My Bridge Tread GRK Screws Literally Broke! - Big Mystery & Simple Solution

[@zigzagzoggo]

I thought you were going to quote from "bridge over troubled water" by Simon and Garfunkel... lol

[@howardkettner]

Hahaha. I wish I’d thought of doing that. Great suggestion.

[@chadbassmaster9084]

1: Tensile strength, not sheer. 2. Yes these are structural, but that bridge is absolutely flexing with the load you are running across it. Incredible weight for what you’re asking of these screws. Its actually amazing the screws held in the wood and sheered while holding and not just lost “tread” and pulled from the wood. Those GRK screws are tough. Time for some concrete and I beams

[@howardkettner]

Thanks for the well-reasoned advice and explanation.

[@thebradleysoncatbirdhill6849]

Howard, that was outstanding! You know I love the technical stuff. I have used GRK screws for years, but never in the same application that you are describing! Great information, as always!👍

[@howardkettner]

Oh man, you are too kind. But I appreciate it! Having been a desk jockey for most of the last 42 years this is all new to me. If you’d asked me 15 months ago what a GRK screw is, all you’d have gotten would have been a blank stare. Here’s to new (albeit sometimes painful) learnings. Huge thanks John and Beth for sharing my journey - and being part of those learnings.

[@mikesveganlife4359]

I'm seeing this problem locally with building inspectors signing off on these GRK RSS screws in places of 1/4 lag bolts and not paying attention the manufactures requirements for closer distances for the fasteners than 1/4 lag bolt would have. They seem to not paying attention on the need for them to be closer together than what a 1/4 lag bolt would need to be particularly in deck ledgers.

[@howardkettner]

Yeah - for sure. Big eye-opener for me. Thanks for tuning in and being part of my journey!

[@shaunanderson8282]

Hi Howard, I hope your solution is holding up 7 months on. I imagine they might be tensile failures rather than shear failures. I see that the overall length of those RSS vs the non-threaded section and the locations of the CEE thread (that reverse thread bit a part way down the RSS) means that this is the wrong screw for the thickness of your running planks (treads) on top. By installing without predrilling the treads, the threads of the screws are binding up on both sections of timber because the thread is partly in both pieces, whereas the intent is that the CEE thread enlarges the hole a bit in the upper piece of timber and the main thread only grips the bottom piece of timber. If the thread is gripping both pieces, this means that the screw cannot torque down properly and develops unintended stresses. When the timber shrinks the top piece is held up on the threads so the screws end up potentially carrying compression forces, or when the timber swells or any other event that causes tension, all the force is happening in the screw right at the joint rather than spread out over the length of the unthreaded section of the screw. Bolts and screws are supposed to be able to stretch to spread the load out whereas in this case they cannot, instead all of the stretching is happing right at that one spot hence you end up with a tensile failure. Cup head bolts with a spike washer is what we use on our road bridges, but depending on where your girders are this might be difficult. Otherwise I'd suggest predrilling the treads with a hole size that won't grab the thread, or preferably using RSS/lags that have an appropriate length unthreaded section to accommodate the tread thickness. Hopefully your fix works just on account of the amount of RSS fasteners, but if you have the same issue again it might pay to consider this.

[@howardkettner]

Wow! Shaun. Best explanation ever for what may be going on with my bridge.. I'm enormously appreciative of the detail and logic of your explanation - and your suggestion. Thank you.

[@shaunanderson8282]

@@howardkettner No worries mate, keep up the good work.

[@es2709]

I've long said that the best info is always sourced from the comments section. Thanks for proving me to be correct!

[@BrianAz]

Hi @Howard Kettner! Im new to your channel but as an exotic/super/hypercar lover I did notice you wearing the Mclaren gear! May I ask if you are and owner, employee, engineer, or just a fan of that awesome automobile?

