4 ms·

Did anybody else laugh at the idea of using 2 grade 5 1" bolts per 2x4? I know wood is strong and all, but it's not that strong! They could have held it all t

Did anybody else laugh at the idea of using 2 grade 5 1" bolts per 2x4? I know wood is strong and all, but it's not that strong! They could have held it all together quite successfully with some 3" deck screws or 3/8" lag bolts. By using such big bolts they actually made the structure less safe because they had to drill very large holes; those holes made the wood weaker than it otherwise would have been. I don't know who at MIT insisted that they use 1" bolts but that person had no business sticking their nose in.

moeamaya 12y ago
Ben mentions his frustrations in his blog: Despite my personal opinion that these changes were unnecessary and frankly absurd (in addition to costly: 1" bolts cost around $10 apiece), we didn't really have any choice but to make the changes. After all, for the Cambridge and MIT to let us build this thing and have people ride it, we absolutely needed our plans to be signed off by a professional engineer. The Roller Coaster used to be an annual tradition but was put on hold the past few years because of EHS regulations and safety concerns. I know they really wanted to build it this year, so they obliged with the request.
- msandford 12y ago
  Seems like that PE maybe shouldn't have a stamp. I'm not saying they're definitely incompetent and should have it taken away, but it kind-of makes you wonder.
  - deleted 12y ago
    [deleted]
  - tonyarkles 12y ago
    I'm guessing it was "err on the side of overkill". Probably better than "err on the side of someone dying" while not wanting to bill too many hours doing detailed analysis to figure out what the cheapest solution would be that would still meet safety requirements.
rtkwe 12y ago
There are some benefits to through bolts over deck screws or lag bolts. It's easier to inspect a proper bolt to make sure it tightened and fix it if it is too loose. A screw could strip out with the vibration and stresses of the ride and it'd be really hard to check and fix.
- msandford 12y ago
  There's some truth to what you're saying, but a 3/8" or 1/2" bolt would be more than sufficient and would have the added benefit of not weakening the structure. If you're using desk screws you can purchase a few impact drivers which have adjustable torque settings and enjoy screwing to a specification. With lag bolts you could actually inspect everything with a torque wrench and the correct size of socket and that would work just fine. I think the biggest advantage to bolts is that they're reusable so you could take the whole thing down and build it again next year if you wanted. Or at least the start tower, say. But oversizing fasteners is only good up to point; beyond that and you're trading off fastener strength which could never ever be used (due to the lack of ability of the wood to sustain those kinds of loads without breaking) against reducing the ability of the wood to sustain ever smaller loads as the holes get bigger.
  - tomsaffell 12y ago
    >But oversizing fasteners is only good up to point; beyond that and you're trading off fastener strength which could never ever be used (due to the lack of ability of the wood to sustain those kinds of loads without breaking) against reducing the ability of the wood to sustain ever smaller loads as the holes get bigger.* That sounds reasonable, and yet I'm not sure it's applicable here. It's a 1" hole in the face of a 5.5" member (2x6). In structural engineering plans that I've seen (see below) that would be allowed (for certain placements) w/o explicit permission of the engineer. From that I deduce that such sized holes do not substantially weaken the member. But perhaps there is more to it. Also consider that a larger bolt means the wood-metal interface is spread over a larger area, so lower pressure - perhaps reduces splitting? >With lag bolts you could actually inspect everything with a torque wrench and the correct size of socket and that would work just fine. True, but unless the lumber is pre-dried, it will shrink after