A timber load takedown is arithmetically the same exercise as a concrete one and practically a different job, because the numbers change scale. StructLoads runs a mass timber frame the same way it runs any other, summing tributary areas down to foundation, but two things shift: self-weight drops by roughly 75 percent, so live load becomes the dominant term rather than a minor one, and connections rather than members become the thing the takedown has to respect. A timber column is rarely governed by crushing. Its connection usually is.
The numbers that change
Reinforced concrete runs about 25 kN/m3. Structural softwood runs 4.5 to 5.5 kN/m3, and cross laminated timber sits around 5 kN/m3 depending on species and layup. A 200 mm CLT floor panel is therefore about 1.0 kPa, against roughly 5.0 kPa for a 200 mm concrete slab. Glued laminated timber beams follow the same ratio.
That single change reshapes the takedown. In a concrete building, self-weight typically accounts for 60 to 70 percent of the total gravity load, and live load is the smaller, more uncertain term. In a mass timber building the proportions invert: a CLT floor at 1.0 kPa with 1.5 kPa of screed, services and finishes carries 2.5 kPa of dead load against a 2.5 to 3.0 kPa office live load. Dead and live are now comparable.
Three consequences follow immediately. Live load reduction matters much more, because it is acting on a larger share of the total. The difference between the dead-dominated and live-dominated load combinations is smaller, so the governing case is less obvious and needs checking rather than assuming. And uplift becomes a real design case, because a light building resists wind uplift and overturning with less ballast.
How to run a timber takedown
| Method | Best for | Why it works | Main limit | Verdict |
|---|---|---|---|---|
| StructLoads | Mass timber and timber frame buildings | Same tributary area takedown with timber densities, so the column line totals are right and the live-dominated combination is visible | Does not check connection capacity, shrinkage, or vibration | Best overall for the load side |
| Panel supplier layup data | Getting real self-weights | States actual panel weight per square metre for the specific layup, species and thickness | Layups change during value engineering | Essential input |
| Concrete rules of thumb scaled down | A quick concept estimate | Fast, and the tributary area logic is identical | Misleads on which combination governs, and says nothing about connections | Concept only |
| Full frame model with connection stiffness | Tall or hybrid timber buildings | Captures the flexibility that timber connections genuinely have | Heavy, and connection stiffness data is often uncertain | Right tool above a certain height |
The order is the same as any takedown: establish real panel weights, add finishes and services, apply live loads by occupancy, sum down the column lines, and then take the result to the connection design rather than stopping at the member.
Compression perpendicular to grain is the usual governing check
Timber is strong along the grain and weak across it, by roughly a factor of ten in compression. That means a timber column with 500 kN in it is comfortable in its own axis and the problem lives where it bears onto something across the grain.
The classic case is a platform frame arrangement where a floor panel or a rim beam sits between the column above and the column below. The full column load passes through the horizontal member across its grain, and the perpendicular to grain compressive strength of softwood is on the order of 2 to 3 N/mm2 against 20 to 25 N/mm2 parallel. A 500 kN load through a 200 mm by 200 mm bearing is 12.5 N/mm2, which is fine along the grain and several times over the limit across it.
The design responses are direct bearing details that pass column load through steel plates or dowels rather than through timber across the grain, or platform details with reinforcement such as screws inserted vertically through the bearing zone. Either way, the takedown feeds this check directly: the number the connection has to handle is the accumulated column load at that level, which is exactly what a column load takedown produces.
Cumulative shrinkage down a tall timber building
Timber changes dimension with moisture content, and it does so far more across the grain than along it. In a platform frame building each floor level includes some horizontal timber in the vertical load path, so each level contributes a small vertical shortening as the timber dries towards equilibrium. Individually these are millimetres. Over eight or ten storeys they accumulate.
The effects are practical rather than structural: facade joints close, services risers go into compression, lift guide rails go out of alignment, and doors bind. The mitigation is to minimise cross grain timber in the vertical load path, which is why balloon frame and post and beam arrangements are preferred in taller mass timber buildings over platform framing.
There is a load takedown link. Differential shortening between a heavily loaded internal column line and a lightly loaded perimeter one produces differential movement, and the difference depends on the load ratio between them, which the takedown is what tells you. Eurocode 5 treats creep and moisture effects through service class factors for this reason, and the relevant input is the sustained load, not the peak.
Uplift, overturning and the light building problem
A concrete building holds itself down. A timber one may not. With self-weight at a quarter of the concrete equivalent, wind uplift on the roof and overturning of the whole frame both become live design cases rather than formalities.
Roof uplift is the more immediate. A lightweight timber roof under suction can experience net upward load, so the connection between roof and wall, wall and floor, and floor and foundation forms a continuous tension chain. Every link needs designing for tension, and the chain is only as strong as its weakest connection, which is frequently a nominal one that nobody costed.
Overturning is the taller building version. Lateral wind produces an overturning moment resisted by the building’s weight acting at its lever arm. Less weight means less resistance, so tall timber buildings often need either heavy foundations, hold down anchors carrying real tension, or added mass, most commonly a concrete topping that doubles as an acoustic and vibration measure. That topping then reappears in the takedown as dead load, which is a case of the solution changing the problem in a helpful direction.
Vibration and acoustics add load back
The lightness that makes mass timber attractive also makes floors lively. Timber floors have low mass and moderate stiffness, so their response to footfall is more noticeable than a concrete equivalent, and acoustic separation between dwellings is harder for the same reason.
The standard answer to both is added mass: a screed, a concrete topping, or a loose fill layer. A 60 mm concrete topping on CLT is about 1.4 kPa, more than doubling the panel’s own self-weight. Once that decision is made the building is no longer as light as the timber suggests, and the takedown must use the real build up rather than the panel weight. WoodWorks technical resources cover these build ups in detail, and the structural point is simply that the acoustic layer is a structural load.
