Broad Group's Building Systems (Part I)
Broad Group's Building Systems (Part I)
What has the firm capable of building a 30 story skyscraper in 15 days been up to?
Broad Group is a large Chinese company, started in 1988 as a manufacturer of air conditioners, chillers and boilers. They’re perhaps best known in the US for their “Broad Sustainable Building” subsidiary, which made headlines a few years ago with a series of skyscrapers built incredibly quickly - 2 or more floors a day.
Broad Group’s buildings are often hailed as the future of building technology, or an example of how backwards US construction is. I thought it would be useful to take a deep look at the building systems they use, and get a sense of their performance.
Though they don’t advertise the connection, there are actually two Broad construction companies, each run by one of the founders of the company. “Broad Sustainable Building”, founded in 2009, is run by Zhang Yue as a subsidiary to the air conditioner business, and is the one associated with lightning fast building construction. “Broad Home”, founded in 2006, is run by his brother, Zhang Jian, and is one of the largest precast concrete producers in China.
Due to length, this edition will be split into two parts. This edition will cover the original Broad Sustainable Building system, as well as it’s new product, B-Core. We’ll finish up next time with a discussion of their new modular system, and of Broad Home.
Original Broad Sustainable Building System
The original Broad system is prefabricated panelized construction, consisting of large (3.9m x 15.6x) steel-framed floor panels. Panels are bordered with steel trusses, and infilled with steel joists, topped with a profiled steel deck. The panels are just over 16” thick, and arrive from the factory with wiring, plumbing, mechanical ducts and drywall all pre-installed, giving them a high level of completion. Roof panels seem to be similar sections, though appear thinner.
Panels are supported on prefab steel square columns with upper and lower braces, forming a sort of eccentric braced frame that is likely used for lateral resistance (as well as shortening the effective length of the panel trusses, possibly allowing them to span farther). Interior columns are about 25ft on center, exterior ones are about 12ft on center.
Panels arrive with built-in attachment points for the curtainwall system, which aids construction speed. One thing to note is the lack of field welding - everything installed on-site is bolted together, making for quicker installs.
Panels arrive from the factory complete with much of the necessary material to build out the floor stacked on top - prefab light gauge partition walls, drywall, columns, bolts, etc. It gets hoisted into position atop the panels as they’re being set. Buildings consist of hundreds of these panels, which are set extremely rapidly (4 or 5 cranes are setting panels simultaneously). Once the panel is set, work begins on building out the floor below. Interior partitions are set, services are connected, and curtainwall (also prefabbed) is flown in.
This system evolved over time. In at least some versions, it uses heavy steel moment frames in lieu of the eccentrically braced columns. And early versions show floor panels framed using castellated beams, while later ones use built-up sections made from angle and channel.
Overall this seems like a perfectly reasonable prefab building system. The panels are deep enough, and accessible enough (panel bottoms are drywall, as opposed to a structural material that couldn’t be removed) that running services through them likely doesn’t cause many difficulties. The braces likely give good lateral resistance, and the steel sections can (to an extent) be scaled up or down as the load requires. The spans are large enough that the interior layout remains flexible for future uses. It seems perhaps somewhat inefficient (using more structural steel than is strictly needed), but it’s hard to be confident of that without knowing the size of the sections they’re using. Materials are arranged for transportation intelligently and efficiently. The buildings are highly repetitive, allowing for more efficient panel production. There’s no reason in principle something like this couldn’t be used in the US (though I find it vanishingly unlikely we could achieve similar construction times with it).
I wasn’t able to get many details on the factory itself, other than it’s size. - 2.5 million square feet, capable of producing over 50 million square feet of panel per year (around 18 Empire State Buildings). Video and photos show a large number of panels stacked on top of each other in the factory, but they appear to be mostly manually built, without any sort of assembly-line, automated production. Construction time for these buildings is impressive, but I’d be curious to know for how long prior (and with how much labor) panels were being assembled.
Broad apparently spent $650 million developing this system, but despite the media coverage it doesn’t seem to have ever been very popular. In 2012 16 buildings had been built with it, and by 2016 only 30 or so had, and Broad had still made no money with it.
