Structure and support · Foundations & Ground
Pad Foundation System
This entry explains the pad foundation as an assembly of pad, formation, base connection and the ties that link pads to one another, and sets out what an owner should discuss with the structural designer before column positions are fixed.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What pad foundation is
A pad foundation sits beneath a single column or post and carries a dual duty: it spreads a concentrated load into the ground, and it provides the point at which the member above is anchored. Because the load arrives at a point rather than along a line, the spreading works outward in all directions from that point, which is why a pad is usually squarish in plan unless something forces it to be otherwise.
The distinction from the nearest sibling, the strip foundation, is visible before anything is built on it. If what stands on the foundation is a single column or post needing a connection to hold it in position, the foundation is a pad. If the thing above is a continuous wall that simply bears down along its length and needs no connection at all, it is a strip. The connection itself belongs to the frame entries, not to this one.
Pads are rarely designed as isolated objects even though each one is isolated in the ground. Ties or ground beams between them, the way the ground floor is carried around them, and the assumption the frame designer has made about whether the base is held rigidly or free to rotate, all change what an individual pad has to be. Identical columns on identical ground can therefore end up on quite different foundations.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- pad footing
- isolated footing
- isolated spread footing
- spread footing
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Spread the concentrated load of a single column or post into the ground over a bearing area larger than the member itself.
- Provide a stable and accurately positioned surface to which the base of that column or post can be connected.
- Resist whatever the frame designer asks of the base, whether that is downward load alone or also overturning and uplift.
- Act with the other pads, where ties or ground beams are provided, so the foundations behave as a set rather than as separate points.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
Order is not meaningful in this system. These roles interact as parts of one whole rather than stacking up in a sequence, so the list below is not a build-up and nothing should be read into the order it appears in.
- Primary supportPad base
The block of concrete that receives the column load and spreads it into the formation. Its plan shape follows the way load arrives - regular under a simple column, elongated where the pad has to be offset from a boundary or shared between adjacent members.
- SubstrateBearing formation
The prepared ground surface each pad is cast against. Every pad sits on its own patch of ground, so variation across a site shows up as differences between pads instead of being averaged out along a continuous element.
- SubstrateBlinding and separation layer
The thin working surface placed over the formation before reinforcement arrives. It is placed so that reinforcement is not set down on soil, so there is a clean face to set out from, and so the formation is not worked over directly between excavation and casting. What it achieves depends on how it is placed and on what happens on it afterwards.
- Primary supportReinforcement mat
Steel arranged so the pad can carry the bending that spreading a point load creates, with further steel where the base has to resist overturning. Its layout follows what the connection above it is asked to do rather than the size of the pad alone.
- AttachmentHolding-down assembly
The cast-in bolts, anchors or sockets that fix a baseplate or post base to the pad. This is the only part of the foundation that has to line up with something fabricated in a workshop, which makes its position the least forgiving setting-out in the assembly.
- Primary supportTie or ground beam connection
Where beams span between pads, the pad receives them as well as the column. That changes its reinforcement and often the pad itself, because a beam brings load and restraint from directions the column standing on it never applies.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
What the frame assumes about the base is what the pad has to deliver. If the base is treated as free to rotate, the column is being held upright by bracing elsewhere and the pad mainly receives downward load. If it is treated as rigid, the pad and its holding-down assembly are what keep the column standing, and both grow accordingly. The same column on the same ground does not always produce the same foundation.
Load reaches the pad through a small contact area before it spreads, so whatever occupies the space between baseplate and concrete is part of the load path. Packing that leaves part of a plate unsupported drives the whole load through the remaining contact, and a base bedded against a surface that was never brought to level transfers that irregularity straight into the pad.
Pads become a foundation rather than a collection of points only when something links them. Left independent, each settles according to the ground immediately beneath it and the frame above has to absorb the difference. Ground beams and ties share that difference out, and they also give the external wall and the ground floor something continuous to bear on in the gaps between columns.
Tolerance is where this assembly usually bites. Holding-down bolts are cast into wet concrete and have to meet a plate drilled elsewhere; correcting one that has moved is a structural repair, because the arrangement being altered is the one that transfers uplift and horizontal force. The connection detail, rather than the pad, therefore tends to govern how accurately the pad has to be built.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Primary support
The spreading block and its reinforcement are commonly encountered in these families. What a particular pad requires is set by the structural design for that column and that ground, and by the documentation issued with the products used.
Attachment
Baseplates, cast-in anchors and holding-down assemblies are commonly encountered in these metal families. Which is appropriate in a given exposure, and how it is protected where it enters concrete, is a matter for the designer and the manufacturer instructions.