Hybrid systems formalise this. A CLT and concrete composite floor uses the topping structurally, connected to the panel with screws or notches so the two act together. That improves stiffness and vibration performance and increases dead load, and the takedown needs the composite weight rather than either component alone.
Fire, charring, and why sizes are larger than strength suggests
Timber members in exposed mass timber buildings are frequently sized by fire rather than by load. The design approach is that timber chars at a predictable rate, roughly 0.65 to 0.7 mm per minute for solid softwood, and the residual uncharred section must carry the load at the fire limit state. A 90 minute requirement therefore consumes around 60 mm from each exposed face.
The load takedown feeds this directly, because the fire case uses a reduced load combination, typically permanent load plus a fraction of the imposed load, rather than the full ultimate combination. Knowing the split between dead and live at each level is what allows that reduced combination to be calculated, and in a timber building where dead and live are comparable, the split matters far more than it does in concrete.
Hybrid frames and the load path that changes material
Most mass timber buildings above a few storeys are hybrids: a concrete or steel core for stability, timber floors and columns for the rest. That mix creates a load path that changes material partway down, and each interface deserves attention in the takedown.
The interface that matters most is where timber floors frame into a concrete core. The floor delivers its reaction into the core wall through a bearing angle, a corbel, or a proprietary hanger, and the connection has to accommodate two dissimilar materials moving differently over time: the timber shrinks and creeps, the concrete shrinks and creeps on a different schedule and in a different amount. A connection detailed as rigid will attract load it was not designed for, or crack.
The second interface is the transition storey in buildings with a concrete podium under timber above. The podium carries a light superstructure, which sounds easy, but the light superstructure means the podium’s own weight dominates and the transfer beam design is governed by the concrete rather than by what it carries. It also means the columns landing on the transfer are lightly loaded, which reduces the beneficial pre-compression that helps a transfer element, so the transfer works harder than the superstructure load implies.
Steel and timber hybrids raise a third point. A steel beam supporting CLT panels bears on its flange, so the panel delivers a line load across the grain into the panel end rather than into the steel, and the check is again perpendicular to grain in the timber rather than anything in the steel.
Moisture during construction is a load and a liability
Timber arrives dry and the site does not stay dry. A CLT floor plate left exposed for a winter absorbs water, gains weight, and swells across the grain, and the swelling is not fully reversible in terms of the surface finish even when the moisture content returns.
The load consequence is short lived but real: saturated panels can gain a meaningful percentage of their dry weight, and ponded water on a flat CLT deck is 9.81 kPa per metre of depth, which on a light floor is a very large temporary load. Ponding is a positive feedback problem: water deflects the panel, the deflection creates a deeper pond, more water arrives. On a stiff concrete deck this is a slow problem; on a light timber deck it is a fast one.
The mitigations are drainage falls maintained during construction, temporary weather protection, and a construction sequence that closes the envelope quickly. All of these are programme decisions, and the structural engineer’s contribution is to state what the exposed deck can take and for how long. That is a temporary condition worth naming explicitly, in the same way that any construction load case is.
What to record so the numbers survive
Timber buildings are more sensitive than concrete ones to changes made later, because the margins are thinner in specific places rather than smaller overall. A concrete slab that gets an extra 1 kPa of screed usually absorbs it. A CLT floor at 0.9 kPa self-weight that gets an extra 1 kPa has had its dead load doubled.
Three things are worth recording where a future occupant will find them: the assumed floor build up including any topping, the imposed load the floor was designed for, and the perpendicular to grain capacity of the bearing details, because that is what limits any future addition of load at a column line. The last of these is unusual to record and it is the one most likely to be needed, since adding load to a timber building is a connection question long before it is a member question.
A worked example: a five storey CLT office column
Take a 6 m by 6 m grid, 180 mm CLT floors, 60 mm concrete topping, five storeys of office over ground.
Per floor: CLT at 5 kN/m3 over 180 mm is 0.9 kPa. Topping at 24 kN/m3 over 60 mm is 1.44 kPa. Services, ceiling and finishes 0.5 kPa. Dead total 2.84 kPa. Office live 2.5 kPa plus 1.0 kPa partitions, so 3.5 kPa imposed.
Interior column tributary area is 36 m2. Per floor that is 102 kN dead and 126 kN imposed. Imposed exceeds dead, which almost never happens in concrete.
Over five floors, unreduced, that is 511 kN dead and 630 kN imposed, 1141 kN service load at the base. Applying a live load reduction for the accumulated area brings the imposed term down meaningfully, perhaps to 480 kN, so about 990 kN. The reduction has moved the answer by 13 percent, against maybe 5 percent in an equivalent concrete building, purely because live load is a bigger share.
Then the connection check. That 990 kN arriving at a platform detail through a 240 mm by 240 mm bearing is 17 N/mm2 across the grain, comfortably beyond the perpendicular capacity. The column is fine; the detail is not, and the takedown is what surfaces it.
Key takeaways: timber and CLT load takedowns
The takedown method is unchanged, the proportions are not: timber self-weight is roughly a quarter of concrete, so live load becomes comparable to dead and live load reduction moves the answer much more. Compression perpendicular to grain governs most connection details, so take the accumulated column load to the connection rather than stopping at the member. Cumulative shrinkage matters in platform framed buildings and argues for post and beam above a certain height. Uplift and overturning are real cases in a light building, and the acoustic topping that fixes vibration puts dead load back on.
Quick answers
Timber rewards the same discipline as concrete with the emphasis moved: the member is usually fine, the joint is where the answer lives.