I am in need of help again!! I am trying to design separate spread foundations for a large sign (3 spread foundations, 1 for each post). I have been following the guidance using the Sign Structures Guide 2010 and Eurocodes but I am unsure whether to carry out the calculation for 1 sign foundation and combine them. Can anyone point me in the right direction? Thanks again.
Provided the posts are positioned symmetrically and the ground is level, you could just divide the wind load by 3 and design each separate foundation for that.
In theory the middle post is subjected to a greater load, but in practice it is usually convenient to make the 3 separate bases all the same size, on the assumption that the sign structure will deflect slightly under extreme loading to redistribute the load in an efficient way. Each separate base should be designed to support the whole of the point load that could be applied to any point on the sign.