Why can’t I place foundations in a slope at different levels?
KeyPOST will accommodate foundations in slopes but can’t be used to model or suggest foundations at different heights. There are good reasons for this.
It is not a simple matter to calculate the effects of placing large concrete masses uphill of another not least since in many cases ground conditions have to be assumed. It is also rare that the ground conditions are homogenous, increasing the complexity, particularly in terms of likely failure modes of the slope. A full geotechnical survey should always be undertaken, and detailed design expertise sought on a case-by-case basis.
In addition, wind forces on an uphill foundation, and the smaller bending moments resulting due to the shorter sign supports, can be transferred disproportionately to the downhill ones. This introduces a rotational effect on the structure that can result, for example for plank constructed signs, in failure of interlocking channels and a ‘fanning’ effect of the sign face, as well as rotation on the concrete blocks. BS EN 12899-1:2007 ducks the issue and gives no guidance, nor does the excellent IHE Sign Structures Guide which covers foundations in some detail.
In many cases it is still valid to place the foundations with different ground cover to avoid these rotational effects, excepting some passively safe systems, but the cases where foundations are required at different heights, along with other limitations of KeyPOST such as gantry signs, should be properly handled by a qualified structural engineer and should fully address known ground conditions.