[@howardkettner]

Huge thanks for the visit and for being part of my journey. I'm actually more of a tractor and heavy equipment guy - going all the way back to my teens. The McLaren 'wear' was a gift from my children, knowing that I never miss F1 weekend qualifying and racing, have attended the Montreal Grand Prix, and intend to attend more races live. Love the McLaren brand and the legacy. When I was a teen I was very intrigued by McLaren and followed anything Bruce McLaren in the car magazines of the day. Love their recruitment of Lando Norris - and also their involvement with Indycar. What's not to like right? 8 F1 Constructors' Championships and 12 Drivers' Championships. And - their production cars. I can only drool - but drool I do. hahaha. Again, thanks for tuning in . . .

[@jamesbruce1183]

I have found a drill is faster than an impact for screws this size. An impact may affect the screw's strength. Have you double checked the bridge design for weight carrying capacity? Maybe be too much deflection for the screws because the bridge is just overloaded.

[@howardkettner]

Thanks for tuning in and being part of my journey. Love the suggestion of using a drill. Had not thought about the effect that an impact might have. I've concluded that GRK's are just wrong for this sort of project because of what you raised - that being the deflection. Fewer long bolts and washers IMO would be a vastly improved solution. Again, thanks for tuning in and being part of the conversation.

[@travissimon7872]

It May have helped if you would've ran the decking boards in the opposite direction. That way the boards are being pressed down on all of the board instead of a seesaw type pressure

[@howardkettner]

Thanks for tuning in and commenting. I definitely have to understand more about bridge decking and tread configuration. Very much appreciate your suggestion. Will definitely explore further.

[@eee2115]

How is it holding up now that you placed them closer? It seems crazy to me to place these things so close to each other. I have used GRK deck screws before and like them alot. But if I were to do your build and now that I know how many screws it would take, I would opt for other options.

[@howardkettner]

The deck is holding fine now, but your assessment is entirely correct. I cut the wood for the deck but hired an apparent professional bridge builder to do the installation. Knowing what I know now, I would definitely vigorously pursue other options.

[@lanedexter6303]

Interesting! I have to wonder about some of these modern fasteners (recently used some TimberLOK screws while building a woodshed). They claim to be better than larger lag screws, surely use better steel, and they are convenient. But depending on the type of movement and stress to which they are subjected, I wonder if they are a bit brittle. The sheer size of the old softer steel lag screws may have its place in some applications.

[@howardkettner]

Huge thanks for visiting and weighing in Lane. I really appreciate your perspective. I would not disagree with you. Totally worth considering.

[@evictioncarpentry2628]

That's not a shear failure. Shear load is put vertically on the screw. (Face load) Your failure of the GRKs is from being stretched beyond capacity. (tensile load) Those should have probably been at least proper grade 8 or higher bolts nuts and washers. I would guess at minimum 1/2 inch or more. That would hold 150,000psi per bolt.

[@howardkettner]

Thanks for tuning in. This is literally the first time I've ever witnessed something like this so I really appreciate your taking the time to comment and make a suggestion to help me in creating a permanent solution. Dump trucks don't swim well last time I checked.😬

[@evictioncarpentry2628]

Yeah just the wrong application for these things. If you look at your cart in the "pull out" side, you'll see Lag bolts have nearly double the pull out strength of GRKs at 1/2 or 5/8. Where as GRKs have higher shear value from a side load. I would suspect even 1/2" & up lags & washers would fair better but the best bet is actual graded hardware that's meant & rated to be stretched. Like I said at 1/2" grade 8 it's 150k psi per bolt. And it goes even higher if you go up to 5/8 or 3/4. That's a lot of weight in the tandem and the wood probably flexes more then you think.

[@mikerayle6103]

From what im gathering its a nice bridge and wasnt correctly fastened. But if i were to plan ahead about driving a 55k (60 tons for us US folks) truck over a bridge made of wood i would be worried. The grk screws are fine just maybe not for that application. Hoping quadrupling the screws solves the problem. And no more issues happen. Its wood its going to flex especially with that kinda weight

[@howardkettner]

Thanks for tuning in and being part of the journey. So far so good. Now that the reno's are done, the heaviest loads coming across the bridge are under 60,000 pounds. Had a crane truck in here two weeks ago - with a lightly loaded Sea Can on it and I took care to watch the flex and did a post crossing check of the GRK's and all appears well.

