strength calculation - need point of view 1

[kingnero]

would like to fasten the frame (see attached drawing) to a concrete block.

But in my calculation, I don't need the height of the pillar. I am overlooking something. Can someone take a look at it, and tell me what I'm doing wrong?
btw, this is my responsability, this information I receive will not be used for anything that might result in personal injury.

Excuse my english, I'm european (belgium) hence the dimensions and weights are also in kg and meter.

My point of view: the weight at the hook will result in a momentum: 250 kg * 9.81 m/s² * 3.5m = 8600 Nm

Will this momentum result in a force at the two opposite bolts: F = 1/2 * 8600 / 0.3 = 14.333 N ?

Tensile area of a Metric 16 bolt = 157 mm²
stress in 1 bolt = 92 N/mm²
yield stress of such a bolt (grade 8.8) is 640 N/mm².
stress should be limited to about 1/3 of the yield stress (correct?), but still 92 N/mm² < 640/3 (213) N/mm², and I still have a safety factor of bigger than 2.

Can anyone tell me why I didn't use the height of the pillar? Where did I miss something?

attachment:

 

[hokie66]

The height has nothing to do with the moment (not momentum). Your calculation is basically correct, although you should include the mass of the jib boom, and don't forget the hoist. I would use a smaller lever arm for the base plate, say 200 rather than 300, as the bolt on the compression side is not the resisting element. Without the boom and hoist, this would make your bolt force 21.5 kN, well within the capacity of an M16 8.8 bolt.

If this is a slewing jib, you will also have to check the bolts when the jib rotates, but in view of the small force, that will probably work as well.

 

[desertfox]

Hi kingnero

As already stated you should include the mass of the components, anyway I have uploaded a file on the way I would calculate the maximum bolt loads and my figure is slightly different than yours at 13.36kN.
Again the height in your case is not important except in terms of adding more mass to the components.

desertfox

 

[kingnero]

Thanks for both your replies.
Your point of the lever arm being shorter than 300 mm is indeed correct, I did not think of that.

I will take mass of the components into account, however it will only be a minor factor as the boom is profiled metal (not an I-beam), and the hoist weighs maybe 5 kg at most.

Can someone please elaborate on the second part of my question?

Tensile area of a Metric 16 bolt = 157 mm²
stress in 1 bolt = 92 N/mm²
yield stress of such a bolt (grade 8.8) is 640 N/mm².
stress should be limited to about 1/3 of the yield stress (correct?), but still 92 N/mm² < 640/3 (213) N/mm², and I still have a safety factor of bigger than 2.

 

[desertfox]

hi kingnero

Your latter part of your question looks okay to me except that there is no need to limit the stress to one third of the yield stress, although you do need a safety factor so your on the right side of caution using the value you have chosen. Remember also that your bolt pre-load should be greater than the external load acting on them otherwise your joint won't hold.
When you account for the mass of the components remember to calculate the centre of gravity for the combined parts as it will likely not be in between or even central between the fixing and you will have to treat it like an offset load like your first part.
Also if you consider the edge of the plate to be the pivot point for over turning it will be slightly larger than 300mm as per my uploaded file.

regards

desertfox

 

[hokie66]

The permissible (working load) capacity of an M16 8.8 bolt is about 58 kN, in accordance with my old Australian safe load tables. So you have plenty of capacity no matter how you figure it. Personally, I would use M20 grade 4.6 bolts, with a permissible tensile capacity of 35 kN. Grade 4.6 bolts are more readily available as anchor bolts.

 

[kingnero]

Could someone please elaborate the bottom plate pivoting point?
If I understand both your replies correctly, Hokie66 says it should be shorter, where desertfox says it's slightly longer, upto the edge of the plate.

Drilling the holes is't really feasable, as no hand drills will go up to 20 mm, and there is no way I can mount the pillar+baseplate in my milling machine. If anything, I was thinking about searching for grade 12.9 M16 bolts. They can be found over here.

 

[hokie66]

Where the compression resultant would be depends on the plate thickness, and you didn't ask us to determine that. The thicker the plate, the longer the lever arm. I was being conservative and limiting the lever arm to the distance from the tension bolts to approximately the edge of the post.

I thought you were planning to use M16 bolts. What size are the existing holes? I just suggested using grade 4.6 bolts because they are more common for anchor bolts, and the development in the concrete will be the controlling factor anyway. M16-8.8 bolts should work fine, provided you can anchor them to take the load.

 

[kingnero]

Plate thickness is 15 mm.

Holes are about 18 mm.

Anchoring will be done using metal cilindrical pieces, drilled and tapped for the M16 bolts, welded together at the correct spacing, and metal bracing (also outwards) will be welded on them at a depth of about .4 meters.

This will be fixed in a hole about 1 by 1 meters of size, and over .6m of depth.

When fixed down at the correct positioning, concrete with metal fibres will be poured into the hole.


