Post-and-Beam Timber Frames

The single most important idea in a post-and-beam frame is a separation of jobs. In a load-bearing masonry building, the walls do two things at once - they hold the building up and they enclose the space - which is why such buildings tend to have thick walls, small openings and rigid plans. A frame splits those jobs apart. A discrete skeleton of columns and beams takes on the whole structural task of carrying gravity to the ground, and that liberates the walls entirely: since they no longer hold anything up, they can be thin, movable, glazed from floor to ceiling, or simply left out. This is the same principle that let early modernism open its facades to glass, and in timber it feels especially generous because the structure itself is beautiful.

Think of it as a grid of strong points rather than continuous walls. Columns stand at the intersections of a structural grid; beams run between the columns to catch the floors and roof; the floor or roof deck spans across the beams to make the usable plane. Load is gathered by the deck, handed to the beams, handed to the columns, and delivered to the foundations - a clean chain of hand-offs we will trace in a moment. Because the load is concentrated into a small number of members, those members have to be strong, which is exactly what engineered timber makes possible without them becoming clumsy.

The design consequence is freedom. An open, column-dotted plan can be subdivided any way you like with non-structural partitions, reconfigured years later without touching the structure, and wrapped in as much glass as the climate allows. For offices, studios, markets, libraries, workshops and homes that want flexibility and daylight, the frame is a natural fit. And because the columns and beams are on show, the structure becomes the architecture - the rhythm of the grid, the meeting of post and beam, the grain of the wood all read as the character of the room. That expressive, flexible openness is the first reason the post-and-beam frame is the classic mass-timber move.

What makes the modern timber frame different from the old barn is the member itself. The workhorse of the mass-timber frame is glulam - glued laminated timber - in which many kiln-dried boards are bonded face to face under pressure into a single large, straight (or curved) beam or column. That simple move solves the historic problems of sawn timber all at once. A glulam member can be far longer and deeper than any log, so it spans halls and open floors a solid piece never could. It is dimensionally stable and predictable, because the boards are dried, graded and the natural defects dispersed and averaged out across the section rather than concentrated in one weak knot. And it can be made to precise, repeatable sizes, so the frame arrives as a set of engineered components rather than variable natural pieces.

In a frame, glulam plays two distinct roles. As a beam, it works in bending - it has to resist sagging under the floor load, so it is made deep in the direction of the span, its depth doing the heavy lifting. As a column, it works mainly in compression - it has to carry the stacked weight straight down without buckling, so its job is to be stiff and well-proportioned for its height. The same material, oriented and sized for the task, does both. Alongside glulam you will meet LVL and other engineered members playing similar structural roles, and in many frames the floor plane is made of CLT panels - but the beams and columns are most often glulam, and the mental model of "engineered posts and engineered beams" holds.

A word of discipline belongs here. The exact size of every beam and column - how deep, how wide, what grade, spanning how far - is a structural-engineering result, not something you eyeball from a rule of thumb. Spans, loads, deflection and vibration limits, and the buckling of slender columns all feed into it, and it is the timber engineer's calculation to code. What you own as the designer is the concept: the grid, the spans you are asking for, the ambition for openness and slenderness - and then you brief the engineer, who turns that into real members. Any dimension quoted in this course is illustrative of the idea, never a value to build from.

A frame of posts and beams is not a building until something spans between the beams to make a floor or roof you can stand on - the deck. In mass timber the deck is usually a solid engineered plane: CLT panels, nail-laminated or dowel-laminated timber, or a timber-concrete composite, laid across the beams. The deck does two things at once. It carries the floor loads across the gap between beams (spanning the short way, beam to beam), and, tied down properly, it also acts as a diaphragm - a stiff horizontal plate that ties the whole floor together and shares out sideways forces, a role that matters enormously once we start thinking about stability.

With the deck in place we can trace the full gravity load path, and understanding it is the heart of reading any structure. Weight - people, furniture, finishes, the deck's own mass - lands on the floor deck. The deck carries that load sideways to the beams it rests on. Each beam gathers the load from the deck on either side and carries it along its span to the columns at its ends. Each column collects the load from the beams meeting it, adds the weight of everything stacked above, and carries the accumulated total straight down. At the bottom, the foundation spreads that concentrated load safely into the ground. It is a chain of hand-offs, and it explains why columns get more heavily loaded lower down a tall frame (they are carrying more storeys above) and why the connections where members meet are so critical - every hand-off happens at a joint.

Those joints deserve respect. Unlike concrete, which is poured monolithic, a timber frame is assembled from discrete pieces, so the connections between beam and column, column and column, and deck and beam are where the structure is made continuous - and where much of the engineering effort and cost sits. Connections are typically steel: plates, brackets, dowels, screws and bolts, concealed or expressed, designed to transfer the calculated forces and to behave predictably in fire. We meet connections properly in Module 4; here the point is simply that a frame is only as good as its joints, and their design - like member sizing - is the engineer's, done to code.

The post-and-beam frame is at its best when you want open, flexible, daylit space and long spans. Because a handful of slender members carry everything and the walls are free, the frame suits offices, studios, education and civic buildings, markets, workshops, showrooms and open-plan homes - anywhere the value is in a clear, adaptable floor plate that can be glazed generously and rearranged over a building's life. It also lends itself to expression: leave the glulam exposed and the grid becomes the room's character, warm and legible. And because the members are prefabricated, a frame goes up fast and cleanly, bolted together like a kit.

But there is one thing a bare frame cannot do, and naming it honestly is essential: a simple post-and-beam frame does not brace itself against sideways forces. Gravity travels neatly down the columns, but wind pushing on the facade and the shaking of an earthquake push the building sideways, and a frame of pin-jointed posts and beams would simply lean over and rack like a wobbly bookshelf under those loads. So every real frame needs a deliberate, separate lateral stability system to resist that sway and carry horizontal load safely to the ground. That system is usually one of three things: bracing (diagonal members, or braced bays, that triangulate the frame), shear walls or a shear core (stiff vertical panels - often CLT, or in hybrids a concrete core - that act like a spine), or moment connections (rigid joints that let the frame resist sway by themselves, harder and costlier in timber). We explore stability fully in Module 4; the lesson here is that lateral stability is a first-order design decision, not an afterthought.

The frame also has honest limits beyond stability. Concentrating load into few members makes connections demanding and fire design specific - the joints and exposed members must be designed for their fire resistance by a fire engineer to code. And where a plan is naturally cellular - many small, repetitive rooms with walls everywhere, as in a hotel or apartment block - a frame can be less efficient than simply making those walls structural, which is exactly the panel system we turn to next. Choose the frame for openness and span; know that its stability and its joints are engineered, deferred to the specialists and the code.