It’s unclear what became of this system. Broad had originally planned on using it to build what would have been the tallest building in the world, a 202 story skyscraper. The government put a stop to this, and the system doesn’t seem to have been used much since then. Nothing more recent appears on their website, or anywhere else online that I was able to find - their marketing material implies that they’ve moved on to a completely new building system: B-Core.
New Hotness: B-Core Panels
B-Core is Broad’s new building material. It consists of stressed skin panels made of two layers of 304 stainless steel plate, joined by a series of thin-walled stainless steel tubes. Using stainless steel gives the panels a high degree of corrosion resistance. The panels will act as sort of a truss, with the top and bottom flanges in tension and compression, separated by the thin-walled tubes [1]. The panels are anywhere from 1.5” to 15” thick, depending on the use case (for buildings, they’re typically 4 or 6” thick).
The void around panels is filled with rock wool to improve it’s insulation, and some shots show plumbing lines being run through it, though that’s likely fairly difficult in practice.
The marketing material for B-Core goes beyond effusive, and approaches the level of surreal. Broad describes it as an amazing, miracle material, and has plans it replacing all other materials in everything from buildings, to bridges, to aircrafts, to ships.
For buildings, B-Core seems to be used in two different structural systems. One is a more “traditional” system where B-Core takes the place of traditional structural elements - floor slabs, but also walls, beams, and columns. With this system, B-core becomes a material just like normal structural steel or concrete, capable of being arranged in a variety of different building configurations. The other system uses B-Core as part of a prefab modular system, which we’ll cover in next week.
Broad’s previous building system seemed to be mostly assembled manually. B-Core, on the other hand, is built using a highly automated (and pretty impressive) robotic factory.
B-Core is a relatively new system. According to Broad, it was developed from 2016 to 2018, and was first released in 2019. The product literature on their website is dated just 3 months ago. For the basic B-Core structural system (not the modular prefab system), the only buildings I’m aware of being built with it are a single test building, and a strange, inverted pyramid building (“Building F”), both on the Broad manufacturing campus.
B-Core Panel Properties
Broad helpfully gives all the specifications for the B-Core panels on their website, letting us calculate it’s performance characteristics. For now, we’ll just calculate it’s capacity as a flooring system. Panels were analyzed with a 75 psf floor load, the same as used in our previous floor system analysis, and assumed to be simple span.
In their product literature, Broad states that B-Core panel can span up to 12 meters (~40 feet). And we can see from the calculations that the panels do have impressive strength capacity, with the heaviest 6” panel being able to span almost 60 feet - and this is using relatively low capacity 304 stainless. A higher grade of steel could go even further.
However, flooring systems consisting of thin, strong materials are almost always controlled by deflection rather than strength [2]. Using the standard deflection limits from the US code (span/240), we see more modest, but still relatively impressive, performance: the maximum span now is 34 feet for a 6” slab, and just under 26 feet for a 4” slab.
A major benefit of the panels is that flooring can be applied directly to the top surface, without the need for concrete or other topping - in their promotional video, Broad shows carpet tiles applied directly on the steel surface. Without a cementious topping, however, a panel this thin and light will have extremely poor vibration performance - the panels will bounce noticeably while walking on them. Preventing this requires reducing the span even further, to approximately 16 feet for the heaviest 6” section, and 12 feet for the heaviest 4” section.
And we can see in practice that Broad limits the systems’ spans to around these values. Both their headquarters ‘F’ building and their prefab modules have interior beams to break up the spans to around 12-14 feet or so [3].
Comparing B-Core to Other Flooring Systems
We can plug this data into our previous floor comparison to see how it compares to other flooring systems (click to embiggen).
First, span vs depth - we can see that the vibration-controlled limit puts B-Core squarely in the range of other low-span, low-depth systems in use such as steel deck and panelized wood systems. It’s performance here does make it among the thinner systems in it’s span category, and of course if you ignore vibration it looks even better (and would in fact be the top performing system past 20 foot spans).
Looking at span vs weight, we see something similar - B-Core is right in line with the weights of the low-span, low weight systems. B-Core is lightweight, but not especially lightweight: B-Core does significantly better than the concrete based systems, but that’s about it.