That should be strong enough even if I ever wanted to use a stronger hoist. In that case, I just might need to drill out the holes and tap a bigger thread, and make sure I will have the same baseplate hole spacing.

 

[hokie66]

That block of concrete is not big enough to resist the overturning by itself. And I wouldn't advise depending on fibre reinforced concrete to resist the bending. Sounds like you need a structural engineer to design this for you. It's not a big project, but it needs to be done correctly.

 

[desertfox]

hi kingnero

I was using the edge of the plate as the pivot because in reality thats were it will pivot from, by using the edge of the plate you can also calculate what small proportion of load is in the front pair of bolts whereas your previous method is assuming they don't carry any load which is conservative.
It is not the thickness of the plate I was referring too but the width, your sketch shows 0.3m betwen hole centres however the material from the hole centre to the edge of the plate you didn't give so I assumed 20mm between the first set of holes on the compression side and the edge of the plate as detailed in my calculation.
Another advantage of the method I give you is that it works for any number of bolt rows ie:- you have 4 bolts spaced on two rows however if you had three or four rows with any number of bolts in a row my method still works but your original method cannot as you have more unknowns.

desertfox

 

[rowingengineer]

I agree with hokie in regards to the distance assumed for bending calculations. This will keep the plate to a reasonable size.

When in doubt, just take the next small step.

 

[desertfox]

hi rowingengineer

But there is no logical basis for using 200mm other then it gives a larger reaction in the bolt on that basis we could use 100mm or 50mm.
So lets say there are three rows of fixings every 150mm apart consisting of two bolts per row what then would you use for a distance in respect of the bending calculations.

desertfox

 

[kingnero]

That block of concrete is not big enough to resist the overturning by itself. And I wouldn't advise depending on fibre reinforced concrete to resist the bending. Sounds like you need a structural engineer to design this for you. It's not a big project, but it needs to be done correctly.

Can I get the necessary information here on this forum, or do you advise me seeking someone local (who I will have to pay) for this?
Again, if I can get the info I need here, I will take full responsability of the work done.
I can still easily expand the size of the hole (I still have to rent a bobcat to dig the hole), if that should be necessary.

 

[rowingengineer]

I would always take the moment from the compression flange of the column. The flange is where the compression force is, to take the compression force to the edge of the plate means a thicker plate.

When in doubt, just take the next small step.

 

[MariusLT]

Do not forget dynamic coefficient (kd=2). When lifting 250 kg, in calculations you should use 500 kg.
Also, for lifting mechanisms, the safety factor should be more than 2, depending on country regulations. I would use 4.
It is strongly recommended to check the concrete base plate for overturning, as someone mentioned above.

To make life easier (and ensure healthy night's sleep), I would:
- dig a hole 1,5x1,5x1 for base slab.
- install 4 J- shaped reinforcement rods L=1000 tapped M24, connected together at 500x500, into concrete.
- make a base plate 600x30 with cutouts (if not holes) for rods at spacing 500x500.
- When installing, one nut should be below plate, two nuts on top. This will help you level the crane. After leveling, you may cover plate and nuts with concrete.

 

[swearingen]

desertfox,

The "tipping point" in reality completely depends on the thickness of the baseplate, as mentioned by others on this thread. Your example of the 150mm spaced bolts is subject to this as well. If in kingnero's case the plate was extremely thin, it would tip at the compression flange of the lift (conservatively approximated by hokie66). If the plate is extremely thick, the tipping point could indeed be at the plate edge, thereby putting all bolts in tension.

kingnero,

I don't know what calcs MariusLT ran, but I get a soil pressure of about 57kPa which is fairly low and should work in most soils. You don't, however, need J-shaped rods (the J has shown to be ineffective in testing, use a nut on the end of a straight, 0,8m rod). Otherwise, everything else seems to be in order from this cursory look, but I'd still try to enlist a local structural guy just to make sure. You don't have any structural friends that you could ask a favor of?


If you "heard" it on the internet, it's guilty until proven innocent. - DCS

http://www.eng-tips.com/supportus.cfm

 

[kingnero]

I am very grateful for the replies so far.
I think I will be able to tackle this further.
I do not have friends with the structural kind of knowledge, I work as an engineer in a metal-processing factory (mostly CNC machinery), where there is no structural engineer.

But with the replies so far, I think I will be able to do this correctly. Better too big than too flimsy. and after all, I will never need the max. capacity of the hoist, nor the max. length of the boom, it's merely for unloading bags of charcoal from a small trailor.

 

[desertfox]

hi swearingen

The original post I responded to gave no dimensions of baseplate thickness and the calculation the OP presented also assumed the baseplate was rigid.
The original question was the how to calculate the size of bolts, in that sense assuming the baseplate is rigid and transferring all the load onto the bolts makes the calculation conservative in the respect of bolt size.
However I appreciate that the baseplate thickness is important and since more information as come to light the thread is taking a new twist.

desertfox

 

[FeX32]

If you make the baseplate too thin it will fail...
Size it and then use desertfox's calcs as a start.


Fe