Loading/Testing

[Random Human]

This forum will be dedicated to Loading/Testing the tower.
With this being said, my first question
As there are multiple "speeds" for how fast you want the sand to be released, what is the best and why?
Couple of thoughts
slow is bad because the tower is under stress for a longer amount of time
fast is bad because there is high stress on the tower because the sand is pulling down more at a faster rate.
Any answers?
Thanks

[SPP SciO]

I think that what's more important than speed is keeping the bucket perfectly still. Your tower was designed to support a static load - any swinging of the bucket will unbalance those forces, leading to premature breakage.

Most teams I've seen use the slow-and-steady method to start, and then accelerate once the bucket is steady. Definitely important for the non-pouring partner to use those stabilizing sticks as permitted by the rules.

I'm wondering if any teams have tested their tower on a hydraulic press. My rudimentary knowledge of physics suggests the tower should hold up just the same, whether you "pull" from the bottom with a bucket of sand or "push" down from above. My hypothesis: the hydraulic press would give you an accurate maximum strength of the tower, and the bucket-loading result would be a certain percentage of that value, depending on how skillfully it was poured/stabilized.

[Balsa Man]

As a practical matter, I think both hydraulic press and hanging bucket will come out very close, in terms of measuring the load at which the structure fails, assuming you control bucket movement to avoid introducing …..off-axis forces. The hydraulic press will/would give you the exact load at failure; the bucket will give you a bit higher result.

The forces the tower sees will be the same, whether the load is hanging from the center of the load block, or pushing down on the center of the load block. With bucket movement controlled/prevented, the only difference in measured max load carried will come from two things: first, reaction time (letting go of the loading lever when failure happens) will mean some additional amount of sand will be let out, and will be in the bucket. Second, if you look at the force on the bucket instantaneously – while there’s sand in the bucket, and a stream of sand falling into the bucket- what’s going on? Once there’s oh, a kg or two of sand in the bucket, the major component of the (downward) force of the bucket is gravity acting on the sand (+bucket). The other downward force comes from the kinetic energy of the falling sand stream. That depends on a) the flow rate (mass per unit time), and b) the distance from the sand release point to the surface of the sand in the bucket. When the falling stream hits the sand surface, its energy is transferred to the bucket. The math to calculate that force …..exists, but its pretty complicated/advanced; I can’t tell you what it is.

But, given what’s going on, I/we can say this: At any point in time/instant when sand is falling into the bucket, the force/load on the bucket is… a little more than just the weight of the sand. At the force at which the tower fails (assuming instantaneous stopping of the falling sand stream), the force from the weight in the bucket is a bit lower than the failure force. Once the falling sand is in the bucket, the weight of the bucket is a bit higher than it was when the tower failed. Again, ‘as a practical matter’ these differences cancel each other out, so that weighing the bucket gives you a number (load carried at failure) pretty close to what the tower was seeing at failure.

So, yes, the approach when loading of starting slowly to….settle the tower, and get the bucket stabilization sticks in-place and working, then smoothly increasing the sand flow to wide open is the optimal way to go. Going slowly, when you’re near the max load the tower will carry, will get you a lower score, because, yes, a load that’s just shy of the max the tower will carry will, given some time, likely cause failure. With the sand flow at max, and being careful to let go of/shut off the sand flow immediately upon failure, the ‘load carried’ the judges will measure (by weighing the bucket +load block and chain) will be slightly above what was in the bucket at the instant of failure)- but, that will (should be) consistently the case for all competitors (that do/are seen to cut flow immediately/quickly). If you’re….clearly slow to shut the flow, the judges should remove what they perceive to be the ‘extra’ amount of sand you allowed to flow (and because it’s a judgement call, you may get a lower, or higher load carried than “actual”). The other aspect to consider, given the physics of loading, and realities of the load measuring process, is that if your tower carries/is scored on full load, you are giving away points- you only get credit for 15kg (or 17kg if meeting the 29cm circle bonus- which, as discussed before, you really want to do- it does/will pay off). So, what you really want to maximize your score is failure at 14.9…. .95…. .98… .99…

[dholdgreve]

I've coached "structures" for 17 years... During this time, I've seen kids try to push the bucket back and forth so that the sand is evenly distributed in the bucket. I've heard coaches specifically tell their kids not to let the bucket tip as the sand is loaded. I've seen kids load so slow that I could beat them with a dixie cup. All of these are incorrect methods. The bucket is connected to the tower (or bridge, or Boom) via a single point... a chain. If the bucket tips to one side or the other, do you think that one side of the chain carries more load than the other? Of course not. If you push the bucket to one side or the other, it will torque the eye bolt and cause an unbalanced load, but so long as the chain is allowed to hang perfectly vertical, the load will remain balanced (as long as the eyebolt itself is vertical, which is a function of the the top of the tower being level as well as the table it is being tested on. The rotation of the bucket has the potential of creating a very small dynamic addition to the load, but to try to keep the bucket from rotating will add way more load.

