It is hardened, thread-formed and has a published design capacity - a wood screw has none of those.

The thread is steeper and deeper, so it cuts its own path and compacts the fibre rather than removing it.

Hardening provides the torsional strength to drive without pre-drilling, which is where an ordinary screw shears.

Most importantly, the manufacturer publishes withdrawal and shear capacities for defined conditions, so the screw can appear in a calculation.

That makes it a substitute for bolts and coach screws in designed connections.

A wood screw with the same head and diameter is a fixing, not a connector, and cannot be used in its place.

Usually not - that is a large part of their value - but dense species, near-end positions and large diameters can still require it.

The sharpened tip and cutting thread are designed to start and drive without a pilot in most structural timber.

Very dense hardwoods can exceed the screw's torsional capacity, and the manufacturer states where a pilot is required.

Close to an end or an edge, a pilot reduces the risk of splitting even where one is not strictly required.

Pilot diameter, where specified, is smaller than for a traditional wood screw because the thread must still engage fully.

Driving speed matters: too fast generates heat and can burnish the hole, reducing withdrawal capacity.

A partial thread draws the members together; a full thread grips both without closing the gap.

With a smooth shank through the first member, the thread engages only the second, so tightening pulls the two into contact.

That is what is wanted when a gap must be closed and the members clamped.

A fully threaded screw engages along its whole length, so it holds both members where they already are.

Full threads are used for withdrawal-governed connections and for reinforcing timber against splitting perpendicular to the grain.

Using a full thread where a gap needs closing leaves the gap permanently, which is a common site error.

By the manufacturer's rules for that product, not by judgement - and they are as binding as for bolts.

Screws too close together along the grain split the timber between them, and the connection fails at a fraction of its calculated capacity.

End distance is usually the most restrictive, because timber has very little strength in tension perpendicular to the grain near an end.

Staggering rows reduces the risk substantially compared with placing them in line.

Penetration depth into the second member is a stated minimum and part of the design value.

The rules differ between products and between timber strength classes, so they come from the datasheet in use.

In many connections yes, and that is exactly what they were developed for.

They install faster, need no through-hole, no washer and no access to the far side, and no nut to tighten.

Published capacities let them be substituted in a calculation rather than assumed equivalent.

They do not loosen as timber shrinks, which bolted connections in solid timber commonly do.

Where a connection must be demountable, a bolt remains the right choice.

The substitution is a design decision, since the capacities and failure modes are not identical.

Matched to the service class and to the timber treatment - and treated timber in wet conditions is the demanding case.

Internal dry timber tolerates a plated finish; external and humid conditions do not.

Preservatives vary in how aggressive they are to zinc and to bare steel, and the compatible finishes are published with the treatment rather than with the screw.

Proprietary multi-layer coatings are common on structural screws and carry their own corrosion test data.

Stainless is used in severe exposure, though hardened stainless structural screws are a specific product rather than any stainless screw.

A corroded structural connector loses capacity invisibly, which is why the finish is specified rather than chosen.

No - design values are product-specific, and substituting invalidates the calculation.

Thread geometry, steel grade, hardening and coating all differ between makers, and all affect capacity.

Two screws of the same nominal diameter and length can have materially different withdrawal values.

Spacing and edge distance rules also differ, so a substitution changes the layout rules as well as the capacities.

Approval documents are issued per product, and a specification that names one product means that product.

Where a substitution is genuinely necessary it is a matter for the designer, with the alternative's own data.