construction. Whether it be lags, hex bolts, or screws, it will need rechecking after the wood settles. All that said, I too would not have used 1" bolts. Seems like 1/2" or 5/8" would suffice. But my main objection to 1" is that it's overkill, and engineering is, after all, the science of achieving the stated objectives with the least resources. I'd love to see the calcs and assumptions of the professional engineer who mandated the 1" bolts. From some structural engineering plans for a house: Do not cut, bore or notch wood members expect where show in the details. Maximum holes at studs to be 0.4x width of stud. Maximum hole diameter at beam joist or rafter to be depth x 0.25, or 2 inches whichever is smaller, and are to be places within the middle third of span. Holes not permitted when depth is 4 inches or less. All other holes require engineers approval
jleader 12y ago
I'm not a structural engineer, but would there be any advantage to the larger bolt spreading the shear load over a larger area of the wood? Imagine a needle-thin (but incredibly strong) bolt; it might just tear through the wood fibers under load. I've seen rings (like http://www.portlandbolt.com/products/others/splitrings.html http://www.portlandbolt.com/products/others/splitrings.html) used to spread the load over a larger area of the wood, with a bolt keeping the wood members pressed against the ring.
- msandford 12y ago
  In reality what you want to do is get a lot of clamp force which will then allow you to use the friction between the two members to carry a lot of the load as well. Handily in that case the load is transferred over a large area meaning that the joint could be as strong as the wood on either side of it. Here's a nice reference for torque vs clamp force for a range of fasteners: http://www.spaenaur.com/pdf/sectionD/D48.pdf http://www.spaenaur.com/pdf/sectionD/D48.pdf If you use a 1/2-13 fastener and torque to 75 ft-lbs it'll provide nearly 5 tons of clamp force (provided that said clamp force doesn't cause the wood to get squished). Now standard friction equation applies: F = u * n And wood-on-wood is between 0.25 and 0.5 so let's be conservative and use 0.25. 9000 lbs * 0.25 = 2250 lbs of frictional force keeping the one wooden member from sliding down the other one. http://www.physlink.com/Reference/FrictionCoefficients.cfm http://www.physlink.com/Reference/FrictionCoefficients.cfm Here you can see the shear strength of a variety of bolts. You'll want to look at the second page under "single shear" as that's the application we're talking about. https://nucor-fastener.com/Files/PDFs/TechDataSheets/TDS_013_Shear_Strength.pdf https://nucor-fastener.com/Files/PDFs/TechDataSheets/TDS_013... If you look here a 2x6 as a vertical column is only good to support about 900lbs so basically all of these equations and speculations are a joke. Grade 2, 1/4" bolts would have 2000lbs of shear strength each, so the only reason to go bigger is for a better interface to the lumber. But that can be largely negated so long as you have a decent amount of clamp force, for 900lbs you would need 3600lbs of clamp force at most. Which you can get with a single 5/16" fastener, and probably a pair of large washers to spread the force out to prevent crushing the wood. https://courses.cit.cornell.edu/arch264/calculators/example7.1/index.html https://courses.cit.cornell.edu/arch264/calculators/example7...
dugmartin 12y ago
Deck screws don't have much resistance to sheering. When any kind of force is involved other than gravity deck screws do well holding things down but not up or on. If you look at a well built deck all horizontal attachments are bolts or nails.
bpowah 12y ago
Yield strength for steel is about 5x that of wood's UTS, and the cross-section of a 2x4 is about 5x that of a 1-in bolt, so yeah wood is 'that' strong. Screws are typically used in a way that they are not carrying a static load -- basically just holding the structural wood in place. The static load in a structure is usually transferred wood-to-wood. When you do see steel carrying a load in a wooden building, it is generally surprisingly beefy-looking.

