Read the Failure, Not the Joint Type
Walk a demolition and the members come out of a failed timber frame straight and sound. The damage sits in the last hand-span at each end: a split running back from a bolt, a crushed seat under a beam, a hanger nail that has ploughed a slot through the flange it was meant to hold. Timber is generous along the grain and mean across it, and every connection asks the load to turn a corner into the weak direction.
Sorting joints by name — mortice and tenon, half lap, bolted splice, proprietary hanger — hides the thing that decides whether they survive. Two joints that look nothing alike fail identically because both put tension across the grain at a sharp corner. Two that look identical behave differently because one was fixed at twelve per cent moisture content and the other at twenty-four.
Everything below follows the ways a joint lets go: splitting along a fastener row, crushing at a bearing, cracking from a re-entrant corner, tearing under restrained shrinkage, withdrawing under uplift, slipping before it loads, and corroding from the inside. Learn the tells for each and the joint announces what it needs before it is ever loaded.
Splitting Along the Fastener Line
The commonest connection failure in a timber frame is a crack that runs from the fastener nearest the end of the member, back along the grain, out to daylight. Nothing yields first. The bolt does not bend and the timber does not crush; a plug of wood between the fastener row and the end grain shears out, and the connection loses most of its capacity in one movement. Group tear-out — the whole block enclosed by the fastener group leaving as a unit — is the same failure at a larger scale.
Geometry governs this, not fastener size. End distance, edge distance, spacing along the grain and spacing across it are all expressed as multiples of fastener diameter, and EN 1995-1-1 Design of timber structures — General — Common rules and rules for buildings and the ANSI/AWC National Design Specification (NDS) for Wood Construction both set them, with values that differ between the two documents, between loaded and unloaded edges, and between fastener types. AS 1720.1 Timber structures — Design methods and CSA O86 Engineering design in wood do the same job in their own territories. Which set applies is decided by the adopted building code and, in Europe, by the National Annex — settle that before copying a spacing off a drawing from another job.
Three habits keep a row intact. Stagger fasteners rather than lining them up on a single grain line, because a straight row is a pre-drawn crack path. Pre-drill in dense or dry material, and in anything within a couple of diameters of an end. And treat a shake or a seasoning check running through the fastener line as a fastener already lost — reposition, rather than bolting across it and hoping.
Adding fasteners is not free. Every extra hole is net section removed, and past a certain number a group stops sharing load evenly, so the end fastener carries far more than its share. Count and spacing have to be settled together, on the section you actually have rather than the one on the drawing.
Fixing the connector count here, before anyone marks out, is the only way to know whether the spacing that keeps the row from tearing out will physically fit in the member on the trestles.
The total design load the joint must transfer.
The rated capacity of a single connector, from the NDS Chapter 12 tables or manufacturer data.
Connector units needed
7 connector units
This is a simple division of total load by your entered per-connector capacity — make sure that capacity comes from the actual NDS Chapter 12 table value (adjusted for species, thickness, and edge distance) or the manufacturer's rated data, not an assumed number.
They open the calculator with your figures already in it
Timber Shear Connector Count Calculator: 7 connector units — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Group action is not applied. The NDS group action factor Cg falls as more connectors are added to a row, so the single-connector capacity you looked up is only valid for the number of connectors you assumed when you looked it up — which may not be the number this page returns. Re-check the tabulated value against the count and iterate until the two agree.
- Direction of load relative to grain is ignored. Split-ring and shear-plate design values differ parallel and perpendicular to grain, and a load at an angle has to be interpolated between them. This calculation divides one number by another with no knowledge of your joint geometry, so a parallel-to-grain capacity applied to an angled or cross-grain load overstates the joint.
- Nothing here checks that the connectors physically fit. End distance, edge distance, spacing along and across the row, the member thickness and width needed to seat a ring or plate, and whether the returned count fits on the contact face at all are all outside this calculation.
