Course · lesson 6 of 6

The apex and the base

The last two joints of the portal frame: the bolted apex, under gravity and reversed under uplift, with the STATIX apex haunch calculator (EN 1993-1-8), and the pinned column base with holding-down bolts and shear transfer, with the base plate calculator (SANS route).

1. Learning outcomes

After this lesson you can:

say which bolts are in tension at the apex under gravity and under uplift;
read the apex page’s check and know which case it covers;
set out the actions on a pinned portal base;
check holding-down bolts in tension and the shear transfer;
know the anchor layouts the base plate page can and cannot check.

2. Before you start

Lesson 5 (the eaves haunch) and Steel Design 1, lesson 7 (base plates: not repeated here); lesson 4 for the actions.
The free apex haunch and base plate calculators.
A pocket calculator and about 50 minutes.

3. The apex

The apex. The two rafters meet at the ridge, each with an end plate; the plates are bolted to each other, usually with a short haunch below to deepen the joint. There is no column: both sides of the joint are plates.

The sign at the apex. Under gravity the apex sags: the bolts near the haunch flange are in tension and the rafter top flange is in compression. Under uplift it hogs and the top bolts take the tension. Both cases need checking.

The ridge bolt layout. The STATIX apex page lays the rows out in two clusters, half near the rafter top flange and half near the haunch flange, so one cluster is in tension whichever way the moment turns.

What the apex page checks. It runs the EN 1993-1-8 component method on the haunched ridge and keeps the sign of the moment: + for gravity (sagging, the bottom cluster in tension), − for uplift (hogging, tension at the rafter top). With the symmetric two-cluster layout the two answers are close, but they are different checks. The ridge plate sits on the plumb cut, so it is longer than the rafter depth. SANS 10162-1 has no end plate method; cross-check bolts and welds by SANS where fair.

The apex haunch follows the eaves haunch rules of lesson 5: the same steel as the rafter, a flange no steeper than 45°, welds checked for the shear flow, and the toe where the flange force turns into the rafter web.

4. The pinned base

The pinned portal base. Most portal columns stand on a plate with four anchors near the column and a grout layer, treated as a pin: it passes axial force and shear but no moment, which keeps the footing small (Foundations 1) and the frame moments as lesson 4 found them.

Actions at a pinned base. Under gravity the column pushes down with a horizontal thrust outward; under uplift the column pulls the base up, and the wind adds to the horizontal force. The holding-down bolts and the shear transfer both need the uplift case.

Holding-down bolts in tension. The plate bends round the anchors (a T-stub, as in Steel Design 1 lesson 7); each anchor needs its steel strength and its anchorage in the concrete: cone breakout and pull-out of the head. SANS 10162-1 sends anchorage to the concrete code; the STATIX page uses EN 1992-4 for it.

Shear transfer. Friction under the plate works only while the base is in compression (SANS gives no coefficient; the page takes 0.20 from EN 1993-1-8). Under uplift the anchors carry the shear in bending and bearing, or a shear lug under the plate does, with its own concrete and geometry rules.

Anchor position. The base plate page needs the tension anchors outside the column flange welds, as in this plan. Many pinned bases put the anchors inside the flanges, close to the web, to keep the joint flexible; the page blocks that layout (NOT CHECKED), so this lesson uses a plate large enough for the anchors to sit outside.

5. The building, complete

The course on one building: from layout and load paths, through the cold-formed purlins and girts, the frame members and the eaves, to the apex and the base that takes it all into the footing.