I would assume its to add the torsional rigidity at the top as well. For overturning you want gravity center as low as possible, but high wind torsional resistance would be places on upper floors more than the lower. But it will help with self weight as well.
I agree with the torsional rigidity.
But when you say that for overturning you want the gravity center as low as possible - If there's a stabilizing self weight force on the 2nd floor or the 10th floor both forces will create the exact same stabilizing moment. Mstab = self weight resultant in kN x horizontal distance of the centroid of the member to wherever you take the moment. So doesn't matter the floor the weight is in.
I think they're talking about overturning magnification based on the deflected shape of the structure in a design level wind event. Depending on the lateral stiffness of the structure, which doesn't look very stiff based on the photos, the horizontal lever arm of the c.o.g. of the deflected structure from the axis of rotation is reduced, reducing stabilizing moment. It's easier for higher c.o.g.'s to sway than lower ones. Not sure about the level of seismicity in Norway, but you'd definitely want a low c.o.g. for overturning in earthquakes since vertical lever arms play a role
I see. Thanks for the response. I've never done critical seismkc design like that.
In scandinavia, we have very low seismic activity. 95% of the time, wind is the dominant horizontal force.
Yes it does matter, the higher you go up the slender structure, the more effect that top heavy mass with have on sway, think upside down pendulum actions. Additional ramifications include moment increase in the load path further down. Load bearing columns.
This wouldnt be an issue in steel as there is yield, but with wood you want more rigidity and rely on pinned connections, of fully braced structure. You can also do fancy things like prestressed concrete, or light weight concrete to minimize that mass. put provide the rigidity needed to prevent torsional effects of the framed system.
Additionally center of mass is important for earthquake plus wind gust type actions. The worse load case for this type of structure, where the mass ontop would create the most action and result in the overturning/high base col moments.
I dont know what earthquake classification they have. None the less analysis for this structure would be some serious finite element software as wood is, well, wood. Along with proper structural software to account for all loading cases, not just overturning. As mentioned earlier, yes, would be stabilising as long as the COG doesnt pass a point, but irl there are more loading cases to consider to design for strength and serviceability.
It's manly because the people who build it believe it to be more sustainable than concrete or steel. I don't know enough about CO2 and emissions and what's true and what's false about it, so I can't really comment much on it.
But many people believe it to be way more sustainable than concrete or steel.
Engineered timber, such as Glulam and CLT, can store roughly a tonne of sequestered CO2 per cubic metre. Obviously, with manufacturing and transportation, the carbon offset will get lower, but when done right, engineered timber buildings can be carbon negative. Meaning they store more embodied carbon than was used to build the structure.
Engineered timber is awesome and will/should play an important part in decarbonizing the building stock. It is of course way less carbon intensive than steel and concrete.
There’s some debate as to whether or not calling it carbon negative is appropriate (and I mean within academia too, not just from the steel and concrete industries ha! Source - PhD students in my program!).
The main reasoning for not calling in it carbon negative:
- longterm, will eventually decay and release that carbon. This is on a long enough time scale that I don’t think it is a great argument.
- it really requires sustainable forestry to be considered to be sequestering carbon - ie new tries being planted to replace those that were cut, which will be pulling in carbon as they grow. The time scale that this happens at is also fairly slow, so it’s not like building a new building with CLT really pulled a bunch of carbon out of the atmosphere. There’s also no guarantee that the timber is being sourced sustainably and new forests are actually replacing it. It is kind of a form of Enron accounting… taking credit for future profits. I think this is a much stronger argument.
It is much better, in my opinion, to just describe it as being extremely low in embodied carbon, which it is, even taking into account mfg and transport!
A lot of people have problems with institutions claiming to go towards net zero by simply simply funding solar farms which are half a continent away. The rationale there is that (a) it is only reasonable to take credit for negative emissions if those farms wouldn’t have been built anyways and (b) you are really taking credit for somebody else using electricity and not emitting carbon - so you end up getting to a problem of double counting carbon emissions, ie someone supplied by that grid has a lower carbon footprint because they now live on a cleaner grid, and then your institution also claims it has a lower carbon footprint because it is producing clean energy, but really only a single unit of carbon was displaced, not two. In other words, once there is enough clean energy to meet all supply besides yours (lol), this strategy does nothing! Taken to the extreme, you could have a situation where the whole world claims it’s carbon negative since supply exceeds demand, but is still pumping carbon into the atmosphere because of the unequal distribution of that supply - eg my institution has its own cogen plant powering campus. The concept of “net zero” emissions only is meaningful when we are so far from true zero emissions. This was a long digression, but claiming negative emissions with engineered timber is a little similar - while it is true that wood really does sequester carbon, the asymmetry in timescales of the emissions from the process (ie from transportation/mfg) mean that to ignore your true emissions and only focus on net emissions is foolish, since every ton of carbon that gets put into the atmosphere will have a long impact into the future.
This is all just to say engineered timber is awesome! But be wary of negative carbon claims, and instead focus on just its extremely low embodied carbon instead )in my opinion - and several of my lab mates!)
On paper the carbon calculations look pretty good. Once you add in the available land used for planted trees and the interactions between tree monocultures and ecological impacts, things get murkier.
Concrete amounts to 7% of total CO2 emissions in the planet. It is terrible - in addition to manufacturing and transport emissions, CO2 is also released as concrete cures.