[@andyking05]

It’s because there will be a small gap between the big pieces of wood but when a heavy vehicle drives over it’s enough to bend the wood but the screws can’t give

[@howardkettner]

Huge thanks for the added info and perspective. Having mostly been a desk jockey for 40 years I’m still figuring this stuff out - with the help of viewers like you. Thanks for being part of my journey and learning.

[@mikesveganlife4359]

To clarify, due to the gaps between the screws, the wood is allowed to bend. If screwed down appropriately it wouldn't bend as much and would not go over the sheer strength of the screws.

[@copperkettlepot]

Hi, I am no engineer, but even without any fasteners - the underlying wood poles either will hold up or fail all on their own - given such a heavy, non-static load. In fact, these fasteners really are fighting the natural "give" or compliancy of the wood. I would have used 1/2' galvanized washered lags (countersunk) to permanently tie the planks to the poles. Sorry, no fooling around with underperforming GRK's. Nice video on real world fastener failure.

[@howardkettner]

I'm clearly no engineer either, but I would tend to agree with you. 1/2-inch galvanized lag screws . . . Huge thanks for tuning in and contributing to the conversation.

[@silascheeseman4371]

Interesting. I just completed building some shelving in a garage using GRK screws. The spacing used was 16" and all seems well... in fact, I think it's over engineered. :)

[@howardkettner]

Thanks for tuning in - and for your comment. Great perspective. GRK screws are the best. And I would have done the same as you for a shelf. It's those 55,000 pound loads across a log base that by it's nature has some flex that called for a visit to the manufacturers specs. That and a bit of math to calculate what each screw is capable of . . .

[@silascheeseman4371]

@@howardkettner I've seen some folks do a test... screw a GRK screw into a chunk of wood, then grab onto the head and pull it out of the wood with a rig that can measure the pounds of force required. But is that a test of the screw... or a test of the wood? I'm not sure it's a fair test of the screw. Things like, type of wood, hard or soft, depth screw is buried, etc. Just something that keeps me awake at night. :)

[@howardkettner]

@@silascheeseman4371 Haha - you and me both! Thanks for sharing your thoughts. We'll get this figured out yet!

[@mainelybuds8543]

Glad you figured it out Howard... Everyone's an expert, until they're not. Lol

[@howardkettner]

Well put! - I need to remember that!

[@michaelcsonka2675]

No your reading that sheet wrong! The sheet lists minimum allowable spacing as in the screws should be no closer then 1.1 inch from the edge, no closer then 3.3 inches from the end, and no closer than 4.5 inches in spacing. Otherwise you split or structurally compromise the wood. It's not 4.5 inches or less! You have 2 issues. First is although GRK have a high shear load they're still vulnerable to repeated bending. You bridge is flexing every time a vehicle goes over it primarily along its length not as much it's width. Hence why the fasteners in the cross timbers haven't yet failed while the lengthwise treads have. More screws will not help and may hurt. Nails, while able to bend more, will still fail eventually and may back out. The solution is most likely carriage bolts if possible which would allow the wood to flex. Issue 2 is for the love of god consult a structural engineer regarding whether that structure can support 50,000 pound trucks safely. I strongly suspect it can not. The threads aren't really a structural element they're more of a ware surface. Screws were most likely used because your just supposed to be keeping the treads from lifting up. If the screws are being subjected to excessive shear forces then it's an indication the beams underneath the bridge which form the actual structure are failing under the loads being applied.

[@madjackgamingandfitness498]

Great comment. Too bad the video is old now. I had a thought the underside didn’t look very supported for such a length. Carraige bolts seem to be the best way to go for heavy impacted structures like this, we used them on docks so I guessed similar principles apply. It seems like a cool project to build a bridge.

