What moment can a poured concrte block deliver?
What moment can a poured concrte block deliver?
(OP)
I want to install a small jib on my private property, which will be used to unload my trailor. (bags of charcoal, small things like that).
I have an existing frame with baseplate that I will be using, in combination with an electrical hoist.
I have calculated all relevant things about the jib, (moment at the baseplate, bolts, ...) however I don't know how to design correctly the concrete block to which the assembly will be bolted to.
Please look at the attached sketch,
Could someone please point me in the right direction?
What I am trying to achieve is a way to calculate the necessary dimensions, so that the block will be able to resist the moment coming from the load.
Many thanks in advance.
I have an existing frame with baseplate that I will be using, in combination with an electrical hoist.
I have calculated all relevant things about the jib, (moment at the baseplate, bolts, ...) however I don't know how to design correctly the concrete block to which the assembly will be bolted to.
Please look at the attached sketch,
Could someone please point me in the right direction?
What I am trying to achieve is a way to calculate the necessary dimensions, so that the block will be able to resist the moment coming from the load.
Many thanks in advance.






RE: What moment can a poured concrte block deliver?
There are 2 things to check:
1. Resistance against overturning - take moment of resistance around the toe of the footing (i.e the edge closest to the load being lifted) and calculate this as dead load times the distance to the centre of the load. This should be at least 50% higher than the appliad moment.
2. Bearing pressure- any soil that is not obviously soft will take 50kn/square metre. This should be just a simple stress calculation of P/A + M/S but check that there is no tension otherwise you will need to use a more complex formula.
RE: What moment can a poured concrte block deliver?
If this is an existing foundation/slab, you can use expansion bolts or epoxy bolts. These are proprietary and the manufacturers have tables with load values and reductions for spacing and edge distance. If you want to use cast-in-place anchor bolts, have fun with Appendix D of ACI!
RE: What moment can a poured concrte block deliver?
If so, thanks very much, I will quickly have my dimensions from your information.
If I am not correct, please correct me...
Bearing pressure will be minimal, I have very small loads but I will check for it anyway.
@ a2mfk: I have calculated everything, I have chosen bolts, the mechanical side is fully covered. I only need to dig out the hole, and pour the concrete.
RE: What moment can a poured concrte block deliver?
RE: What moment can a poured concrte block deliver?
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: What moment can a poured concrte block deliver?
Yes, the moment you calculate in red should be more than 1.5 times the original moment you calculated.
You need to realise that at the base it is not the strength of the bolts that is critical but their anchorage into the concrete.
RE: What moment can a poured concrte block deliver?
This frame will be placed in the dug out pit, at correct height, properly fixated, than concrete (reinforced with fibers) will be poured over it.
I don't worry about pull out, as this frame is massively oversized.
@ msquared: The Jib rotates at the top, so I cannot place the base plate at one side. However I have taken a moment into acocunt with a very large COS, and I will take care when using the thing. For the safety purists: if it fails (which it most likely will not), I will be the only one underneath it.
I will also make sure the concrete base is large enough, more for the leverage than for the bearing pressure (which isn't a large %)
Thanks everybody for the helpful replies, I can go on with this now...
RE: What moment can a poured concrte block deliver?
RE: What moment can a poured concrte block deliver?
RE: What moment can a poured concrte block deliver?
RE: What moment can a poured concrte block deliver?
How close will your trailer be to the edge of your concrete footing. You may have to consider the loads from your trailer wheel on you soil pressures, too.
Also, add at least a small dynamic factor for load lifting.
Good luck.
Bob
RE: What moment can a poured concrte block deliver?
Good luck.
RE: What moment can a poured concrte block deliver?
Also, add at least a small dynamic factor for load lifting.
The jib is rated for 1.5 tonne at the end (= three meters distance from the vertical pole), so I did all calculations with that in mind, however the electrical hoist I have can only lift 500 kg. That means, IF i should ever use the full capacity of the hoist, I will still have a COS = three.
I am quite confident I will be on the safe side.
RE: What moment can a poured concrte block deliver?