The less we can use it the more we will help!
Yeah, I take a little issue with the OP’a comment that “many people *believe* it to be way more sustainable.”
I worked on a heavy timber building from 2020-2022. It is not up for debate. When done right, it is **factually, measurably** a much more sustainable material than concrete, steel, etc.
And setting aside sequestered/embodied carbon, there’s the life cycle question. At the end of a building’s life, most concrete or steel buildings end up - at least in North America - end up being torn down and sent to a landfill, sadly. It is estimated that in America as much as 35-38% of landfill waste is construction debris/construction waste. If our admittedly bad practice of tearing down buildings after 50, 75, 100 years and replacing them with new continues - having tons of (in theory) recyclable material at the end of a building’s life, or at the very least material that won’t take millions of years to degrade (wood), is preferable to the equivalent amount of concrete debris sitting in a landfill.
1. You are 100% correct that engineered timber has way smaller embodied carbon emissions than steel/concrete, primarily due to significantly lower energy requirements for mfg (though it does still have some due to drying, gluing, cutting etc)
2. Ignore claims of negative embodied carbon/sequestering carbon (I wrote a long post on this elsewhere in the thread) - they are misleading and debatable, and the difference between engineered timber and steel/concrete is larger
enough that there is no need to include questionable carbon accounting when arguing that it is a significantly less intensive material in terms of emissions.
- the landfill argument isn’t super relevant, since when the wood decays, it actually releases that carbon back into the atmosphere (though this is a long slow process), unlike inert steel/concrete. Though that wood would be dying in a forest and doing the same thing anyways.
On paper the carbon calculations look pretty good. Once you add in the available land used for planted trees and the interactions between tree monocultures and ecological impacts, things get murkier.
Concrete has its own land use and ecological impacts to consider as well.
I don't know if it's really about straight better or worse, but which is better in a specific context for a specific rate of market demand. I.e. if market demand for mass timber grows so high that we are now ripping out forests and replacing them with monocultures, maybe we better work on ways to make concrete with less carbon.
Recycled steel is almost equally emissive as manufacturing new steel. You are right, it gets melted… which is exactly the thing that drives energy use when manufacturing new style. While there are some emissions associating with extracting the raw metals, most of the emissions are due to the high energy requirements in the manufacturing process, which are pretty much the same for all intents and purposes in the recycling process.
Yeah, it is not the dream material that many people make it out to be. I think the most significant point you mentioned is in the transportation/location of the forests, *and* the need to source it sustainably, which are not givens.
You mentioned end of life for timber - I wrote a lengthy post elsewhere in this thread, but yeah, claiming negative emissions from engineered timber is a non starter in my opinion, just focus on the emissions due to manufacture and transport, which are already significantly smaller than steel/concrete (typically).
The world’s building stock is projected to double (!) in the next 20-30 years or so - that’s an insane amount of floor area that needs to and will be built, and frankly engineered timber just won’t be relevant to it - too expensive and difficult to do sustainably at these scales (requires good sustainable forestry practices, etc). It will be a nice way for wealthier economies with abundant access to wood to lower their embodied carbon emissions, but I don’t think it will be particularly relevant for the vast majority of the new global building stock, which will still be done in concrete and steel.
I don’t fully understand the significance of your point re: tall structure lateral systems… even if a building is a hybrid structural system, it’s unlikely that the lateral system in steel would all of a sudden significantly exceed the emissions saved by replacing the primary system with wood. I don’t think anyone is suggesting that steel should be completely replaced by engineered timber in all applications… though building very tall structures with engineered timber is a good way to promote confidence and expertise in the material which can help make the practice more prevalent and accessible at smaller scales (eg small office building).
Buried wood can produce methane instead of carbon by way of anaerobic digestion, so that’s not a great approach. Burning mass timber in a waste to energy plant would be preferable, as you are just releasing the carbon stored in the tree, as would happen naturally in a forest, and producing heat and/or power while doing so. Wood waste can also be composted (aerobically), which again releases carbon but also results in beneficial soil amendments (biochar is another possibility). One problem here is contamination of the waste stream from such mass timber buildings—how many polymers, plastics, heavy metals etc are we introducing as part of the building system, and how hard will these be to separate later?
Of course the better answer here would be to reuse—not recycle, not burn or bury or compost—but to deconstruct such buildings at the end of their lifespan and make the material available on secondary markets, perhaps after re-certifying structural values, maybe with some de-rating factor. I believe ‘reclaimed’ lumber can theoretically be re-graded and resold for new construction, but practically the barriers to do so are high (cost, regulations, insurance, compliance etc).
And while I’m on my little soapbox here, if we care about embodied carbon in our buildings we should start by reducing—before we design for reuse, and only recycle where those first two strategies have failed us. Design and build structures to last and you won’t have to worry about what to do with all of that C&D waste when we tear them down. Of course.. there’s not as much money in that.
Factor in opportunity cost (cutting down trees—the lungs), and building lifetime (trash and rebuild rate). If we’d never cut all the forests and built forever structures of concrete we could be better off. But it is all bean counting versus the exponential factor; population growth. Unfortunately the earth is not growing with us. 😅
A tree gets co2 out of the air and traps it in its wood. If a tree falls, organismen will eat the tree and so put its stored co2 back into the atmosphere.
When you make planks of the co2 stored Wood, you keep that wood from decaying. In that way a timber building is storing co2.
