Wall Structure

While I was starting some structural checks on the wall I realized we have another unanswered question. How much flex is tolerable in a climbing wall? We have all seen this: a boulderer in the gym lunges to a jug and everyone cringes as they see the whole wall bow perceptibly. The climber has no idea the hold moved.

A steel frame, like the one for the prototype, will be able to flex a great deal before you are anywhere near permanently deforming or breaking the frame. So the main concern when designing the structure is how much flex should we allow. I believe that if the climber doesn’t notice the flex then it should be fine, and the less steel we use the better. How much do you think a climbing hold can move before it will become noticeable to the climber? Take the new survey and make your opinion heard. From some experimentation in Kyle’s garage I am taking 1/8″ as our maximum deflection.

All the images in this post represent an finite element analysis (FEA) of the entire steel wall frame. The first check I ran is shown below. This analysis shows the stress applied to the wall with loads representing the full ~300lbs of wood planks weighting on the wall, a 250lb climber hanging off the upper right corner of the frame, and a 30lb side load applied to that corner as well. This seemed like a reasonable but aggresive loading scenario to initially verify that the frame wouldn’t collapse under normal use, and looking closely we can see that there are no points of stress higher than ~5ksi. So we have a generous factor of safety against failure, roughly 9 assuming we will use 11ga, 44w steel. I had guessed that 10ga would be necessary but after some initial checks I reduced the thickness to 11ga. We could easily go thinner but 12ga saves surprisingly little in cost and 13ga just seems too thin.

load 2.JPG
Stress in frame

The figure below shows the expected deformation with just the climbers weight and the side load applied. This provides an estimate of how much the climber will see the wall move if they suddenly grab the top corner, because the bending due to the weight of the planks and frame will already be present. In this case, with the structural bits being 11ga steel, we will see ~1/32″.

load 1.JPG
Deformation of frame, with base stuck to floor

In this simulation the bottom of the frame is assumed to be stuck to the floor. This seemed to me like a reasonable assumption since the ~500lb weight of the thing should keep the base from sliding around. If the floor is assumed to be frictionless, the frame shifts on the floor as shown below and the top corner will move 1/4″ (twice our permissible limit) due to the side load, as shown below. (note that the deformation is not to scale, it is exaggerated for illustrative purposes) This is mainly due to the bottom of the frame deforming out a square.

t000000top.JPG
deformation of frame sliding on floor

The addition of a cross brace, below, will bring the deflection within the acceptable  range. However, I am inclined to leave the brace out and see if the prototype feels too wobbly and add it in if needed, because the unit will probably behave somewhere in between the fixed and the frictionless case. Spots like the top of the base frame might also require additional cross bracing when we finally get to pulling on a prototype and seeing how wobbly it feels.

t000000-2
deformation of frame with brace added to base

 

From these simulations it also emerged the lower members tying the two halve of the lower frame together don’t need to be formed angles. We can get away with them just being flat bar without significantly changing the deflection. This would be ideal as your crashpad wouldn’t be sitting on the angle making the landing surface uneven. This will also be cheaper since it eliminates 2 parts that require box bends, during forming. A process that requires a change of tooling, making them much more expensive than they would be otherwise.

t000000change.JPG
Base frame without and with the unnecessary flange
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