- This is a connector count, not a connection design, and it does not replace one. The member itself is unchecked: net section at the connector groove, tension or compression capacity of the wood, row and group tear-out, and shear and bearing on the bolt through the ring or plate are separate calculations. A split ring cuts a groove that removes cross-section, so the member can govern before the connectors do.
- Service and installation conditions are not applied. Wet service, elevated temperature, preservative or fire-retardant treatment, load reversal or cyclic loading, and whether the connector sits in one face or both all change the value you look up — none of them change the division done here, and none are checked.
Crushing at the Seat
Compression perpendicular to the grain never bangs. A beam end bearing on a wall plate settles into it a fraction of a millimetre at a time, and the first report is a door that binds or a floor that has developed a fall towards the wall supporting it. By then the fibres are crushed and no recovery is coming.
Bearing length carries the whole argument. A hanger that seats a member on forty millimetres of steel and a joist running a hundred millimetres onto a plate are different details even though the joist is identical. EN 1995-1-1 and the NDS both check bearing on the contact area with an adjustment for how short that bearing is and how close it sits to the member end, and both reduce the permitted stress where the timber will be wet in service. Softwood plates under heavy point loads want a steel bearing plate, a hardwood block, or a densified engineered bearing insert.
Wane at the seat is the quiet killer of this detail. A piece graded acceptable overall can carry its permitted wane exactly where it sits down, so the contact area drawn is not the contact area delivered. Turn the piece, or pack the bearing with a full-width plate — never with a taper wedge, which concentrates load onto one arris.
Platform framing stacks the problem storey by storey. Each floor passes load through a sole plate, a rim board and a head plate: three cross-grain bearings per level. Over four storeys the accumulated shortening is enough to bow a cladding rail or shear a rigid service riser, so allow movement in those details rather than discovering the total at second fix.
Three cross-grain bearings in one floor line
- Upper sole plate — the first of the three cross-grain crossings, and its dressed thickness rather than its nominal name is what the load is squashing Lumber Dimensional Size Calculator
- Floor deck — sheet goods laid across the joists, counted by sheet rather than area once you allow for the cuts at every wall line Plywood and OSB Sheet Calculator (Subfloor, Wall and Roof)
- Rim board and joist ends — closes the joists and hands their reaction down as a continuous line load, so quantity follows the floor perimeter and not its area Rim Board Linear Footage Calculator
- Wall top plates — the crossing that quietly settles under a concentrated load; contact area against the perpendicular-to-grain stress is the whole check Timber Post Bearing Area Calculator
- Studs below — loaded end to end along the fibre, which is timber's generous direction, so buckling over the unrestrained height governs instead of crushing Timber Column Euler Buckling Calculator
Contact area settles this failure mode on its own, so divide the load by what the timber may take across the grain and see whether the seat drawn on the section is actually long enough.
The compressive axial load the post transfers into the beam below it.
The species/grade's allowable compression stress perpendicular to grain, from the NDS design values.
Required bearing area
20.67 in²
They open the calculator with your figures already in it
Timber Post Bearing Area Calculator: 20.67 in² — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 20.67 in² — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- The Fc-perp figure is used exactly as typed and no NDS adjustment factors are applied to it. The bearing area factor Cb, which raises the allowable value in the form (bearing length + 9.5 mm) divided by bearing length for bearings under about 150 mm long set back at least about 75 mm from the end of the member, is not credited, and the wet service and temperature reductions are not taken off either. Enter a value already adjusted for the edition and service conditions you are designing to.
- Tabulated Fc-perp is governed by a deformation limit of roughly 1 mm (0.04 in) of indentation in the standard test, not by fibre rupture. Meeting the required area therefore controls how far the joint beds down, and exceeding it produces progressive crushing and a settling floor rather than a sudden collapse. Creep is not modelled at all, and perpendicular-to-grain deformation under sustained load keeps growing well beyond the initial value.