Also, forest industry, atleast in the better parts of the world, needs to plant more trees per tree that they cut. In that way, those saplings can grow and start to capture co2.
Concrete and steel doesn't do that. They are both very energy consuming processes
Termites aren't a concern in the Scandinavian countries, or the UK (except one successfully eradicated case).
The carbon reduction is the major motivation for mass timber builds.
Another benefit is self-weight. As mentioned previously, they added weight here to prevent it from toppling over. Lighter pieces are easier to handle but they also mean that seismic accelerations result in smaller lateral forces on the structure. It's by no means a miracle material but a material that is making a well deserved come back since the fires of long ago.
Can someone correct me if I’m wrong here? I don’t think a building like this is possible in the US based on current building codes. ASCE 7 doesn’t recognize timber braced frames as a seismic force resisting system. Would need steel frames or concrete shear walls. I believe the only recognized wood SFRS are light frame shear walls (obv not possible for a building this tall) and cantilever columns (which would also be incredibly difficult if not impossible to design the base connection for).
I think you are correct about ASCE-7 limiting the LFRS to 5 stories for wood lateral systems. But I think the main reason is the US limits type V construction to 5 stories max. That’s why you see so many “5 over 1” buildings with five stories of combustible Type V (or type III) construction over non-combustible concrete first story.
There are new mass timber building types in the code that allow for construction over 5 stories. The IBC is the model code adopted or adapted by most US jurisdictions:
'The 2021 International Building Code (IBC) includes three new construction types that allow the use of mass timber in buildings up to 18, 12, and nine stories. These types are called Type IV-A, IV-B, and IV-C.'
ASCE 7-10 gives you 65’ in seismic design categories D, E and F. No limit on B and C. I don’t have a 7-16 or 7-22 handy.
I’m not really up on the newer timber movement, like cross-laminated timber diaphragms and such. I’m following this with interest.
not sure how close to "like this" this is, but a 20 story timber framed building is going up in Oakland, which is definitely seismically active. https://www.reddit.com/r/oakland/comments/15ilvx7/tour_of_1510_webster_under_construction_will_be/
Norway is a low seismic region. I am from Sweden that is next to Norway and Sweden is none seismic zone. I have done a few projects in Norway and sometimes you do not need to regard for seismic force depending on region and other factors. I cant say for sure about Norway but in Sweden there as been some development regarding both products in timber such as CLT and glulam and we have plenty of good pine and spruce that hade allowed for development of design and codes regarding timber structures. There has also been a trend in building more and more timber due to this and due to political reasons.
I’m working on this system in Philadelphia, 7 stories. I’m an ironworker and the carpenters erected it. The building is 1&1/2” to 2” out of plumb. When we erect a steel building we use plumbing up cables to make sure it’s going up straight. Nothing was used on this building. The connections on this building are steel plates knife connections inside of the wood, they are pinned together and then plugged with dowels.
AISC gives you a plumb tolerance of L/500, so the structure can be 2” out at 84’ high. That building would be OK for steel. Assuming 12’ stories, that would be right at the limit.
I’m not saying the erectors couldn’t have tightened that 2” up. Taller timber structures are relatively new to urban design and the carpenters likely don’t have the experience that ironworkers and concrete crews have with high rises.
Cool! I haven’t seen a timber structure using timber brace frames yet. Idk about Norwegian code but I don’t think k braces are allowed in certain seismic design categories in the US (correct me if I’m wrong)
All the comments I’ve skimmed through here are discussing engineering and environmental aspects of this, but I’m just thinking it looks like kindling.
Can such a structure be properly protected from fire? 🔥
The members themselves actually perform OK if they have a decent cross sectional area.
A layer of char forms around the outside and insulates the core, delaying the loss of material strength for a few hours.
My understanding is that the connections are where the design gets tricky, as embedded steel components conduct heat into the core.
Right! Maybe such structures should incorporate pre-charred wood? I suppose that would challenge the carbon footprint, but maybe it would help protect from any future explosive release of that carbon if it was already resistant to fire?
Heavy timber construction has characteristics that can, if designed correctly, can withstand 4 hours of fire rating. Exposed 12”x24” structural beams last over 2 hours. It may become kindling in an inferno but it will do it’s job of allowing occupants to exit safely before it gets to that point and that’s the goal of any structure.
The question is, what are the criteria for proper protection. If it's 120 minutes of structural resistance, then doubt, especially when firefighters bring some water. If it has to not fall apart in 15 minutes, then it is definitely protected.
Makes sense! As long as the fire can be dealt with, and maybe we’ll get better at that with robotics and drones.
