Because it would bear on one edge only, putting the bolt into bending and giving an unpredictable preload.

A flat washer needs a bearing surface square to the bolt axis. On a sloping flange it sits at an angle to the head.

Contact is then along a line rather than across an annulus, so the pressure is concentrated and the washer indents.

The bolt is bent as it is tightened, adding a bending stress to the tension it was designed for.

Torque then produces less clamping force than expected, and how much less depends on the angle.

In fatigue-loaded connections the bending stress at the head fillet is where cracking starts, which is why the detail matters.

Taper opposing the flange slope, so the two cancel and the bolt head bears on a surface square to its axis.

Fitted the wrong way the tapers add, doubling the misalignment and making the joint worse than with no washer at all.

The check is simple: the exposed face the head or nut bears on should be perpendicular to the bolt.

Square washers make the orientation easier to see and harder to get wrong than round ones.

Where washers are used at both ends, each is oriented to its own bearing surface.

On a connection being inspected, a bolt head visibly canted is the sign the washer is reversed or missing.

From the section - beveled washers are made to the standard flange slopes of rolled profiles rather than to arbitrary angles.

Channel sections and certain beam profiles have a defined flange taper, and washers are produced to match.

Those standard slopes differ between regional section series, so a washer for one market's channel may not suit another's.

The section designation is the reliable starting point, and the washer supplier will state which profiles a taper suits.

Parallel-flange sections need no beveled washer at all, which is one of the reasons they became dominant.

For a non-standard angle, a spherical washer set is the correct product.

A matched convex and concave pair that accommodates a range of angles rather than one fixed slope.

The two faces slide on each other, so the assembly self-aligns as the bolt is tightened.

That suits situations where the angle is unknown, variable, or changes in service - adjustable supports, tie rods, and repairs to distorted material.

They are usually machined rather than stamped, and correspondingly more expensive.

The pair must stay together; a single half is useless.

Angular capacity is stated by the manufacturer and is typically several degrees rather than tens.

No - anywhere the bearing face is not square to the fastener has the same problem.

Cast components frequently have draft angles on their surfaces from the moulding process.

Timber connections at an angle, such as bracing meeting a member obliquely, present the same geometry.

Repairs to distorted or bent material often need the fastener to bear squarely on a surface that is no longer flat.

In each case the choice is a matched taper if the angle is fixed and known, or a spherical set if it is not.

The underlying rule is the same: the bolt should be in tension, not bending.

For high-strength structural bolts, yes - and their thickness follows from the taper, which conveniently answers the stiffness question too.

The pressure under a high-strength bolt head will emboss a soft washer, and an embossed washer has lost preload into its own deformation.

Structural beveled washers are supplied to the same hardness requirements as other structural washers.

Their thickness, which follows from the taper, makes them stiff enough to spread load without dishing.

General-purpose beveled washers exist for lighter work and should not be substituted into a designed connection.

As with all structural washers, the specification comes with the connection design rather than from the site.

The bolt is bent, the preload is unknown, and in a fatigue-loaded joint the fixing has a shortened life.

It will usually look acceptable and hold under static load, which is why the omission persists.

The head or nut bears on an edge, indenting the flange and itself.

Torque-derived preload becomes meaningless because an unknown share is going into bending the bolt.

Under cyclic load the bending stress concentrates at the head fillet and thread run-out, which are already the highest-stressed features.

In a designed connection the washer is part of the specification, so leaving it out is a departure from the design rather than a shortcut.