[@LuminairPrime]

I agree that finding broken screws on a bridge calls for an engineer! It appears that the design of this bridge was using those screws like deck screws, just holding boards down, rather than as a serious structural element. For them to break means that the structural element has failed, and the bridge is now operating WAY outside its safety margin. Good thing wood is nature's greatest resource and can bend back and forth really well.

[@joshuawiedenbeck6944]

General rule is that nails are much more resistant to any force that "bends" them (they are pliable), and screws have better holding power as long as they don't move (they are more brittle than nails). SPAX makes screws simular to GRK and I have never seen a SPAX fail.

[@howardkettner]

Clearly I have my homework cut out for me. I was unaware of SPAX but have just looked up that product and see that they haver a Canadian supplier. Great perspective on nails vs screws also. Huge thanks for being part of my journey and learnings.

[@joshuawiedenbeck6944]

@@howardkettner If I remember correctly, SPAX are a German-made fastener. I tend to be partial to anything German made (except for cars).

[@howardkettner]

@@joshuawiedenbeck6944 On the same page exactly. My ancestry is German. Agreed on the cars. Had owned three over the years. I’m a slow learner apparently. Operative word “had”.

[@oflores7457]

Grk ?? Would have better with Sheetrock screws never fail

[@cyberpunkspike]

That chart of distances is unclear, if it was required maximums between screws... why wouldn't it say that on the chart? If it is minimum distances, as in ... you can't place two screws closer together than the chart says, which makes more sense IMO, why does SPF allow smaller distances? I suspect you are misinterpreting the chart, regardless the contractor still did not use enough fasteners or the correct diameter. 12" spacing seems way to large a span in this situation.

[@howardkettner]

I don't doubt for a minute that I misinterpreted the chart. And I would do a deeper dive if this was but one of many such projects. What I've done, being that bridge is essentially a 'one-off' situation for me, is simply overbuilt it by putting GRK's in with 6" maximum spacing. I've had a couple of 50,000 lb trucks across it and no broken screws. I also put two more full length 12" wide 2-1/2" thick treads right down the middle - which seem to give the deck additional support. Thanks for tuning in and for weighing in with your perspectives. Much appreciated!

[@cyberpunkspike]

@@howardkettner It's OK, as long as you didn't do much less distance than the minimums, the wood shouldn't be compromised.... it's basically the maximum amount of screws you can use, before the wood itself becomes weakened, that is kinda what the chart is showing... believe me, it confused me also, they should have added more clarifying information.

[@karensamscaldwell9149]

That is crazy

[@howardkettner]

hahaha Everyday on this property has been a learning day, but I wouldn't trade it for anything. Great to hear from you as always!

[@metricdeep8856]

Maybe tension rather than shear is their application. 16 penny nails would probably work. I bought ($110) of FastenMaster stuff today.....They need a proprietary bit to drive them. Can't use Torx. What a joke. Can't even buy the drive bit from HomeDepot. No one makes what the consumer wants. I repeat...No one makes what the consumer wants. NO ONE meets the consumers needs anymore. NO ONE. My dollars will remain in my pocket until this changes.

[@howardkettner]

I would agree with you - likely a tension application rather than shear. You and I have had the same experience. Bought a box of non-GRK screws and had to visit my Snap-on Dealer for a bit. Couldn't readily find it elsewhere. Thanks for tuning in.

[@es2709]

Give em hell!

[@jstephan03]

Tensile strength and shear strength are two different failure modes. Your failure is tensile related, not shear. Better rework your math.

[@howardkettner]

Thanks for tuning in and pointing me in the right direction towards a complete and permanent solution. Sounds like I've got some more work to do!

[@willysnowman]

No ductile flex. Breaky breaky.

[@howardkettner]

Yeah - knowing what I know now, I would have sought out opinions other than those of my 'professional bridge builder' and gone a completely different route.