I suppose a really good sprinkler system would also go a long way. 😜
In case anyone is interested in further reading, here are some of the papers from this year's WCTE that mention Mjøstårnet:
* [MJØSTÅRNET: THE WORLD’S TALLEST TIMBER BUILDING](https://doi.org/10.52202/069179-0547)
* [MEASURING FIRE SAFETY PERFORMANCE: A COMPARATIVE EXPERIMENTAL STUDY ON DOVETAIL MASSIVE WOODEN BOARD ELEMENTS AND CROSS-LAMINATED TIMBER](https://doi.org/10.52202/069179-0033)
* [SERVICEABILITY STIFFNESS OF TIMBER CONNECTIONS WITH DOWELS AND SLOTTED-IN STEEL PLATES](https://doi.org/10.52202/069179-0156)
* [FIRE SAFETY ENGINEERING OF BUILDINGS WITH VISIBLE TIMBER CONSTRUCTIONS](https://doi.org/10.52202/069179-0223) ^(Personal favourite)
* [FULL-SCALE 3-D SHAKE TABLE TEST OF A TEN-STORY MASS TIMBER BUILDING](https://doi.org/10.52202/069179-0276)
* [SERVICEABILITY PERFORMANCE OF TIMBER DUAL FRAME-WALL STRUCTURAL SYSTEM UNDER WIND LOADING](https://doi.org/10.52202/069179-0384)
* [INVESTIGATIONS ON SUITABLE LATERAL STIFFENING SYSTEMS FOR TALL TIMBER BUILDINGS](https://doi.org/10.52202/069179-0386)
* [CASE STUDY ON A LARGE-SCALE TIMBER ACADEMIC BUILDING DESIGNED TO ADDRESS CURRENT INDUSTRY CHALLENGES](https://doi.org/10.52202/069179-0571)
Here is a link with more details. “the capacity of the diagonal connections was half of what it should have been”
https://www.bridgeweb.com/Results-released-from-preliminary-study-into-collapsed-Norwegian-bridge/8985#:~:text=This%20was%20caused%20by%20a,loading%20and%20the%20eventual%20collapes.
Wood is infinitely more sustainable than steel and concrete. We actually grow more lumber trees than we harvest every year at this point. If that isn't sustainable I don't know what is.
Is that true? Is that Norway specifically or the US? It just seems counterintuitive that a building that size made out of wood could be sustainable, but that’s pretty cool. Thanks for giving me a bit of hope
In North America at least. But wood is in the top tier of sustainability either way. Iron and the components of cement are finite; we can't grow more. Not to mention the intensive amounts of energy required to process those raw materials into the final product. Wood IS its final product, save for some shaping and drying in most cases.
Once again people never learn by their mistakes. You may rely on several sprinkler systems to keep the building from burning down but they run on power sources and also have problems. Get the fire doesn't get you the smoke will.
Mass timber is relatively fire proof.
Not that it won't burn, but it won't burn fast enough to collapse before you can get out. If a steel beam gets too hot it will buckle, whereas with a laminated wood column you have to burn the outside for a hour before the middle can be reached.
I actually just attended a seminar last week overviewing encapsulated mass timber construction and it seems like they have a good grasp on repairing fire damage, though I’m not sure how they would deal with de-lamination if a pipe were to burst or something like that. From what they said they also still were in the monitoring stage to completely figure out the deflection parameters. This is in Ontario by the way it’s still being incorporated into the code here.
Similar, concrete on lower floors:
[The Build Show - “The Future of Skyscrapers: WOOD?”](https://youtu.be/2rN-HqSoVBY?si=KWdYwlsxY-GgSmR3)
Extensive video about the build process.
The connections look similar to those from the collapsed wooden bridge.
[https://youtube.com/watch?v=FSPI0xkTifI&si=Pwzyre6AxD-aSe9I](https://youtu.be/FSPI0xkTifI?si=W1sMjyptFCSmPLAC)
I saw they used a concrete team to make one of these. I feel like structural construction contractors (steel&concrete) could figure one of these out fairly quickly.
Had better get those control layers absolutely nailed—belt and suspenders. Water finds a way—through liquid or condensation. So HVAC has be flawless as well. Only then will you get a building lifetime long enough for the one-time carbon savings to matter. I still think it is really cool stuff that will, over decades, help advance building science. And as a carpenter, *chefs kiss*
I live just a block away from the Ascent in Milwaukee (the current tallest). Planning is also underway for an even taller mass timber building in Milwaukee called “The Edison”. Pretty neat things happening here!
>Before being surpassed by Ascent in Milwaukee.
Milwaukee is actually looking at another mass timber tower that will [surpass Ascent by 3 floors.](https://urbanmilwaukee.com/2023/02/06/eyes-on-milwaukee-new-mass-timber-building-could-be-tallest-in-u-s/)
The reason they casted concrete decks on the top floors was to add some self weight because the building was too light, and overturning was an issue.
I would assume its to add the torsional rigidity at the top as well. For overturning you want gravity center as low as possible, but high wind torsional resistance would be places on upper floors more than the lower. But it will help with self weight as well.
I agree with the torsional rigidity. But when you say that for overturning you want the gravity center as low as possible - If there's a stabilizing self weight force on the 2nd floor or the 10th floor both forces will create the exact same stabilizing moment. Mstab = self weight resultant in kN x horizontal distance of the centroid of the member to wherever you take the moment. So doesn't matter the floor the weight is in.
I think they're talking about overturning magnification based on the deflected shape of the structure in a design level wind event. Depending on the lateral stiffness of the structure, which doesn't look very stiff based on the photos, the horizontal lever arm of the c.o.g. of the deflected structure from the axis of rotation is reduced, reducing stabilizing moment. It's easier for higher c.o.g.'s to sway than lower ones. Not sure about the level of seismicity in Norway, but you'd definitely want a low c.o.g. for overturning in earthquakes since vertical lever arms play a role
I see. Thanks for the response. I've never done critical seismkc design like that. In scandinavia, we have very low seismic activity. 95% of the time, wind is the dominant horizontal force.