*) I have no eccentricity (the jib will be mounted in the center of the concrete block).
*) Also no My moment (for the sake of keeping it simple, as the jib can only be turned towards one side at the time, and the footing will be square, so Mx = My)
Do I need to add the M-load (moment due to the load on the jib) and the M-block (moment due to the weight of the footing) in the formula, or not?
See attached sketch for further info.
Thanks one more time...
RE: What moment can a poured concrte block deliver?
M = Fload*e where e is the eccentricity of the load from center of footing. The footing has no moment about its own centroid.
A = l2 since footing is square
S = l3/6 since footing is square.
This is applicable if P/A >= M/S. Otherwise, an adjustment is needed. That is because you cannot have a tension stress on the underlying soil.
In this analysis, the available pressure on the side of the footing has been ignored which is a bit conservative, but okay.
BA
RE: What moment can a poured concrte block deliver?
RE: What moment can a poured concrte block deliver?
However, even when I leave that one out of the equation, M/S is still larger than P/A.
(see attached sketch, again)
You say: in that case, an adjustment is necessary. I think this means I need to design a larger footing?
However, see csd72:
1. Resistance against overturning - take moment of resistance around the toe of the footing (i.e the edge closest to the load being lifted) and calculate this as dead load times the distance to the centre of the load. This should be at least 50% higher than the appliad moment.
This condition is met. Would I still need a larger footing?
Mfooting=40 kN x 0.75m = 30 kNm which is larger than Mload=17.5 kNm x 150% => 26.25 kNm.
I am terribly sorry for all these questions, I can assure you I'm better in my field of experience as I can only imagine what you guys think of me now, having all those problems with a simple formula.
I have googled for this, and have found http://www.ce-ref.com/ftg_size.htm but there only eccentricity larger than 1/6 x footing length and smaller than 1/2 x footing length is given. I believe my problem is because my eccentricity is much larger than the footing size.
@ TXBRIDGEENG: Greatly appreciate your help, as I would like to understand this better as this really is a fascinating problem.
For your information, it is clayground, and a rather wet area as the groundwater level is not very deep. However as I am also constructing a large storage shed, I have hired a local engineer who will do the stability calcs for it (required by our local government). I will consult him for the allowable bearing pressure.
Thanks again for all your help.
RE: What moment can a poured concrte block deliver?
There are two parts to the calculation...Factor of Safety against overturning and soil pressure. In my earlier post, I was referring to soil pressure.
If M/S > P/A you do not necessarily require a larger footing. What you must do is find the effective size of your footing.
Working backwards;
L = 2*0.75 = 1.5m
Fload = 1.5 tonne = 1500 kg (mass) or a force of approximately 15 kN.
Fload occurs at 17.5/15 = 1.17m outside ftg.
P = 40 + 15 = 55 kN (neglecting weight of jib)
A = 55/2.25 = 24.4 kN/m^2
M = 17.5
S = 1.5^3/6 = 0.5625
M/S = 17.5/0.5625 = 31.1 kN/m^2 which is greater than P/A
Find eccentricity of load, i.e. e = M/P = 17.5/55 = 0.318 from center of ftg, or 0.75 - 0.318 = 0.432m from edge of ftg.
Effective footing is 1.5m x 3(0.432) = 1.5 x 1.29
Effective area = 1.94m^2
P/A = 55/1.94 = 28.3 kN/m^2
P/A +-M/S = 56.6 or 0 (triangular variation)
For clay soil, 56.6 kN/m^2 or about 1180 psf seems okay, so footing size is adequate.
BA
RE: What moment can a poured concrte block deliver?
Made a mistake in my last post.
Say F is load and W is footing weight.
I am assuming that F = 15 kN and W = 40 kN.
Length of jib arm = 17.5/15 + 0.75 = 1.917m (modify this if wrong)
P = W + F = 55kN (neglecting weight of jib)
c.g. of load @ 15*1.917/55 = 0.5227m from center of ftg. which is outside the middle third of the footing, so M/S > P/A.
