Foundation reactions are where a takedown ends: the total load each column and wall delivers to the ground. They are the bottom row of the takedown, the numbers the foundation and geotechnical engineers actually use. A takedown produces them by accumulating the floor loads down every support, and you pass the service values to the soil check and the factored values to the footing design. StructLoads reports both for every support.
What a foundation reaction is
A foundation reaction is a support’s load at the lowest level, the accumulated structural load of every floor it carries, arriving at its footing. For a column it is a point load; for a bearing wall it is a line load per metre. It is the destination of the gravity load path: everything that started as a pressure on the floors concentrates here before spreading back into the soil.
Reading the reactions
The base reactions are usually tabulated, one per support, alongside the total.
| Support | Service reaction | Factored reaction |
|---|---|---|
| C1 interior | 819 kN | 1070 kN |
| C2 edge | 410 kN | 535 kN |
| Sum of all | equals total applied | drives the footings |
The sum of the service reactions is the most useful single check: it should equal the total load applied across all floors. The interior to edge ratio gives a second quick check, the same one used throughout the column loads.
Service for the soil, factored for the footing
A foundation reaction is used twice. The service value sizes the footing plan area against the allowable soil bearing pressure, because the soil feels the real, unfactored pressure; as FHWA’s Soils and Foundations reference manual sets out for shallow foundations, the allowable pressure is the ultimate bearing capacity already divided by a factor of safety, so the margin lives on the soil side and factoring the load again would double-count it. The factored value designs the footing’s own thickness and reinforcement. This is the same split detailed in how to calculate footing loads, and it is why a takedown that reports only one value forces you to recompute the other.
Adding the local weight
The takedown reaction is the load from the superstructure. To get the full footing load you add the self weight of the column and the footing itself, a small but real addition, in line with general foundations practice. For a shallow foundation this self weight is minor next to the column reaction, but it should not be dropped, especially for lightly loaded pads.
The equilibrium check
The strongest check on the whole takedown lives here: the sum of all foundation reactions should equal the total load applied on every floor, because force is conserved. If the floors carry, say, 6000 kN in total and the base reactions sum to 6000 kN, the takedown is in equilibrium. A mismatch means load was lost or added somewhere, and it is the first thing to verify before trusting the reactions, as in how to check a load takedown.
Key takeaways: foundation reactions
Foundation reactions are the base support loads a takedown produces, the load each column and wall delivers to its footing. Use service values for the soil bearing check and factored values for the footing strength, and add the local column and footing self weight. Their sum equals the total applied load, the key equilibrium check. StructLoads reports both as preliminary figures a qualified engineer and a geotechnical assessment confirm.
Quick answers
What are foundation reactions in a load takedown?
Foundation reactions are the support loads at the lowest level of the building, the total load each column and wall delivers to its footing. A takedown produces them by accumulating the floor loads down every support to the base. They are the end product of the takedown and the starting point for foundation design, given as both service and factored values.
How do you turn takedown reactions into foundation loads?
Take each support’s base reaction and add the self weight of the column and footing. Use the service reaction to size the footing plan area against the allowable soil bearing pressure, and the factored reaction to design the footing strength. The takedown gives the column reaction; the footing calculation adds the local self weight and the soil check.
Should foundation reactions be service or factored?
Both, for different checks. Sizing the footing against the soil uses service reactions, because soil bearing is a check on real pressure. Designing the footing’s own thickness and reinforcement uses factored reactions. A good takedown reports both, so each foundation check uses the correct value rather than one number for everything.
How do you check the foundation reactions are right?
Sum them. The total of all foundation reactions should equal the total load applied on every floor, because force is conserved on the way down. If the sum of the base reactions matches the sum of floor pressure times floor area across all levels, the takedown is in equilibrium. A mismatch means load was lost or added somewhere.