Freestanding Concrete Foundation - Shear Angle of Resistance

The information that you provided is grossly insufficient to give you a concise answer.

You need to provide much more information to us, such as:

1. What are the dimensions of the concrete footing? We need the footing diameter and depth.

2. Is there a concrete pier attached to the top of the circular footing? We need the dimensions and shape: height, diameter (if round) or side dimensions (if square or rectangular).

3. We need the height, shape and diameter of the attached antenna. We also need it's overall dead weight.

4. We need to know where in the world you are located in order to calculate the design wind speed and design wind force acting on the antenna, the concrete pier (if present), and the side of the footing.

5. We need to know the characteristics of the underlying soil mass obtained by geotechnical laboratory testing and analysis to obtain the soil fiction value (for determining resistance against sliding) and the design bearing capacity (to compare the combine pressures due to overturning + gravity load).

The "shear angle of resistance for the underlying soil" doesn't enter into the analysis and design of this foundation, so exactly what do you mean by term?

Finally, why a mobile antenna foundation in the first place? How in heck are you going to pick-up this concrete monstrosity and transport it? Why not drive a steel pipe of adequate strength and size down into the ground to a required proper depth and just slip the base of the antenna into the pipe...much easier and cost effective.

Oppps, I forgot to add the following:

6. Need to know the weather conditions where you are located. Do you experience winter ice (freezing rain), sleet, and snow conditions ever?

Hi - thank you for quick reply

I have the information such as dimensions, force etc, but maybe I phrased the terms in my question a bit wrong. I haven't done any calculations on concrete structures so far - sorry about that.

The problem is that the foundation is freestanding and can be moved from time to time. Therefore I do not know the exact characteristics of the underlying soil, nor the placement.

Let me try to reformulate the question a bit:

I still want to verify the bearing capacity though - if possible, and my initial approach is to use Terzaghi's formula.
My major problem is maybe, that I find it hard to know the difference between drained and undrained bearing capacity. I have seen an example of Terzaghi's formula used on undrained condition for foundation in clay.

And because the drained condition needs an angle of friction of the soil (and not shear angle of resistance as i mistakenly wrote), I was looking for a good conservative estimate .. But maybe I can just assume undrained condition for a conservative guess of different grass fields ?

Hope I have clearified the problem a bit. Thank you anyway

Hello srosendal,

You're very welcome. I'm assuming by your statements that you're an engineer of some sort, and that you are familiar with soil mechanics and structural design?

Using Terzaghi's theorem to check bearing capacity of your temp antenna foundation is a bit of overkill IMO, unless of course this is a very tall antenna or you are near buildings etc that could be potentially damaged in the case of a failure.

If you use a "Presumptive Allowable Soil Bearing Pressure" of 1,000 psf you will most likely be safe, unless you plant your antenna foundation on clayey soils, organic-rich soils, peat and the like. Make sure you place the foundation on well-graded, well- confined, and well-compacted gravely soils and you'll most likely be erring on the safe conservative side.

I don't know if your area experiences winter conditions or not, but if it does I'd include in my weight calculations the inclusion of ice weight loading on the antenna. You can find this type of loading, plus design wind loading calculations, within ASCE-7 standard. You'd be surprised how much ice really can accumulate on an antenna during the right matrix of weather circumstances.

Sum Paxial = antenna dead load + ice live load + foundation dead load

horizontal design wind loadings will very dramatically from the ground level to the tip of the mast, as well as the actual cross sectional shape of the antenna mast, but the aggregate wind loadings will be used to determine the Overturning Moment acting at the foundation base plus the design sliding force (ie, horizontal reaction). If you're employing antenna glide wires, then you only need to determine what the magnitude of the design horiz. wind reaction is and compare that to the presumptive sliding resistive value of the soil, using a minimum SF=2.0. You could use a conservative soil friction value between 0.25 to 0.30 for sandy and gravelly conditions, and much much less for organically rich soils...it'd be a good idea to find a soils or foundation design textbook to nail down the friction values for various soil types/

rho (max., min.) = [Sum Mwind * c / I foundation] +/- [Sum Paxial/A foundation]

Just curious, but why the need to have a temporary antenna in the first place?