Load Path: How Buildings Carry Loads to the Ground

A load path is the route a force takes through a structure, from the point where it is applied to the ground that ultimately resists it. Every load, the weight of a person, a gust of wind on a facade, the self-weight of a slab, has to find a continuous chain of members and connections to the foundations. If any link in that chain is missing or too weak, the structure has to find another route, or fail.

Understanding load paths is the most useful single idea in structural engineering for non-engineers. It explains why some walls cannot be removed, why columns line up from floor to floor, and why seemingly small alterations can have large consequences.

The intuition: follow the weight downwards

Picture a book resting on a shelf. The book’s weight pushes on the shelf. The shelf bends and passes the load to its brackets. The brackets pass it to the screws, the screws to the wall, the wall to the floor, and so on down to the ground. Remove any one of those pieces and the book falls.

Buildings work the same way, only with more links. A typical gravity load path in a reinforced concrete frame looks like this:

Load on the floor is carried by the slab.
The slab spans to beams, or directly to columns in a flat slab.
Beams carry the load to columns or load-bearing walls.
Columns and walls carry the accumulated load down through each storey.
Foundations, such as footings, rafts or piles, spread the load into the ground.

The kinds of load involved are summarised in our guide to types of structural loads.

Gravity load paths

Gravity loads are the easiest to follow because they always act downwards. The most efficient load path is the most direct one: columns stacked vertically from roof to foundation, with each floor delivering its load straight down. Every deviation from that line costs material.

When a column above does not land on a column below, for example where a podium car park needs a wider grid than the tower above it, its load has to be carried sideways by a deep beam or thick slab before it can continue downwards. These are transfer structures, explained in our guide to the transfer beam. They work, but they are heavy, and they concentrate a great deal of responsibility on a few members.

Walls can be part of the gravity load path too. In many older houses and some apartment blocks, the walls themselves carry the floors above. Our guide to load-bearing walls explains how to recognise them.

Lateral load paths

Wind, and in some regions earthquakes, push buildings sideways. Lateral loads need their own load path, and it is often less obvious than the gravity one.

Wind pressure acts on the facade.
The facade transfers it to the floor slabs at each level.
The slabs act as stiff horizontal plates, called diaphragms, distributing the load to the vertical bracing elements.
Cores, shear walls, braced bays or rigid frames carry the load down to the foundations.
The foundations resist both the horizontal shear and the overturning effect.

The diaphragm step is easily forgotten. A large opening cut through a slab next to a core can sever the connection between the floor and the element that keeps the building stable. The structure may carry its gravity loads perfectly well and still lose part of its lateral load path.

A worked example

Consider a two-storey house with a 4 m long internal wall on the ground floor, carrying a first-floor beam and slab. Suppose the beam’s tributary width is 3 m and the first-floor load, including dead load and imposed load at service level, is about 7 kN/m². The wall also supports a roof load of, say, 3 kN/m² over the same width.

Load per metre from first floor: 7 × 3 = 21 kN/m.
Load per metre from roof: 3 × 3 = 9 kN/m.
Total on the wall: about 30 kN/m, or 120 kN over its 4 m length.

If the owner wants an open-plan space and the wall is removed, those 120 kN do not disappear. They need a new load path, typically a new beam spanning the opening, sized for both strength and deflection, and new columns or posts at each end, which in turn need foundations able to take the concentrated reactions. Each end might carry around 60 kN plus the self-weight of the new beam, a point load that the original strip footing under the wall was never designed for.

Common mistakes that break load paths

Removing walls without identifying what they support. The most common and most dangerous alteration error.
Cutting openings in slabs and beams. New stairs, lifts, skylights and service ducts can interrupt both gravity and lateral load paths.
Adding load where there is no path. A new water tank or roof garden placed mid-span rather than over beams and columns.
Ignoring the foundations. A new column may concentrate load onto ground or footings that cannot take it.
Weak connections. A load path is only as strong as the connections between members. Missing ties, poor anchorage or inadequate fixings break the chain.
Temporary works. During construction or demolition, the permanent load path may be incomplete and temporary propping must provide one.

The 1968 partial collapse of Ronan Point in London, where a gas explosion removed a load-bearing wall panel and the floors above progressively collapsed, is still taught as a lesson in what happens when a structure has no alternative load path. Modern codes require robustness and tying so that the loss of one element does not lead to disproportionate collapse.

Why you need an engineer

Tracing a load path in an existing building is detective work. Drawings may be missing or inaccurate, previous owners may have made undocumented alterations, and some members carry far more than their size suggests. A Professional Engineer has to establish how the building actually works before deciding what can change, and design a replacement path where one is interrupted.

In Singapore, structural alterations such as removing walls, cutting slabs or adding significant loads generally require a Professional Engineer’s design and BCA approval. If you are unsure whether your project crosses that line, our guide on whether you need a structural engineer will help, and our structural engineering consultancy can trace the load path in your building before any work begins.

Frequently asked questions

What is a load path in a building?

A load path is the continuous chain of structural members and connections through which a load travels from where it is applied to the foundations and ground. For gravity loads, it typically runs from slabs to beams to columns or walls and down to the foundations.

What happens if a load path is interrupted?

The load must find another route through the structure. If an adequate alternative exists, members may simply carry more stress and deflect more. If not, members can crack, sag or fail, and in the worst case a progressive collapse can occur.

How can I tell if a wall is part of the load path?

Clues include walls that run beneath beams or joists, walls stacked above each other on successive floors, and walls marked on structural drawings. None of these is conclusive on its own, so a structural engineer should confirm before any wall is removed or altered.

Do lateral loads follow the same load path as gravity loads?

Not entirely. Lateral loads from wind travel through the facade into the floor slabs, which distribute them to cores, shear walls, bracing or rigid frames before reaching the foundations. A building needs both a gravity and a lateral load path to be stable.

Planning to remove a wall, cut an opening or add significant load? Talk to our structural engineering consultancy or get in touch.