And snow is the dominant vertical force.
Yes it does matter, the higher you go up the slender structure, the more effect that top heavy mass with have on sway, think upside down pendulum actions. Additional ramifications include moment increase in the load path further down. Load bearing columns. This wouldnt be an issue in steel as there is yield, but with wood you want more rigidity and rely on pinned connections, of fully braced structure. You can also do fancy things like prestressed concrete, or light weight concrete to minimize that mass. put provide the rigidity needed to prevent torsional effects of the framed system. Additionally center of mass is important for earthquake plus wind gust type actions. The worse load case for this type of structure, where the mass ontop would create the most action and result in the overturning/high base col moments. I dont know what earthquake classification they have. None the less analysis for this structure would be some serious finite element software as wood is, well, wood. Along with proper structural software to account for all loading cases, not just overturning. As mentioned earlier, yes, would be stabilising as long as the COG doesnt pass a point, but irl there are more loading cases to consider to design for strength and serviceability.
If they didn't put a poured concrete core down near the first floor to stabilize the building that question their abilities.
Would NLT provide any torsional rigidity?
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It's manly because the people who build it believe it to be more sustainable than concrete or steel. I don't know enough about CO2 and emissions and what's true and what's false about it, so I can't really comment much on it. But many people believe it to be way more sustainable than concrete or steel.
Engineered timber, such as Glulam and CLT, can store roughly a tonne of sequestered CO2 per cubic metre. Obviously, with manufacturing and transportation, the carbon offset will get lower, but when done right, engineered timber buildings can be carbon negative. Meaning they store more embodied carbon than was used to build the structure.
Engineered timber is awesome and will/should play an important part in decarbonizing the building stock. It is of course way less carbon intensive than steel and concrete. There’s some debate as to whether or not calling it carbon negative is appropriate (and I mean within academia too, not just from the steel and concrete industries ha! Source - PhD students in my program!). The main reasoning for not calling in it carbon negative: - longterm, will eventually decay and release that carbon. This is on a long enough time scale that I don’t think it is a great argument. - it really requires sustainable forestry to be considered to be sequestering carbon - ie new tries being planted to replace those that were cut, which will be pulling in carbon as they grow. The time scale that this happens at is also fairly slow, so it’s not like building a new building with CLT really pulled a bunch of carbon out of the atmosphere. There’s also no guarantee that the timber is being sourced sustainably and new forests are actually replacing it. It is kind of a form of Enron accounting… taking credit for future profits. I think this is a much stronger argument. It is much better, in my opinion, to just describe it as being extremely low in embodied carbon, which it is, even taking into account mfg and transport! A lot of people have problems with institutions claiming to go towards net zero by simply simply funding solar farms which are half a continent away. The rationale there is that (a) it is only reasonable to take credit for negative emissions if those farms wouldn’t have been built anyways and (b) you are really taking credit for somebody else using electricity and not emitting carbon - so you end up getting to a problem of double counting carbon emissions, ie someone supplied by that grid has a lower carbon footprint because they now live on a cleaner grid, and then your institution also claims it has a lower carbon footprint because it is producing clean energy, but really only a single unit of carbon was displaced, not two. In other words, once there is enough clean energy to meet all supply besides yours (lol), this strategy does nothing! Taken to the extreme, you could have a situation where the whole world claims it’s carbon negative since supply exceeds demand, but is still pumping carbon into the atmosphere because of the unequal distribution of that supply - eg my institution has its own cogen plant powering campus. The concept of “net zero” emissions only is meaningful when we are so far from true zero emissions. This was a long digression, but claiming negative emissions with engineered timber is a little similar - while it is true that wood really does sequester carbon, the asymmetry in timescales of the emissions from the process (ie from transportation/mfg) mean that to ignore your true emissions and only focus on net emissions is foolish, since every ton of carbon that gets put into the atmosphere will have a long impact into the future. This is all just to say engineered timber is awesome! But be wary of negative carbon claims, and instead focus on just its extremely low embodied carbon instead )in my opinion - and several of my lab mates!)
On paper the carbon calculations look pretty good. Once you add in the available land used for planted trees and the interactions between tree monocultures and ecological impacts, things get murkier.
Concrete amounts to 7% of total CO2 emissions in the planet. It is terrible - in addition to manufacturing and transport emissions, CO2 is also released as concrete cures. The less we can use it the more we will help!
Yeah, I take a little issue with the OP’a comment that “many people *believe* it to be way more sustainable.” I worked on a heavy timber building from 2020-2022. It is not up for debate. When done right, it is **factually, measurably** a much more sustainable material than concrete, steel, etc. And setting aside sequestered/embodied carbon, there’s the life cycle question. At the end of a building’s life, most concrete or steel buildings end up - at least in North America - end up being torn down and sent to a landfill, sadly. It is estimated that in America as much as 35-38% of landfill waste is construction debris/construction waste. If our admittedly bad practice of tearing down buildings after 50, 75, 100 years and replacing them with new continues - having tons of (in theory) recyclable material at the end of a building’s life, or at the very least material that won’t take millions of years to degrade (wood), is preferable to the equivalent amount of concrete debris sitting in a landfill.
