How Base Steel Thickness and Strength Affect Screw Performance

The screw is only one part of the connection

The steel receiving the thread is part of the fastening system and often controls the failure mode. Its thickness and mechanical properties affect both installation and connection resistance.

Changing the steel thickness

A thicker supporting member generally gives more thread engagement and can increase pull-out resistance. For a self-drilling screw, however, the same increase in thickness also increases the work required from the drill point. A stronger connection after installation is of little use if the selected point cannot drill the support reliably.

Changing the steel strength

Higher-strength steel can provide more resistance around the engaged thread, but it can also be harder to drill and form. Test data or drilling limits established in one steel grade should not be transferred automatically to a substantially different grade.

Thickness and strength cannot be considered separately

A thick low-strength sheet is not necessarily equivalent to a thinner high-strength sheet. Connection equations and test programmes normally account for both geometry and material strength because both affect the failure mechanism.

Typical failure modes

• thread pull-out from the supporting steel;
• pull-over or local deformation of the attached sheet;
• screw tensile or shear failure;
• interaction between more than one connection mode;
• failure during drilling or installation before the connection is properly formed.

Drilling and resistance are separate checks

Do not use a structural capacity check as proof that the screw can drill the specified build-up. Drilling capability has to be verified separately for self-drilling products.

When the substrate changes

If the support thickness or steel grade changes, review both the drilling range and the connection resistance. A value established for another substrate should only be used where the applicable calculation method or technical documentation permits it.