Calculate Grillage Foundation axial capacity

[Adam0917]

Hello forum members,

I'm currently working on analyzing a 30-years old telecommunication tower. The tower was built on top of a hill so the original tower designer used a grillage foundation (details can be found in my attachment) instead of spread footing. In addition, instead of using concrete to fill the voids between beams, designer used sand.
I'm just wondering if there's any guide/code or design standards to follow in determining the capacity of such a type tower foundation.

I searched here and there, found an old post from this forum back in 2003 discussing the grillage foundation:

https://www.eng-tips.com/viewthread.cfm?qid=35439

In addition, I found one of our forum members posted a link to a collection of publications. One of which provides useful information, but it was published back in 1923 (page 252).
Here is the link:

http://www.slideruleera.net/miscellaneous.html

Simply by using the allowable soil bearing, I know the foundation is good. However, are there any other factors I should consider since the foundation has been placed into the soil for 30 years.... (corrosion for example)

Please provide me with suggestions and guidelines (if any) to use. Any input would be greatly appreciated!

Adam

 

[atrizzy]

Typically, if the slats of the grillage are spaced properly, you can rely on the gross footprint of the grillage, same as you would a footing.
Same goes for uplift, in my experience.

 

[Adam0917]

Thanks Atrizzy,

From the drawing layout, 1st tier beams have a spacing of 153mm, which is okay as per the publication I mentioned in my post.
There's a concrete pad below it so I would check the bearing using the same method as I would for normal footing fnd.

So I would assume the same thinking can be applied to the deadman anchor foundation too? My tower has grillage foundation for anchors too.

 

[atrizzy]

Your deadman anchors are in tension, so while you have a bearing area, your upwards and lateral component resistances would typically relate to the soil weight above and passive soil pressure, respectively.

 

[Adam0917]

But is the grillage type anchor foundation would engage the soil the same way as the concrete deadman? In my case, the beams are not covered by concrete but just sand.

 

[atrizzy]

Well that may depend somewhat on your soil types, but generally, yes. Ask your geotech what kind of vertical and horizontal soil pressures you should be using for the deadman anchors.

 

[Adam0917]

Alright, thank you atrizzy!