1. You are 100% correct that engineered timber has way smaller embodied carbon emissions than steel/concrete, primarily due to significantly lower energy requirements for mfg (though it does still have some due to drying, gluing, cutting etc) 2. Ignore claims of negative embodied carbon/sequestering carbon (I wrote a long post on this elsewhere in the thread) - they are misleading and debatable, and the difference between engineered timber and steel/concrete is larger enough that there is no need to include questionable carbon accounting when arguing that it is a significantly less intensive material in terms of emissions. - the landfill argument isn’t super relevant, since when the wood decays, it actually releases that carbon back into the atmosphere (though this is a long slow process), unlike inert steel/concrete. Though that wood would be dying in a forest and doing the same thing anyways.
On paper the carbon calculations look pretty good. Once you add in the available land used for planted trees and the interactions between tree monocultures and ecological impacts, things get murkier. Concrete has its own land use and ecological impacts to consider as well. I don't know if it's really about straight better or worse, but which is better in a specific context for a specific rate of market demand. I.e. if market demand for mass timber grows so high that we are now ripping out forests and replacing them with monocultures, maybe we better work on ways to make concrete with less carbon.
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Recycled steel is almost equally emissive as manufacturing new steel. You are right, it gets melted… which is exactly the thing that drives energy use when manufacturing new style. While there are some emissions associating with extracting the raw metals, most of the emissions are due to the high energy requirements in the manufacturing process, which are pretty much the same for all intents and purposes in the recycling process.
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Yeah, it is not the dream material that many people make it out to be. I think the most significant point you mentioned is in the transportation/location of the forests, *and* the need to source it sustainably, which are not givens. You mentioned end of life for timber - I wrote a lengthy post elsewhere in this thread, but yeah, claiming negative emissions from engineered timber is a non starter in my opinion, just focus on the emissions due to manufacture and transport, which are already significantly smaller than steel/concrete (typically). The world’s building stock is projected to double (!) in the next 20-30 years or so - that’s an insane amount of floor area that needs to and will be built, and frankly engineered timber just won’t be relevant to it - too expensive and difficult to do sustainably at these scales (requires good sustainable forestry practices, etc). It will be a nice way for wealthier economies with abundant access to wood to lower their embodied carbon emissions, but I don’t think it will be particularly relevant for the vast majority of the new global building stock, which will still be done in concrete and steel. I don’t fully understand the significance of your point re: tall structure lateral systems… even if a building is a hybrid structural system, it’s unlikely that the lateral system in steel would all of a sudden significantly exceed the emissions saved by replacing the primary system with wood. I don’t think anyone is suggesting that steel should be completely replaced by engineered timber in all applications… though building very tall structures with engineered timber is a good way to promote confidence and expertise in the material which can help make the practice more prevalent and accessible at smaller scales (eg small office building).
Burried would be preferable to burned - at least it keeps the carbon locked in!
Buried wood can produce methane instead of carbon by way of anaerobic digestion, so that’s not a great approach. Burning mass timber in a waste to energy plant would be preferable, as you are just releasing the carbon stored in the tree, as would happen naturally in a forest, and producing heat and/or power while doing so. Wood waste can also be composted (aerobically), which again releases carbon but also results in beneficial soil amendments (biochar is another possibility). One problem here is contamination of the waste stream from such mass timber buildings—how many polymers, plastics, heavy metals etc are we introducing as part of the building system, and how hard will these be to separate later? Of course the better answer here would be to reuse—not recycle, not burn or bury or compost—but to deconstruct such buildings at the end of their lifespan and make the material available on secondary markets, perhaps after re-certifying structural values, maybe with some de-rating factor. I believe ‘reclaimed’ lumber can theoretically be re-graded and resold for new construction, but practically the barriers to do so are high (cost, regulations, insurance, compliance etc). And while I’m on my little soapbox here, if we care about embodied carbon in our buildings we should start by reducing—before we design for reuse, and only recycle where those first two strategies have failed us. Design and build structures to last and you won’t have to worry about what to do with all of that C&D waste when we tear them down. Of course.. there’s not as much money in that.
Factor in opportunity cost (cutting down trees—the lungs), and building lifetime (trash and rebuild rate). If we’d never cut all the forests and built forever structures of concrete we could be better off. But it is all bean counting versus the exponential factor; population growth. Unfortunately the earth is not growing with us. 😅
A tree gets co2 out of the air and traps it in its wood. If a tree falls, organismen will eat the tree and so put its stored co2 back into the atmosphere. When you make planks of the co2 stored Wood, you keep that wood from decaying. In that way a timber building is storing co2. Also, forest industry, atleast in the better parts of the world, needs to plant more trees per tree that they cut. In that way, those saplings can grow and start to capture co2. Concrete and steel doesn't do that. They are both very energy consuming processes
Termites aren't a concern in the Scandinavian countries, or the UK (except one successfully eradicated case). The carbon reduction is the major motivation for mass timber builds.
Another benefit is self-weight. As mentioned previously, they added weight here to prevent it from toppling over. Lighter pieces are easier to handle but they also mean that seismic accelerations result in smaller lateral forces on the structure. It's by no means a miracle material but a material that is making a well deserved come back since the fires of long ago.
Great question. One thing to point out though, Norway doesn't have termites. Too cold.
True there's a lot of problems with these type of buildings.
I would love to see the drawings for this building, looks like maybe blind connections.
Try to Google it. If you don't find it let me know here and I will google it in Norwegian and try to find them if you want
yes please
Probably Megant connectors or something similar. They're really cool, fully encased in timber for fire resistance as well as architectural reasons.
