slowly learned and repeated many many times.
There was even one of my favorite code calls that I made about an elementary school to coordinate local fire alarm requirements. It was only right after the call late on a Friday afternoon that I found out that the fire marshal I just spoke with was hired onto our team and was starting the following Monday. They say fire protection is a small world, right? He turned out to be one of the most knowledgeable people I know and one of my favorite people to work alongside.
The Joys & Pains of Code Calls
Code calls also come in many different flavors.
Sometimes I'm just shocked by how friendly and helpful code authorities are. I once made a call at 15 minutes till 5pm on a Friday to a small town in Arkansas, thinking I would just leave a voicemail. After my questions, I asked if the department conducted flow tests, and while he said they did, he apologized that because of a prior commitment he couldn't do it then but would be happy to do it first thing Monday morning. I almost fell out of my chair. Very helpful and caring people in this field.
On the contrary, sometimes the hardest part about a code call is just finding the right person to speak with who is actually responsible for plan review of fire protection systems and getting a few minutes of their time. Not to pick on New York City because I love the people there and speak with a handful of you regularly, but if you're trying to get a hold of someone to verify or coordinate a few particulars of your system... well... good luck! Maybe it's because they knew I can't stand the Yankees.
I also sometimes get AHJs who simply say all they do is 'per code' and they aren't interested in talking specifics. The whole point of the call is filling in the gaps where a code or standard does not direct but rather defers decisions to the AHJ.
Want a siamese fire department connection with national thread, or a Storz-type? Either way is code compliant. As an engineer I can make either way work.
Is a wall-mounted FDC permissible, or does it need to be freestanding? Either location is compliant, but NFPA 13 says the location needs to be coordinated with the AHJ.
What I've gathered and refined over hundreds of code calls is my cheatsheet I currently use today. Just like the design cheatsheet, if you're using the Toolkit you can quickly highlight categories for your record keeping.
What's even better about this tool, though, is that you can quickly fill in the content (while on the call) and then right after save as a PDF and email to the AHJ themselves. Want them to have a record of the call and a quick way to verify your notes? Great! You now have a logged code call and the AHJ has an opportunity to review your notes.
The process of calling, taking notes, and composing the email used to take close to an hour total. This tool alone brings that total time to about 15-20 minutes. That's three-quarters of an hour you could save on every job you make the call!
A Radical Big-Picture Concept
One of my longer big-picture ideas to help the industry is to beta test and, if successful, open up a larger code-call database. I envision this as a database that brings designers and code authorities together to make local requirements clear and help jurisdictions get installations that reflect their preferences and mandates.
Want to know what hydraulic safety factor is required for sprinkler systems in Springfield, Illinois? Great - a quick query in the database reveals that and a clean list of other local requirements.
Want to know what type and location for FDC's that Tucson, Arizona requires? Great, we'd have that too.
This would clearly have a huge value for designers and engineers - but what I'm really curious about is how to incentivize code authorities to take the survey or help us populate the database. If you're an AHJ, email me (email@example.com) or comment below about whether you'd be open to the idea of making your local requirements public in a database.
I would have to think that AHJ input would only help local authorities get installations that match their needs - but I also know that getting action out of anyone is only possible with mutual benefit and sometimes incentives.
Just like the Design Cheatsheet posted a couple weeks ago, this form is integrated into the updated version of the MeyerFire Toolkit ready for download today. Below is a blank and filled-in template.
If you're already a Toolkit user, you can download the code call cheatsheet today by logging in here. If you're not using the Toolkit, you might consider joining in on what's quickly becoming what some consider the best tool for fire sprinkler design under $200. See more about it here.
The Questions on My List
The current code call checklist I use today has had items added and scratched over years of finding out what's important and what questions always get the same answers.
That being said, there's no real one defined list that matches everyone's preferences. What questions do you ask that you feel are important to the design that's not explicit in code? Comment below.
Join the Cause
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First - thank you for such a warm response to last week's article on a major and thorny topic of using sprinklers alongside glazing in rated assemblies. I genuinely appreciate and am motivated by so many thoughtful people in our industry.
As I mentioned last week, below is a link to the original article with the new PDF summary. It compares rated window assemblies, use of closely-spaced sprinklers for atrium enclosures, and the use of window sprinklers across many important categories.