IMHO, the correct way to load any structure, begins with proper set up...
1) make sure the loading block is positioned perfectly in the center. For towers, site from column to column, caddycorner (at a 45 degree angle, and make sure the eyebolt is in the line of site, then look at the other diagonal and do the same. When the eyebolt is perfectly within the line of site of both diagonals, it is centered.
2) Hang the bucket high enough that if it rotates of tips a little, it won't touch the floor, but not so high as to interfere with the loader, or touch anything.
3) Now go back and double check that the block remained centered.
4) begin loading slowly... don't slam the gate open... this can cause the sand to hit with a thump, causing the loading system to "bounce, and throw the loading block off center.
5) Over the course of 20 to 30 seconds, gradually open the flow to wide open.

If you are loading slowly and the tower breaks, the event supervisor will almost certainly take out more sand than he should have. If you are wide open, the reverse is true. Wide open works to your advantage. Towers, as with all structures, have a "lifespan." Old English castles may last a 1000 years. American homes may be good for a 100 years... Seems like baseball stadiums are only good for 30 years. Roads? Maybe 10 years... You get my point. People typically only consider weight of the tower, and load held... We should be considering duration as well.

[dholdgreve]

I think there is a very subtle difference between a gravity load versus a hydraulic press load. I think hydraulic presses are generally "fixed in the X and Y planes, and are only movable in the Z plane, unlike a loading block, that is free to shift left or right, front or back, should a brace or column fail near the top.. I've seen a number of towers fail with the top end of a column (or 2) doubling over. The telltale sign of this is the loading block falling off to one side, and strafing all the bracing down one side of the tower. I don't think you'd see this kind of failure with a hydraulic press, as it has no opportunity to slide to the side....
Also, with a press, there is less chance of not having the load perfectly centered. With a loading block, there is inherent error centering the loading block.

My guess is that you would see between 10 and 15% higher load results on a hydraulic press that a free hanging loading block, but it's just that... a guess.

[Balsa Man]

Point well taken on the freedom of the load block to move in X and Y directions.

[baker]

Great topic... I've always taught start slow (not painfully slow) to get the 'bed' set and then gradually open to full. "Get it all in there before it breaks". Because we usually build and test several bridge, tower, booms (BTB) the team is comfortable with the pour. One observation we have noticed thru the years is the (BTB) doesn't really start to stress until half full, from that point on the real action begins. Git er done....

[Random Human]

Thanks for replying, some interesting insights.
If the bucket is tilted, there is no off-axis compression, in my opinion, because as the sand is falling, gravity pulls the sand toward one side of the bucket. The momentum from the sand coming out does push the bucket putting stress on the tower.

[BuildingFriend]

Personally, I feel that speed is more and more important since the switch from Bridge Building to Towers. You now have to thread the whole chain through the tower, center, place the bucket and s-hook, recenter, and then start loading. We have towers dedicated to practicing loading the chain in under a minute as that takes up so much time. Starting slow is hard to do now and can be compensated by lowering the height at which you start pouring (PE=mgh) and making sure your partner is stabilizing really well. Usually when we max the time is around 5:30 ish so some leeway but quite difficult. A word of caution- the faster you load the more prone you are to tipping and/or touching the bucket. If you do so at any time, you are at the mercy of the supervisor disqualifying any further sand due to you "supporting the tower" or you knocking the bucket and making the load move and thus breaking the tower prematurely. Best of luck!

[dholdgreve]

Thanks for replying, some interesting insights.
If the bucket is tilted, there is no off-axis compression, in my opinion, because as the sand is falling, gravity pulls the sand toward one side of the bucket. The momentum from the sand coming out does push the bucket putting stress on the tower.

As far as I know, gravity only works in one direction... down. I have not yet seen a SDU (Sand Dispensing Unit) that flows so fast at wide open that it "pushes" the bucket and chain off it's vertical axis. I have seen buckets tilt or tip, but as I stated earlier, as long as it is suspended by a single member (chain) it is not an issue. It cannot pull harder on one side of the chain than the other. I think we are on the same page here, but I was a little confused on your wording.