Distance from c.g. to edge of ftg = 0.75 - 0.5227 = 0.2272m. Effective length of ftg = 3*0.2272 = 0.6816m
A(effective) = 1.5*0.6816 = 1.022m^2
P/A = 55/1.022 = 53.8 kPa
Max soil pressure = 2*53.8 = 107.5 kPa (2250 psf)
Min soil pressure = 0
The pressure may be okay if the clay is fairly stiff, but it is getting a bit on the high side.
BA
RE: What moment can a poured concrte block deliver?
The problem with P/A +M/s is that you will get tension in the soil if the M/s is greater than P/a and unlike steel, soil cannot take tension.
The result is that instead of having 2 stress triangles of opposite mmagnitude (one tension and one compression) you end up with one compression triangle and a section of footing beyond this that has zero soil pressure. The centre of the triangle will coincide with the centre of the reaction. The rest is just basic mathematics.
RE: What moment can a poured concrte block deliver?
I now understand that, if the case of Ptotal falls out ouf the middle third of the footing, the formula differs from the original:
It becomes: Qmax = 2x P/Aeff ; so the +-M/S part falls away.
It's not that hard, however it helps if someone walks you through that scenario.
Thanks again for your time and explications.
RE: What moment can a poured concrte block deliver?
Thats P/Aeff where Aeff is the width of the loaded area I mentioned above.
RE: What moment can a poured concrte block deliver?
I wouldn't say +-M/S falls away. It becomes 2*P/Aeff because M/S = P/A. Thus, P/Aeff + or - M/Seff becomes 2*Peff/A or 0 (triangular variation of soil pressure across effective length)
BA
RE: What moment can a poured concrte block deliver?
but:
S = width x length² /6 ; with length = 3 times the distance edge to c.g. of load (S= 1.5 x (3 x .2272)² / 6 = .11622 m³)
so
M/S = 17.5 kNm / .11622 m³ which is 150 kN/m².
So I still must be interpreting something wrongly.
@csd72: I understood everything up until Aeff (incl.), however I am having more problems with the second half of the formula, especially when I saw BA took the P/A part twice... didn't see that one coming!
RE: What moment can a poured concrte block deliver?
The moment of 17.5kNm is the moment of F(load) about the edge of the footing.
The c.g. of load occurs 0.2272m inside the edge of the footing. That means the eccentricity from the center of the effective footing is 0.2272/2 = 0.1136m. The moment you want is P*e = 55*0.1136 = 6.248 kNm and M/S = 6.248/0.11622 = 53.8 kPa which is equal to P/A.
BA
RE: What moment can a poured concrte block deliver?
The total load of footing, crane and load is P which occurs at the third point of footing, i.e. at B/3 from the edge.
A = L*B
S = L*B2/6
e = B/2 - B/3 = B/6 where e is the eccentricity of total load from the center of the effective footing
P/A = P/L*B
M/S = P*B/6 * 6/L*B22 = P/L*B
So P/A = M/S Q.E.D.
BA
RE: What moment can a poured concrte block deliver?
BA
RE: What moment can a poured concrte block deliver?
A = L*B
S = L*B2/6
e = B/2 - B/3 = B/6 where e is the eccentricity of total load from the center of the effective footing
P/A = P/L*B
M/S = P*B/6 * 6/L*B2 = P/L*B
So P/A = M/S Q.E.D.
BA
RE: What moment can a poured concrte block deliver?
kingnero,
The reason why you double it is because it is a trangular distribution and you maximum stress is twice the average.
Just remember that the centroid of the triangle is one third from the largest end and that this needs to coincide with the location of the applied reaction.
RE: What moment can a poured concrte block deliver?
I've tried this with varying loads and surfaces, just to practice this, and it went just as it should. I feel confident now to show my calculations to (obligatory by the government) hired engineer, to have him comment on the allowable soil bearing pressure.
I hope he won't make a problem out of this (as he is only paid to give his findings/calculations about the shed foundations to the government, not to me).
Thanks again for writing this up, I think this post qualifies excellently for a how-to or faq, for future reference for people asking similar questions. Maybe a thought for the moderators?