How the two differ in kind
A bolted connection is checked against several independent failure modes, and its capacity is the LEAST of them. Adding bolts raises one and eventually stops helping, because a different mode has taken over.
The BOLTS fail two ways. In SHEAR, through the bolt shank across the faying surface, at a capacity set by the bolt's grade, its area and whether the shear plane passes through the threads. And in BEARING, where the bolt presses against the side of its hole and the plate deforms or tears — which is a plate property expressed per bolt, and is why bearing capacity depends on plate thickness and on edge and spacing distances.
The PLATE fails three further ways, and none of them improves by using stronger bolts. NET SECTION rupture: the plate tears across the line of holes, at a capacity based on the material remaining after the holes are deducted. BLOCK SHEAR: a block of plate containing the bolt group tears out, rupturing along one line and shearing along another — for example the end of a beam web tearing away below a bolted connection. And edge TEAR-OUT, where a bolt too close to an edge simply shears its way out through the material in front of it.
BLOCK SHEAR is the mode people miss, and the reason is that it is a property of the bolt GROUP's GEOMETRY rather than of the bolts or of the gross plate. It depends on the shape of the block the bolt pattern defines and on the lengths of the tension and shear planes around it — so it appears when a connection is made compact to fit, and it recedes when the bolts are spread out.
That produces the counter-intuitive result worth the page: adding a bolt to a crowded connection can make it WEAKER, because the extra hole reduces the net section and shortens the tear-out path more than the extra bolt adds in shear.
The factors that actually differ
Show
| Bolt capacity | Plate capacity | |
|---|---|---|
| What fails | The bolt, in shear through the shank. | The plate — net section, block shear, or tear-out at an edge. |
| What governs the capacity | Bolt grade, area, and whether the shear plane is through the threads. | Plate thickness, material, hole size, and the bolt group's geometry. |
| Effect of adding a bolt | Increases capacity, proportionally. | Can REDUCE it — another hole shortens the net section and the tear-out path. |
| Effect of a stronger bolt grade | Increases capacity directly. | None at all. Plate modes do not care what grade the bolts are. |
| Block shear | Not a bolt mode. | The one most often missed — it depends on the group's shape, not on the bolts or the gross plate. |
| Edge and end distance | Affects bearing capacity per bolt. | Affects tear-out and block shear directly, which is why minimums exist. |
| Slip-critical connections | Capacity comes from friction under bolt pretension, not from shear — a different check again. | The plate modes still apply at the ultimate condition. |
| Where the design is decided | Bolt count, size and grade. | Plate thickness, hole positions, edge distances and the shape of the group. |
| What to do when it governs | More bolts, larger bolts, or a higher grade. | A thicker plate, longer edge distances, or spreading the bolt pattern out. |
| Both must be checked | Yes — the capacity is the least of all the modes. | Yes, and the plate modes are the ones a bolt schedule does not show. |
Which one, and when
Choose bolt capacity when…
- Selecting the bolt size, grade and count for a connection.
- Where the plate is generous and the bolts are the constraint.
- Checking whether the shear plane passes through the threads, which changes the capacity.
- Designing a slip-critical connection, where pretension and the faying surface govern instead.
Choose plate capacity when…
- Checking any connection with a compact bolt group, which is where block shear appears.
- Where the plate is thin relative to the bolts, so bearing and tear-out govern.
- Coped beam ends and short connection plates, the classic block shear geometry.
- Before adding a bolt to a crowded connection, which can reduce its capacity.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- How can adding a bolt make a connection weaker?
- Because it adds a hole, and holes reduce the plate. The extra bolt contributes its own shear capacity, which is a gain. But it also removes material from the net section across the line of holes, and it changes the geometry of the block that could tear out — usually shortening the shear planes around the group or bringing the pattern closer to an edge. If either of the plate modes was already close to governing, that loss can exceed the gain, and the connection's capacity, which is the least of all modes, goes down. It is counter-intuitive because a bolt schedule shows more bolts and looks stronger, and it is the specific reason adding a bolt to a crowded connection is a design change to be checked rather than a safe improvement.
- What is block shear?