If you find this useful, please consider sharing with others who also may be interested in the content. If you're not already subscribed, you can get this and many other similar resources for fire protection design, inspection, review, & engineering by subscribing, for free, here.
Thanks & have a great weekend!
Please don't take me to be an elitist.
I know what you’re thinking – yeah, right – this chump is from St. Louis and he’s not full of himself!?
It’s true I was raised in and have since circled back to St. Louis, Missouri. If you've heard of St. Louis it's probably because we're the most dangerous city in Missouri, or the Midwest, or the world or something. It's really not that bad if you don't have kids or go outside after sunset.
The other knock on St. Louis is that we're secretly wishing we were Chicago and, in terms of self-inflated ego, consider ourselves the last city on the East Coast.
Yeah that's also true.
While I am proud of our baseball team and beer production, please don't take me to be an elitist.
For instance, if you and I are talking fire protection and you use any of the following common wrong terms, I won't correct you. I don't even see a need to correct any of these terms, but I do find it interesting that several slightly incorrect phrases have been so pervasive in the fire protection industry.
Here’s my top list of misnomers in fire protection:
6. "Semi-Recessed" Sprinklers
I mentioned that I don't try and correct terms. The "semi-recessed" sprinkler is the reason I don't.
I had some very knowledgeable and technical counterparts in a prior role that were driven a little crazy with the term "semi-recessed".
A sprinkler can be concealed (only coverplate showing), can be fully-exposed pendent, or it can be recessed (deflector located closer to the ceiling as to show less).
If you consider a concealed sprinkler to be "fully-recessed," then I understand the logic that gets us to the "semi-recessed" designation.
However, some read "semi-recessed" a redundant term along the lines of "we want this partially exposed, but only partially." Putting the semi- is asking for partial of the partial.
5. Goose Necks
“Fire sprinkler return bend” is not as much fun as saying "goose neck," yet the two terms are synonymous for fire sprinkler systems. Of any of the odd or misappropriated terms in fire protection this one is the least offensive and is technically not even a misnomer.
That being said, three sticks of pipe and a couple elbows off a pipe outlet just doesn't scream goose to me...
The Goose Neck, also known as a fire sprinkler return bend.
4. Everything Is Awesome... and apparently is a Fire Wall
I enjoy working with architects. I do, and I'm not just saying that because my mortgage depends upon their continued blessing.
One requirement I'm convinced is part of the architect licensing is to call any rated separation a “Fire Wall.”
Fire Barrier, Fire Partitions, Smoke Barrier? Nope.
If it's not a rated wall that extends through the entire building and allows structural collapse on either side, it's probably not a Fire Wall and is more than likely a Fire Barrier. If it’s a separation wall in I-1 or some residential occupancies, tenant space separation, corridor walls, elevator lobbies, or egress balconies it may even only be a Fire Partition.
3. Spelling the "E" word
I never won a spelling bee as a child, but I'm confident enough I don't often run spellcheck.
My run as an ever-confident fire protection engineer ended when a peer review from another firm pointed out my incorrect spelling of "escutcheon". I'd like to pretend it was a one-off instance, but the misspelling was in a standard detail I had created and had been using for a couple years. Swing and a miss.
Es-cutch-eon: a flat piece of metal for protection or covering around a hole or void. And a tough one to spell.
2. Fully-Sprinkled Building
Your preschooler's dream: a fully-sprinkled building.
This one can be a little hard to spot, but did you notice there’s an “er” missing in this term?
When it happens I just imagine raining sprinkles through a building. Sounds tasty, but have you ever tried sprinkles plain? Not good.
1. Sprinkler Head
Lastly, this is the big one.
In the origins of fire sprinkler systems, a "sprinkler head" was a designation to the device that forced water distribution at the tip of the system.
Over 100 years later the fire sprinkler is still around, but the sprinkler has since adapted to the name "sprinkler" in lieu of "sprinkler head".
A sprinkler head.
Need evidence? It takes NFPA 13, the leading authority on fire sprinkler systems, 22 chapters before even mentioning the term "sprinkler head", and even then uses the term in talking about cabinet installation for nitrate film protection. Hardly a reining endorsement or common usage of the term.
In fact, Chapter 22 is the only chapter that even uses the term "sprinkler head". All other references are to the "sprinkler" or "fire sprinkler" throughout the entire standard.