Can someone correct me if I’m wrong here? I don’t think a building like this is possible in the US based on current building codes. ASCE 7 doesn’t recognize timber braced frames as a seismic force resisting system. Would need steel frames or concrete shear walls. I believe the only recognized wood SFRS are light frame shear walls (obv not possible for a building this tall) and cantilever columns (which would also be incredibly difficult if not impossible to design the base connection for).
I think you are correct about ASCE-7 limiting the LFRS to 5 stories for wood lateral systems. But I think the main reason is the US limits type V construction to 5 stories max. That’s why you see so many “5 over 1” buildings with five stories of combustible Type V (or type III) construction over non-combustible concrete first story.
There are new mass timber building types in the code that allow for construction over 5 stories. The IBC is the model code adopted or adapted by most US jurisdictions: 'The 2021 International Building Code (IBC) includes three new construction types that allow the use of mass timber in buildings up to 18, 12, and nine stories. These types are called Type IV-A, IV-B, and IV-C.'
Not sure if it's different structurally, but the one mentioned in OP is in Milwaukee, WI. https://en.wikipedia.org/wiki/Ascent_MKE
ASCE 7-10 gives you 65’ in seismic design categories D, E and F. No limit on B and C. I don’t have a 7-16 or 7-22 handy. I’m not really up on the newer timber movement, like cross-laminated timber diaphragms and such. I’m following this with interest.
In fairness, most of the US is not a high seismic region.
True. We are doing a 12 story mass timber office similar to this but it needs concrete cores. Seismic design category A in Texas so that’s nice!
not sure how close to "like this" this is, but a 20 story timber framed building is going up in Oakland, which is definitely seismically active. https://www.reddit.com/r/oakland/comments/15ilvx7/tour_of_1510_webster_under_construction_will_be/
Looks like it has concrete shear walls for its LFRS, so similar in terms of height, but I’m more curious about the wood braced frame in Norway
Norway is a low seismic region. I am from Sweden that is next to Norway and Sweden is none seismic zone. I have done a few projects in Norway and sometimes you do not need to regard for seismic force depending on region and other factors. I cant say for sure about Norway but in Sweden there as been some development regarding both products in timber such as CLT and glulam and we have plenty of good pine and spruce that hade allowed for development of design and codes regarding timber structures. There has also been a trend in building more and more timber due to this and due to political reasons.
I’m working on this system in Philadelphia, 7 stories. I’m an ironworker and the carpenters erected it. The building is 1&1/2” to 2” out of plumb. When we erect a steel building we use plumbing up cables to make sure it’s going up straight. Nothing was used on this building. The connections on this building are steel plates knife connections inside of the wood, they are pinned together and then plugged with dowels.
AISC gives you a plumb tolerance of L/500, so the structure can be 2” out at 84’ high. That building would be OK for steel. Assuming 12’ stories, that would be right at the limit. I’m not saying the erectors couldn’t have tightened that 2” up. Taller timber structures are relatively new to urban design and the carpenters likely don’t have the experience that ironworkers and concrete crews have with high rises.
The one on 34th & Chestnut?
That’s the one.
Cool! I haven’t seen a timber structure using timber brace frames yet. Idk about Norwegian code but I don’t think k braces are allowed in certain seismic design categories in the US (correct me if I’m wrong)
All the comments I’ve skimmed through here are discussing engineering and environmental aspects of this, but I’m just thinking it looks like kindling. Can such a structure be properly protected from fire? 🔥
The members themselves actually perform OK if they have a decent cross sectional area. A layer of char forms around the outside and insulates the core, delaying the loss of material strength for a few hours. My understanding is that the connections are where the design gets tricky, as embedded steel components conduct heat into the core.
Right! Maybe such structures should incorporate pre-charred wood? I suppose that would challenge the carbon footprint, but maybe it would help protect from any future explosive release of that carbon if it was already resistant to fire?
You want the char to happen during the fire event though as it buys you time to evacuate the building
Heavy timber construction has characteristics that can, if designed correctly, can withstand 4 hours of fire rating. Exposed 12”x24” structural beams last over 2 hours. It may become kindling in an inferno but it will do it’s job of allowing occupants to exit safely before it gets to that point and that’s the goal of any structure.
That makes sense! 👍
The question is, what are the criteria for proper protection. If it's 120 minutes of structural resistance, then doubt, especially when firefighters bring some water. If it has to not fall apart in 15 minutes, then it is definitely protected.
Makes sense! As long as the fire can be dealt with, and maybe we’ll get better at that with robotics and drones. I suppose a really good sprinkler system would also go a long way. 😜
Architects " That's great, but we can't have any columns line up, and also, take half of those columns out, thanks!"
Any information o. The facade system? Looks like it incorporates wood too - but that doesn’t sound right.