Looking now at span vs cost, we see what we’re sacrificing by choosing stainless steel as a material. Stainless steel is expensive (2-3x the cost of normal steel), and B-Core uses a lot of it, and doesn’t use it particularly efficiently. Composite steel deck, for instance, only uses steel in the bottom, tension flange (in it’s final loading condition). At the top, it resists compression forces using concrete, which is around 1/10th the strength of steel, but also around 1/150th the cost (by volume). B-Core, on the other hand, only uses stainless steel.
The above prices are based on pure raw material cost - no addition has been made for manufacturing, labor, or transportation costs, which means these costs are probably underestimating the actual cost to the tune of 30-50%. The result is that B-Core is perhaps the most expensive flooring system in existence. No matter how efficient the robotic factory to assemble these panels, there’s no real way around this (and until they reach volume production, that factory is likely adding a great deal of cost).
And it’s unclear what using stainless is getting them. Stainless is virtually never used in building construction, for this exact reason. The only exception would be extremely corrosive environments - for normal exposure, galvanized (or even simple paint!) will get you all the protection you need, egregious lack of maintenance notwithstanding. There are 100+ year old steel buildings around the world still in use that can attest to that fact [4].
B-Core for Other Structural Elements
Broad implies B-Core can be used for any structural element - walls, beams, columns, etc. There’s no reason in principle this shouldn’t work, but it’s unclear what’s gained by doing so - the arrangement of the B-Core section isn’t especially efficient.
For beams, the culprit is the lack of a top and bottom flange when panels are arranged vertically. It means even MORE steel needs to be attached to make it an effective beam section (we can see from sections that this is fact what’s done). On the prefab modules, they opt to use normal steel channel sections instead of B-Core for the interior beams.
For walls and columns, the arrangement of the cross tubes means they aren’t taking any vertical load - they simply keep the flanges separate and transfer out of plane bending. Overall, B-Core isn’t making very efficient use of it’s very expensive component materials.
B-Core and Structural Tradeoffs
The unfortunate reality is that structural sections are generally a poor place for building system innovation. Structural systems aren’t like say, drug discovery, where chemistry allows for an entire universe of potential compounds just waiting to be found. Structural system performance is almost entirely governed by material properties and relatively simple geometric relationships. There’s simply not much room for radical advancements outside of dramatically improved materials [5]. Improvements mostly come in the form of things like dovetail deck, relatively minor rearrangements of material giving a slight boost to performance at greater cost.
Mostly, the best you can do is choose the sort of tradeoff you want to make in the span, cost, depth, and weight space. We can see that B-Core hasn’t really enlarged or changed the tradeoff space. It’s just staked out a new, and frankly somewhat strange position in it.
Discussion of Broad’s building systems will conclude next week!
[1] - Technically more of a frame than a truss, as the tubes are arranged vertically only, and so can’t transfer all the load via axial forces.
[2] - In a cruel twist of fate, deflection isn’t influenced at all by a material’s strength- only it’s moment of inertia (geometrical arrangement), and it’s modulus of elasticity (how much it lengthens under a given load). Modulus of elasticity is largely independent of material strength - structural steels range in strength from 36 ksi to 270 ksi or more, but all have the same modulus of elasticity.
[3] - The 6” A-1.5 seems to be the “standard” floor panel, which vibration analysis gives a span of only 9 feet for. However, a panel spanning continuously over interior supports will have better performance characteristics (for vibration, deflection and strength), likely allowing the approximately 13 foot span that’s typically shown.
[4] - One possibility is that a high degree of corrosion protection is required for anything exterior facing, as the B-Core might have no other covering beyond paint (or nothing!), and might exist in a highly corrosive environment. It’s unclear whether China’s high level of air pollution results in significant corrosion, but this paper suggests it does.
[5] - There are exceptions to this. Prestressing genuinely reduces deflection limitations by inducing an “upward deflection” that load then brings back down towards zero. Engineered lumber takes existing wood material and arranges it in such a way that it has dramatically improved performance characteristics.
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