- The output is a single area, not a bearing length, a plate size or a post section, and it assumes that whole area is loaded uniformly. Area cannot be won by spreading past the edge of the supporting member, and a thin plate that cantilevers beyond the post face does not deliver uniform stress unless it is stiff enough in bending, which is a separate check. An out-of-plumb, cupped or partially seated bearing contacts over only part of the face and sees a peak stress well above load divided by area.
- Only crushing of the supporting member at one interface is checked. The post's own capacity in compression parallel to grain with the column stability factor CP, the supporting beam's bending and shear under the concentrated load, uplift and lateral restraint at the connection, and the same bearing check repeated at every interface further down the load path are all outside what this page computes.
- Fc-perp from the NDS supplement is an allowable stress, so the axial load entered must be an unfactored service load combination; putting a factored LRFD load such as 1.2D + 1.6L (0.42 gal) against it oversizes the bearing by roughly the load factor. Fc-perp is also one of the few timber design values the load duration factor does not adjust, so a short-duration wind or seismic case does not justify a higher allowable stress here.
The Re-entrant Corner
Cut a notch in the underside of a beam and you have not merely made it shallower — you have built a crack starter. Shear along the grain and tension across it arrive together at the inside corner, and tension perpendicular to grain is the lowest number in timber's whole profile. The crack opens at that corner and runs horizontally along the member, sometimes the length of a bay, under service load, with no noise to warn anyone.
Position and shape matter more than depth alone. A notch on the tension face close to a support is the severe case; the same cut on the compression face, or well away from the support, is a milder thing entirely. Tapering the cut instead of squaring it, and drilling the corner before sawing so it ends in a radius rather than a kerf point, both blunt the concentration. Where depth cannot be avoided, fully threaded screws driven across the potential crack plane, or plywood or steel side plates, hold the split shut.
Engineered members change the rules outright. Glulam is still timber and still splits, but a laminated veneer lumber rim or an I-joist flange is off limits — an I-joist flange must never be notched at all, and web holes are governed by the manufacturer's evaluation report rather than carpentry judgement. Product standards such as EN 14080 Timber structures — Glued laminated timber and glued solid timber — Requirements describe what the member is, not what may be cut out of it on site.
A notch someone has already cut, or one the services drawing insists on, needs its reduction put to a number at this point in the sequence — before the beam is loaded and the answer arrives as a split you cannot close.
The full, un-notched depth of the beam.
How deep the end notch is cut into the beam, measured from the notched face.
Shear stress amplification factor
1.452 ×
- Net depth after notch
- 9.75 in
They open the calculator with your figures already in it
Timber Beam Notch Reduction Factor Calculator: 1.45 × — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- This returns the amplification factor on its own. It never asks for the shear force at the support, the beam breadth or the species allowable shear value, so it cannot say whether the notched beam is adequate — a 1.44 factor is harmless on a lightly loaded joist and decisive on one already sitting close to its allowable shear stress.
- The factor multiplies the shear stress worked out on the net depth below the notch — 3V ÷ 2b·dn — not on the full section. Apply it to a gross-section stress of 3V ÷ 2bd instead and the answer falls short by a further full-depth to net-depth ratio: for the 300 mm (12 in) beam with a 50 mm (2 in) notch used as the example, the correct stress is 20 per cent higher than that mistake gives.
- Nothing here enforces how deep a notch is allowed to be. The inputs will accept a 150 mm (6 in) notch in a 300 mm (12 in) joist and return 4.00 without objection, whereas end notches in sawn bending members are normally capped near a quarter of the depth, with a shallower limit away from the support and no notching through the middle third of the span; the exact fractions depend on the code and edition you are working to.
- The squared ratio assumes a square notch with a sharp re-entrant corner and gives no credit for tapering or bevelling the cut, which is the usual way of easing that stress concentration. It also frames the problem as a shear stress, when the failure that governs is a split running along the grain from the corner in tension perpendicular to the grain — a seasoning check already present at that corner can start it below the calculated stress.