moeamaya 12y ago

Ben mentions his frustrations in his blog: Despite my personal opinion that these changes were unnecessary and frankly absurd (in addition to costly: 1" bolts cost around $10 apiece), we didn't really have any choice but to make the changes. After all, for the Cambridge and MIT to let us build this thing and have people ride it, we absolutely needed our plans to be signed off by a professional engineer. The Roller Coaster used to be an annual tradition but was put on hold the past few years because of EHS regulations and safety concerns. I know they really wanted to build it this year, so they obliged with the request.

msandford 12y ago
Seems like that PE maybe shouldn't have a stamp. I'm not saying they're definitely incompetent and should have it taken away, but it kind-of makes you wonder.
- deleted 12y ago
  [deleted]
- tonyarkles 12y ago
  I'm guessing it was "err on the side of overkill". Probably better than "err on the side of someone dying" while not wanting to bill too many hours doing detailed analysis to figure out what the cheapest solution would be that would still meet safety requirements.

rtkwe 12y ago

There are some benefits to through bolts over deck screws or lag bolts. It's easier to inspect a proper bolt to make sure it tightened and fix it if it is too loose. A screw could strip out with the vibration and stresses of the ride and it'd be really hard to check and fix.

msandford 12y ago
There's some truth to what you're saying, but a 3/8" or 1/2" bolt would be more than sufficient and would have the added benefit of not weakening the structure. If you're using desk screws you can purchase a few impact drivers which have adjustable torque settings and enjoy screwing to a specification. With lag bolts you could actually inspect everything with a torque wrench and the correct size of socket and that would work just fine. I think the biggest advantage to bolts is that they're reusable so you could take the whole thing down and build it again next year if you wanted. Or at least the start tower, say. But oversizing fasteners is only good up to point; beyond that and you're trading off fastener strength which could never ever be used (due to the lack of ability of the wood to sustain those kinds of loads without breaking) against reducing the ability of the wood to sustain ever smaller loads as the holes get bigger.
- tomsaffell 12y ago
  >But oversizing fasteners is only good up to point; beyond that and you're trading off fastener strength which could never ever be used (due to the lack of ability of the wood to sustain those kinds of loads without breaking) against reducing the ability of the wood to sustain ever smaller loads as the holes get bigger.* That sounds reasonable, and yet I'm not sure it's applicable here. It's a 1" hole in the face of a 5.5" member (2x6). In structural engineering plans that I've seen (see below) that would be allowed (for certain placements) w/o explicit permission of the engineer. From that I deduce that such sized holes do not substantially weaken the member. But perhaps there is more to it. Also consider that a larger bolt means the wood-metal interface is spread over a larger area, so lower pressure - perhaps reduces splitting? >With lag bolts you could actually inspect everything with a torque wrench and the correct size of socket and that would work just fine. True, but unless the lumber is pre-dried, it will shrink after construction. Whether it be lags, hex bolts, or screws, it will need rechecking after the wood settles. All that said, I too would not have used 1" bolts. Seems like 1/2" or 5/8" would suffice. But my main objection to 1" is that it's overkill, and engineering is, after all, the science of achieving the stated objectives with the least resources. I'd love to see the calcs and assumptions of the professional engineer who mandated the 1" bolts. From some structural engineering plans for a house: Do not cut, bore or notch wood members expect where show in the details. Maximum holes at studs to be 0.4x width of stud. Maximum hole diameter at beam joist or rafter to be depth x 0.25, or 2 inches whichever is smaller, and are to be places within the middle third of span. Holes not permitted when depth is 4 inches or less. All other holes require engineers approval

jleader 12y ago

I'm not a structural engineer, but would there be any advantage to the larger bolt spreading the shear load over a larger area of the wood? Imagine a needle-thin (but incredibly strong) bolt; it might just tear through the wood fibers under load. I've seen rings (like http://www.portlandbolt.com/products/others/splitrings.html http://www.portlandbolt.com/products/others/splitrings.html) used to spread the load over a larger area of the wood, with a bolt keeping the wood members pressed against the ring.

msandford 12y ago
In reality what you want to do is get a lot of clamp force which will then allow you to use the friction between the two members to carry a lot of the load as well. Handily in that case the load is transferred over a large area meaning that the joint could be as strong as the wood on either side of it. Here's a nice reference for torque vs clamp force for a range of fasteners: http://www.spaenaur.com/pdf/sectionD/D48.pdf http://www.spaenaur.com/pdf/sectionD/D48.pdf If you use a 1/2-13 fastener and torque to 75 ft-lbs it'll provide nearly 5 tons of clamp force (provided that said clamp force doesn't cause the wood to get squished). Now standard friction equation applies: F = u * n And wood-on-wood is between 0.25 and 0.5 so let's be conservative and use 0.25. 9000 lbs * 0.25 = 2250 lbs of frictional force keeping the one wooden member from sliding down the other one. http://www.physlink.com/Reference/FrictionCoefficients.cfm http://www.physlink.com/Reference/FrictionCoefficients.cfm Here you can see the shear strength of a variety of bolts. You'll want to look at the second page under "single shear" as that's the application we're talking about. https://nucor-fastener.com/Files/PDFs/TechDataSheets/TDS_013_Shear_Strength.pdf https://nucor-fastener.com/Files/PDFs/TechDataSheets/TDS_013... If you look here a 2x6 as a vertical column is only good to support about 900lbs so basically all of these equations and speculations are a joke. Grade 2, 1/4" bolts would have 2000lbs of shear strength each, so the only reason to go bigger is for a better interface to the lumber. But that can be largely negated so long as you have a decent amount of clamp force, for 900lbs you would need 3600lbs of clamp force at most. Which you can get with a single 5/16" fastener, and probably a pair of large washers to spread the force out to prevent crushing the wood. https://courses.cit.cornell.edu/arch264/calculators/example7.1/index.html https://courses.cit.cornell.edu/arch264/calculators/example7...

dugmartin 12y ago

Deck screws don't have much resistance to sheering. When any kind of force is involved other than gravity deck screws do well holding things down but not up or on. If you look at a well built deck all horizontal attachments are bolts or nails.

bpowah 12y ago

Yield strength for steel is about 5x that of wood's UTS, and the cross-section of a 2x4 is about 5x that of a 1-in bolt, so yeah wood is 'that' strong. Screws are typically used in a way that they are not carrying a static load -- basically just holding the structural wood in place. The static load in a structure is usually transferred wood-to-wood. When you do see steel carrying a load in a wooden building, it is generally surprisingly beefy-looking.