What's the beef with using the term "sprinkler head"? Well, sprinklers don't have heads. They have frames and deflectors, but no heads. Besides, a whole lot of peeping “heads” throughout a building would just be creepy.
Hollywood already does our industry enough disservice, we don't need people thinking there's heads in the sprinklers, right?
Have you come across any of these? What are your pet peeves? Comment here.
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Not all code revisions are more conservative.
The 2018 Edition of NFPA 101 has updated the long-held occupant load factor of 100 sqft per person to 150 sqft per person. If you don’t live in the life safety arena, this change allows the calculated occupant load for a business space to be notably less, thereby requiring less exit width, stair width, potentially the number of exits, and other means of egress requirements.
The 1934 Building Exit Code first incorporated the density of 100 sqft per person, which was based upon a 1922 recommendation from the Building Exit code committee. It has since carried through over eight decades of code revisions and has lasted through many differences in office design.
Concentrated Business Use Introduced in 2015
The Life Safety Code introduced the Occupant Load Factor for “Concentrated Business Use” at 50 sqft per person in the 2015 Edition.
The goal with determining occupant loads has always been to provide the means of egress for a maximum probable number of occupants, and the introduced higher density was intended to address higher-density spaces than would normally be expected in a business occupancy. Annex material in NFPA 101 states that this should be applied where occupant concentrations are maximized, such as business call centers, trading floors, or data processing centers.
Modern open office concepts have changed the way we congregate and occupy buildings
Challenges with High Occupant Loads
This 2015 Edition change, according to testimony in committee hearings for 2018, has brought increased scrutiny and sometimes higher occupant loads to business occupancies by review authorities.
Increased occupant loads impact egress capacity, additional exiting, and can be very difficult to achieve higher occupant loads in existing buildings. Without additional horizontal exiting or plumbing fixtures, many existing office buildings cannot accommodate redesigns under higher occupant loads.
Open office concepts have also introduced new challenges. Collaborative spaces are sometimes being reviewed as assembly, even though these small rooms are intended and often used by the same people that are no longer at their workstations. The occupant load could effectively double-count the same occupant for two different work areas.
While terminology for the collaboration rooms is not entirely defined, modern office buildings are often labeling these as huddle rooms, quiet rooms, focus rooms, enclave rooms, or other owner-specific terms. These type spaces appear to meet the intent for the new collaborative room load factors identified below.
Collaboration rooms, often labeled as huddle, quiet, focus, or enclave rooms, are often used for smaller group activities by people who otherwise occupy the open office space. These smaller spaces function differently than traditional conference rooms.
Researching New Load Factors
The NFPA Fire Protection Research Foundation sought to study the appropriateness of the business occupant load factor for modern buildings in 2012.
Two studies stemmed from their initiative; a WPI Student Research project studies office building designs, modern changes in the workplace, and occupancy impacts of flexible employee scheduling and telecommuting. This study suggested it would be reasonable to increase the load factor to 150 sqft per person.
The second study, by Gilbert Group at the University of Canterbury in Spain, found average load factors for modern office buildings averaged 181 sqft per person. Both studies summarized that the 100 sqft per person occupant load was considered conservative.
Further testimony in the committee hearings suggested that at least ten research studies on office buildings conducted since 1935 have indicated that the Occupant Load Factor for businesses was conservative at 100 sqft per person.
The research, motions, and resulting voting brought a few major changes to the 2018 Edition of NFPA 101. Business use occupant load factor has increased from 100 sqft to 150 sqft per person; the “Concentrated Business Use” load factor has remained from the 2015 edition; and lastly small collaboration rooms and large collaboration rooms (with a threshold at 450 sqft) are given occupant load factors of 30 sqft and 15 sqft per person, respectively:
NFPA 101 Updates to Business Occupancies by Year
While the discussion above considers the process for NFPA 101 Changes, the International Building Code has similar provisions for the 2018 Edition for Business Occupant Load Factors as well as definitions of net and gross floor areas (Table 1004.1.2 and Chapter 2 Definitions, respectively).
Lastly, occupant loads of 50 people or more in a single space would still consider the space to be assembly and not a business occupancy. Large presentation rooms, training areas, or lecture halls can quickly introduce assembly occupancy requirements that are unaffected by these changes.
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Joseph Meyer, PE, is a Fire Protection Engineer in St. Louis, Missouri. See bio on About page.