- The provision behind this factor is written for solid rectangular sawn timber. Glulam, LVL and I-joists are governed by separate rules — I-joist flanges must not be notched at all, and tension-face notches in glulam have their own provisions — yet the calculator will still return a number if you type those dimensions in.
- An end notch also moves the bearing onto the underside of the cut, and compression perpendicular to the grain over that shortened seat is not checked here at all. A notch that passes the shear amplification check can still crush at the support.
Restrained Shrinkage
Timber moves across the grain as it dries and barely at all along it. Bolt a deep member top and bottom to a rigid steel plate and the wood tries to shrink between those two fixings; it cannot, so it splits. That split usually shows in the first heating season after handover and gets blamed on the material rather than the detail.
Detail the joint so the shrinkage has somewhere to go. Concentrate fasteners on a single line across the depth of the member wherever the connection allows it, and where two lines are unavoidable, slot the secondary holes vertically. Keep the vertical extent of any rigid fastener group as small as the force permits: a group three hundred millimetres deep in a green section is a certainty, not a risk.
Moisture content at the moment of fixing decides how much movement is still to come. Stock fixed at delivery moisture into a building that will be heated to a dry indoor equilibrium has the entire journey ahead of it, while kiln-dried material fixed in a conditioned space has almost none. EN 1995-1-1 handles this through service classes and the NDS through wet-service adjustment, and EN 335 Durability of wood and wood-based products — Use classes describes the exposure a detail will actually see. Record the meter reading at fixing; it is the cheapest evidence you will ever collect when a split is disputed.
Withdrawal and Prying
Gravity joints forgive a great deal. Uplift joints forgive nothing, because they convert shear into tension along the fastener axis, and axial grip is where timber is weakest and most variable. A nail driven into end grain has almost no withdrawal value and should never be counted on for any.
Most uplift failures are schedule failures rather than design failures. A proprietary hanger or strap is tested and rated with every hole filled, using the specific fastener its evaluation report names — the short square-twist nail, not the round wire nail in the fixer's pouch, and not six holes out of ten because a rafter was in the way. Straps that wrap a member need fixing on both faces to develop the wrap; a strap nailed on one leg only is a bracket.
Prying is the second mechanism, and it hides inside the first. Where a plate or strap can lever off the face, the fastener sees tension amplified by the lever arm, and the failure appears as a head pulling through thin steel or a cone of fibre lifting around the shank. Pull the plate hard to the timber, keep fasteners close to the point of bearing, and size heads for the gauge of steel they pass through. EN 14592 Timber structures — Dowel-type fasteners — Requirements and the corresponding evaluation reports define what a fastener may be credited with; a screw of the same diameter out of a different box is not automatically a substitute.
Slip Before Strength
A joint can be strong and still be useless. Bolts in oversized holes take up their clearance before they take any load, and a truss or portal with a dozen such connections in series will move visibly on first loading while every joint remains well inside capacity. Serviceability, not strength, is what the client notices.
Fit answers most of this. Drill dowel and bolt holes to the tolerance the design assumes rather than to whatever bit is loose in the case, because a hole a millimetre oversize per joint accumulates into a sagging ridge. Dowels in tight-fitted holes are markedly stiffer than bolts in clearance holes, which is why engineered connections controlling deflection reach for them. Where slip cannot be designed out, report the number to the engineer rather than quietly absorbing it.
Glued site joints deserve a separate warning. Adhesive connections are brittle and give no notice, and their performance depends on clamping pressure, glue line thickness, temperature and moisture content that a factory controls and a scaffold does not. Glued laminated and finger-jointed components arrive with a factory production control regime behind them; reproducing that with a cartridge gun on a wet morning is a different activity wearing the same name.