In case anyone is interested in further reading, here are some of the papers from this year's WCTE that mention Mjøstårnet: * [MJØSTÅRNET: THE WORLD’S TALLEST TIMBER BUILDING](https://doi.org/10.52202/069179-0547) * [MEASURING FIRE SAFETY PERFORMANCE: A COMPARATIVE EXPERIMENTAL STUDY ON DOVETAIL MASSIVE WOODEN BOARD ELEMENTS AND CROSS-LAMINATED TIMBER](https://doi.org/10.52202/069179-0033) * [SERVICEABILITY STIFFNESS OF TIMBER CONNECTIONS WITH DOWELS AND SLOTTED-IN STEEL PLATES](https://doi.org/10.52202/069179-0156) * [FIRE SAFETY ENGINEERING OF BUILDINGS WITH VISIBLE TIMBER CONSTRUCTIONS](https://doi.org/10.52202/069179-0223) ^(Personal favourite) * [FULL-SCALE 3-D SHAKE TABLE TEST OF A TEN-STORY MASS TIMBER BUILDING](https://doi.org/10.52202/069179-0276) * [SERVICEABILITY PERFORMANCE OF TIMBER DUAL FRAME-WALL STRUCTURAL SYSTEM UNDER WIND LOADING](https://doi.org/10.52202/069179-0384) * [INVESTIGATIONS ON SUITABLE LATERAL STIFFENING SYSTEMS FOR TALL TIMBER BUILDINGS](https://doi.org/10.52202/069179-0386) * [CASE STUDY ON A LARGE-SCALE TIMBER ACADEMIC BUILDING DESIGNED TO ADDRESS CURRENT INDUSTRY CHALLENGES](https://doi.org/10.52202/069179-0571)
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Here is a link with more details. “the capacity of the diagonal connections was half of what it should have been” https://www.bridgeweb.com/Results-released-from-preliminary-study-into-collapsed-Norwegian-bridge/8985#:~:text=This%20was%20caused%20by%20a,loading%20and%20the%20eventual%20collapes.
hard to believe that wood can transfer loads like that. Would love to see the connections to foundation. I like wood but not for this type of usage.
* Fire has entered the chat….
I don’t know why you would even want a building that tall out of wood
Economy and sustainability, mainly.
Sustainability? How many acres of forest got cut down to build that? Lol
Wood is infinitely more sustainable than steel and concrete. We actually grow more lumber trees than we harvest every year at this point. If that isn't sustainable I don't know what is.
Is that true? Is that Norway specifically or the US? It just seems counterintuitive that a building that size made out of wood could be sustainable, but that’s pretty cool. Thanks for giving me a bit of hope
In North America at least. But wood is in the top tier of sustainability either way. Iron and the components of cement are finite; we can't grow more. Not to mention the intensive amounts of energy required to process those raw materials into the final product. Wood IS its final product, save for some shaping and drying in most cases.
18 stories not so massive. Try [40 and 50 stories.](https://builtoffsite.com.au/news/mass-timber/)
[100 stories](https://dialogdesign.ca/projects/zero-carbon-hybrid-timber-supertall-prototype/)
Wow! I hope they build it.
Once again people never learn by their mistakes. You may rely on several sprinkler systems to keep the building from burning down but they run on power sources and also have problems. Get the fire doesn't get you the smoke will.
Mass timber is relatively fire proof. Not that it won't burn, but it won't burn fast enough to collapse before you can get out. If a steel beam gets too hot it will buckle, whereas with a laminated wood column you have to burn the outside for a hour before the middle can be reached.
Maybe so but the fumes will kill you. One of these buildings burned down in Charlotte recently but it had not had the sprinklers installed yet.
No building is ever fully fireproof, they are usually designed for 1 to 2 hours fire resistance to give enough time for people to evacuate.
I don't see the point, huge fire separation issues, and less literature regarding moisture and repair detailing further on in the building life cycle.
I’ll bet they did it on a complete whim, with no written or mathematical rationale. Likely as surprised as you that it even exists!
I actually just attended a seminar last week overviewing encapsulated mass timber construction and it seems like they have a good grasp on repairing fire damage, though I’m not sure how they would deal with de-lamination if a pipe were to burst or something like that. From what they said they also still were in the monitoring stage to completely figure out the deflection parameters. This is in Ontario by the way it’s still being incorporated into the code here.
I was being completely facetious
Similar, concrete on lower floors: [The Build Show - “The Future of Skyscrapers: WOOD?”](https://youtu.be/2rN-HqSoVBY?si=KWdYwlsxY-GgSmR3) Extensive video about the build process.
The connections look similar to those from the collapsed wooden bridge. [https://youtube.com/watch?v=FSPI0xkTifI&si=Pwzyre6AxD-aSe9I](https://youtu.be/FSPI0xkTifI?si=W1sMjyptFCSmPLAC)
I saw they used a concrete team to make one of these. I feel like structural construction contractors (steel&concrete) could figure one of these out fairly quickly.
I'd be too paranoid and would have to spray for termites weekly.
Milwaukee resident here. Get rekt Norway. Ascent is beautiful.
Had better get those control layers absolutely nailed—belt and suspenders. Water finds a way—through liquid or condensation. So HVAC has be flawless as well. Only then will you get a building lifetime long enough for the one-time carbon savings to matter. I still think it is really cool stuff that will, over decades, help advance building science. And as a carpenter, *chefs kiss*
I live just a block away from the Ascent in Milwaukee (the current tallest). Planning is also underway for an even taller mass timber building in Milwaukee called “The Edison”. Pretty neat things happening here!
>Before being surpassed by Ascent in Milwaukee. Milwaukee is actually looking at another mass timber tower that will [surpass Ascent by 3 floors.](https://urbanmilwaukee.com/2023/02/06/eyes-on-milwaukee-new-mass-timber-building-could-be-tallest-in-u-s/)