- A failure in which a block of plate containing the bolt group tears out of the member — rupturing in tension along one line of the block and shearing along another. The textbook case is the end of a beam web at a bolted connection: the web tears across the line of holes in tension and shears down along the bolt line, so a rectangle of web comes away with the bolts still in it. Its capacity depends on the geometry of that block — the lengths of the tension and shear planes the bolt pattern defines and the material's strength on each — rather than on the bolts or on the plate's gross area. That is why it appears in compact connections, why coped beam ends are the classic case, and why it is missed when a check considers only bolt shear and gross section.
- Why do edge and end distances have minimums?
- Because a bolt too close to an edge tears out through the material in front of it rather than bearing against a hole that is confined on all sides. The bolt pushes against the plate, and if there is insufficient material between the hole and the edge in the direction of the force, that material shears out in two planes and the bolt exits — a failure that happens at a fraction of the bolt's shear capacity. Minimum end and edge distances exist to prevent it, and bearing capacity itself is expressed as a function of those distances rather than as a fixed value, so a bolt at the minimum has a lower bearing capacity than one further in. The same distances feed into the block shear calculation, which is why a connection detailed to the bare minimums has all its plate modes near their limits at once.
- Does the shear plane matter?
- Yes, and it is one of the easier things to get wrong on a drawing. A bolt's shear capacity depends on the cross-sectional area resisting the shear, and that area is smaller through the threaded part of the shank than through the plain shank. Codes therefore distinguish the two cases — shear plane through the threads, and shear plane through the plain shank — with different capacities, and the design has to state which applies. Whether it does depends on the grip length: a bolt whose threads run into the joint has its shear plane in the threads. The conservative assumption is threads-included, and it is what most detailing assumes, but a design taking credit for the plain shank depends on the bolt length and the plate thicknesses actually used matching what was assumed.
- What is a slip-critical connection?
- One in which the load is carried by FRICTION between the connected plates rather than by the bolts bearing in their holes. The bolts are tightened to a specified pretension, which clamps the plates together, and the friction developed at the faying surfaces resists the applied shear — so the connection does not slip into bearing at all under service loads. It is used where movement cannot be tolerated: connections subject to load reversal or fatigue, oversized or slotted holes, and where slip would affect the structure's behaviour. Its capacity depends on the pretension actually achieved and on the faying surface condition, which is why surface preparation is specified and why bolt tightening is a controlled procedure with a verification method rather than a matter of tightening until it feels right.
- What should be done when the plate governs?
- Change the plate or the geometry, not the bolts, because a stronger bolt grade does nothing for a plate mode. The options are a thicker plate, which improves bearing, net section and block shear together; longer edge and end distances, which improve tear-out and block shear; spreading the bolt pattern out, which lengthens the shear planes around the block and reduces the number of holes across any one section; and staggering the holes, which lengthens the net section path across the plate. Which is available depends on the connection's geometry — a coped beam end has limited depth to work with, which is exactly why those connections govern on block shear so often. Recognising that the plate governs is the point; the remedy is then straightforward.
- Do welded connections avoid all this?
- They avoid the bolt modes and they introduce their own, so the principle is the same: capacity is the least of several modes. A welded connection is checked for the weld itself — throat size, length, and the strength of the weld metal — and for the base metal at the weld, including the shear and tension capacity of the material adjacent to it. There is no net section reduction from holes, which is a genuine advantage where a member is near its capacity. Against that, welding is a site or shop process with its own quality control, inspection requirements and distortion behaviour, it is less tolerant of fit-up tolerance than bolting, and site welding is slower and weather-dependent. Most structures use both: shop welding and site bolting, which is why the connection checks are the ones this page covers.
- Who checks the plate modes?
- The connection design, and whether that is the engineer or the fabricator's detailer depends on the contract — which is worth establishing, because it is exactly the boundary at which checks get missed. Where connections are designed by the engineer, the drawing specifies the plates, the bolt pattern and the edge distances, and the checks have been done. Where the fabricator designs the connections to a schedule of forces, the responsibility and the competence sit there, and the engineer's role is to state the forces and to review. The failure mode of an unclear split is a connection detailed to fit rather than to a check: a compact group inside a coped end that satisfies bolt shear and has never been checked for block shear. Naming who does it is a contract question with a structural consequence.