The Wet Interface
Decay and corrosion both begin where two pieces meet, because that is the one place water gets in and cannot get out. A post base sitting flat on a slab, a beam end pocketed into masonry, a lapped joint with an upward-facing shoulder — each holds water against end grain, and end grain drinks.
Detail the connection to shed and to breathe. Stand posts off the slab on a base that lifts the end grain clear and drains, weather or chamfer any upward-facing surface inside the joint, and leave a ventilated gap rather than a sealed one where timber meets masonry or concrete. Sealant run across the top of a joint with no drainage below traps precisely the water it was meant to exclude.
Fastener metallurgy has to match both the timber and the exposure. Copper-based preservative treatments are aggressive to plain steel and to some coatings, and that corrosion happens inside the joint where nothing is visible until a head shears off under a spanner. Hot-dip galvanizing to EN ISO 1461 Hot dip galvanized coatings on fabricated iron and steel articles — Specifications and test methods, or ASTM A153/A153M Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware, is the usual minimum for treated exterior work, with austenitic stainless where exposure is severe or the connection will never be inspectable again. Mixing metals in one joint sets up a galvanic cell, so keep each connection to a single metallurgy.
Walking the Frame
An inspection that starts at the joints finds problems while they are still cheap to fix. Give each connection ninety seconds and look in a fixed order, because the eye skips whatever it is not actively expecting.
Two joints deserve a second look every time: the one nobody detailed, and the one somebody changed. An unspecified connection was made by whoever reached it first, and a changed one — a hanger swapped, a notch deepened for a duct, a bolt shifted to miss a knot — carries an assumption that has stopped being true. Photograph both, with a tape in frame, before the ceiling closes.
- Measure the shortest end and edge distance on the joint, not the one that merely looks tightest.
- Confirm every hole in a hanger or strap is filled, with the fastener the evaluation report names.
- Treat any split passing through a fastener line, or leaving a notch corner, as a defect rather than character.
- Check the bearing: full width in contact, no wane at the seat, no taper packers.
- Check notches and holes for position relative to the support, corner shape, and whether the member may be cut at all.
- Look for upward faces holding water, end grain on a hard surface, bare steel in treated timber, or two metals in one joint.
| Failure mode | What you see | Correction |
|---|---|---|
| Row shear and group tear-out | Crack from the end fastener running back along the grain | Increase end distance, stagger the row, pre-drill |
| Bearing crush | Beam settled into the plate, binding doors, wrinkled fibre at the seat | Lengthen the bearing or introduce a plate or hardwood block |
| Notch cracking | Horizontal split leaving the inside corner of a notch or dap | Taper and radius the corner, screw or plate across the crack plane |
| Restrained shrinkage | Vertical split between two rigid fixing lines in a deep member | Single fastener line, slotted secondary holes, drier stock |
| Withdrawal and prying | Nail heads standing proud, strap lifting, cone of fibre at the shank | Named fastener, all holes filled, plate pulled tight to the face |
| Corrosion at the interface | Rust staining at heads, dark wash running below the joint | Matched coating class, one metallurgy, drain the detail |
Take-off by connection, not by member
Quantities for timber frames are usually taken off in metres of member; the risk sits in the schedule of joints, so book these alongside the cutting list.
- Fastener schedule per connection type — Diameter, length, coating class and count per joint, with the evaluation report reference for anything proprietary.
- Connector count and spacing envelope — Number of shear connectors per splice, and the end, edge and spacing dimensions the count implies in the section supplied.
- Bearing area at every seat — Length and width of contact at each support, plus any bearing plate, hardwood block or densified insert needed to achieve it.
- Notch and hole register — Every notch, dap and service hole with its depth, position relative to the support and reduction factor, signed off before cutting.
- Moisture content at fixing — Meter readings by delivery batch, recorded on the day the connections are made, with the intended service class noted against them.
- Corrosion class by location — Which joints are external, treated, or permanently concealed, and the coating or stainless grade each of those groups requires.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.