Structural Engineering & Statics
where the load goes
Concept

Tied arch — where it appears

An arch whose horizontal thrust is carried by a tie between its springings rather than by the ground. The thrust is unchanged — it is still wL²/8f — but it is internal, so the foundations carry vertical load only and the structure can stand on bearings rather than on abutments.

Named by 4 essays across 2 fields — each of them below, with the objects they name alongside it.

The tie is a redundancy, so its stiffness decides the thrust. Thrust and rib bending for a 60 m tied arch of 0.15 rise ratio, against the stiffness of its tie. Cut the tie and the structure is a curved simply supported beam, so the tie force is the one redundant and the force method gives it: with a rigid tie the answer is 2227 kN, within 1.0 per cent of the funicular wL²/8f, and the shortfall is the arch's own axial shortening. A real tie stretches 68 mm and returns 2166 kN — 2.7 per cent of the flexibility is the tie — and whatever thrust the arch does not get, it carries as bending: 760 kNm at 30 m. A tenth of the tie stiffness is not a tenth of the problem; it is a different structure.

The thrust that never reaches the ground

The arches so far have all ended at the same sentence — the foundation is where an arch is really decided. A tie changes the sentence without changing the arithmetic: the horizontal force is still there, still the same size, and it now closes on itself through a bar at deck level.

structures · Tied arch Cross the hangers and the chords stop bending. The same tied arch, the same sixteen hangers, the same load on half the span — hung vertically and hung as a network. Vertical hangers make the two chords a Vierendeel frame, which has no truss action at all, so a partial load is carried by bending: 3316 kNm in the tie and 6234 in the arch. Inclined hangers can carry the shear between the chords axially, and the same load gives 686 and 831 — factors of 4.8 and 7.5. The thrust is identical in both, because that is decided by the span and the rise and nothing else.

Cross the hangers and the bending goes

A tied arch with vertical hangers is a Vierendeel frame with a curved top chord — it has no truss action at all, so a load on half the span is carried by bending. Incline the hangers so they cross and the same two chords become a truss.

structures · Network arch A hanger that would push is a hanger that is not there. The force in each hanger of a tied arch of 100 m span and 17 m rise on 32 crossed hangers, carrying 60 kN/m of dead load over the span and 120 kN/m of live load over its left half, against the hanger's position along the tie. Solved as though every hanger could push (pale bars), 4 come out in compression, the worst at 185 kN. Solved with those hangers taken out until no remaining one is compressed (dark bars), 8 are slack — more than the linear solution has in compression, because each hanger that goes passes its share to its neighbours and some of them follow.

The deck that keeps the hangers in the truss

Crossing the hangers of a tied arch turns it from a frame that bends into a truss that does not, as long as every hanger is pulling. A hanger cannot push, and under a load on half the span some of them would have to. Solved as it really is — with those hangers taken out, and the ones that follow them — the arch keeps the linear answer exactly until the live load on half its span is about equal to its own dead weight, and past that the tie's moment doubles and then quadruples. The arch's own weight is the pretension nobody had to apply, and a heavier deck is a stiffer network.

structures · Network arch Hung from its deck, the rib's bending is not amplified at all. The bending moment along a 60 m parabolic rib at a 12 m rise (EI 250,000 kN·m²) under 25 kN/m of dead load and 10 kN/m of live load on the left half. Standing free on pinned springings (solid) it peaks at 1,303 kN·m, against 576 at first order (dashed): the thrust amplifies the half-span bending 2.26 times. The same rib hung over a deck 0.10 times as stiff as the rib on vertical hangers (thick) peaks at 524 kN·m, 0.99 times its first-order 528, and the deck (thin) carries 59. The deck closes the thrust in tension, and the tension it carries straightens the deck as much as the thrust bends the rib.

The rib that cannot lean on its own tie

A free arch rib under a half-span live load bends in the shape of its own buckling mode, and its thrust amplifies the bending more than twofold. Hang a deck from the same rib on vertical hangers and close the thrust in the deck, and the amplification disappears: the thrust and the tie's pull move together, so the couple they make keeps its lever arm, and the only second-order term left is the small difference between the rib's deflection and the deck's. The critical load rises sixteenfold, and the deck's share of the bending is simply its share of the stiffness.

stability · Arch buckling

Named alongside it

The objects these essays reach for when they reach for this one.

HangerLoad arrangementThrustArchAmplification factorAxial shorteningBending momentBucklingCableCompatibilityDead loadDeck

All concepts