Protection
Protective coatings from these families are commonly encountered on steelwork at and just below the base, where wetting and drying concentrate. Whether any coating is appropriate depends on the exposure assessed by the designer and on the product documentation.
Junctions
Where this system meets others
Interfaces are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves a junction is a detail designed for the specific building — not a rule of thumb.
Steel column base
The baseplate arrangement sits between frame and foundation and belongs to the frame design, while the anchors it uses are cast into the pad. Fabrication tolerance and cast-in tolerance meet here, and neither party can adjust the other after the concrete has set.
Read about Steel Frame →Timber post base
Timber posts are commonly held clear of the pad on a metal base so the end grain is not standing in water collecting on the concrete. How that base is fixed, and how water is kept from ponding around it, is part of the same detail rather than an afterthought.
Read about Post and Beam Frame →Portal frame base
A portal frame delivers horizontal thrust as well as vertical load at its bases, so the pads and any ties between them are resisting the frame spreading. The base condition assumed in the frame analysis has to be the one the foundation actually provides.
Read about Portal Frame →Floor slab passing around pads
A ground floor cast around isolated pads meets a stiffer element at each column, and whether the slab is separated from the pad or continuous with it changes where cracking is likely to appear. Barrier and insulation layers have to negotiate the same obstruction.
Read about Ground-Bearing Slab →Stability route to ground
Bracing, shear walls and moment frames all have to finish somewhere, and where they finish on pads the foundation is receiving forces unrelated to the weight above. Which pads are part of the stability route is a question for the structural designer.
Read about Lateral Stability →
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Movement
- Each pad responds to its own patch of ground, so differential settlement between columns is the characteristic risk rather than uniform settlement of the whole. Ties, ground beams or a stiffer frame are the usual responses, and choosing between them is an engineering decision.
- Durability
- The base of a column is a wetting and drying zone where water from the floor, from rain and from cleaning collects. Detailing that lets the base sit clear of standing water, and protection of the embedded metalwork, are the durability questions that recur here.
- Buildability
- Isolated excavations, each needing its own formation and its own set of cast-in items, are more numerous and more error-prone than a single continuous trench. Accurate setting out matters more here than anywhere else in shallow foundation work.
- Interfaces
- A pad is defined by what it connects to. The frame above, the ties beside it, the slab around it and the drainage below it all impose requirements, and the pad tends to be sized by whichever of them asks for most rather than by the column load alone.
- Documentation
- Setting-out drawings, the assumed base condition and the record of what each excavation was founded on are what allow a later alteration to be assessed. Without them, adding load to an existing frame means investigating pads individually.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- Is each column base assumed to be free to rotate or held rigidly, and does the foundation reflect that assumption?
- Are the pads tied together, and if not, what is absorbing the settlement difference between them?
- How does the ground floor negotiate each pad, and does the slab bear on it or stay separate from it?
- Who is responsible for reconciling the cast-in anchor positions with the fabricated baseplates?
- Do any pads carry stability forces from bracing or a moment frame as well as vertical load?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- A pad is not simply a short piece of strip foundation, because it has to receive a connection and to spread load in every direction rather than sideways only.
- Making a pad larger does not compensate for a base connection that was designed on a different assumption about how the column is held.
- Isolated pads do not make a foundation on their own; whether they behave as a set depends on what links them and on the frame above.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What base condition did the frame design assume, and what does the foundation have to resist as a result?
- What ground was each pad founded on, and were the excavations inspected before concrete was placed?
- Are ground beams or ties proposed between pads, and what are they there to control?
- How are the cast-in items positioned and checked, and what is the process if one is found out of place?
- How is the steelwork at and below the base protected against the wetting it will see in service?
What this page does not do
- Pad sizing, reinforcement and the base connection are structural design matters for a qualified engineer working with the frame designer.
- Nothing here states any bearing capacity, pad size, anchor arrangement or tolerance, because all of them depend on the ground and the frame in question.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Alternatives
Different systems answering the same need. Listing them together is not a comparison and does not suggest one is better — which, if either, suits a project is a design decision.
Meets these systems
Systems this one physically meets. The junction is usually where the design problem lives, so these are worth reading together.
Commonly built alongside
Systems routinely present in the same building without necessarily touching this one.
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Inspiration
Related Ideas Library pages
Design directions where this system commonly appears.
Foundation and Ground Interface Systems
Assemblies where a building first meets undisturbed ground — strip, pad, raft and piled foundations, pile caps, ground improvement and ground gas protection.
Browse all foundations & ground entries →