Concept

Composite section — where it appears

A section made of two materials or two pours bonded together and bending as one, such as a precast beam with a slab cast on it. Its stiffness is found by transforming one material into the other, and each part creeps and shrinks on its own clock.

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

Also named here as restraint moment — the same set of essays touches all of them, so they are one junction rather than several.

The joint's moment takes the sign of the day it was cast. For two 12 m pretensioned beams, 300 mm wide and 700 mm deep, with a 160 mm in-situ slab 1.2 m wide, in 60 per cent humidity, made continuous over their pier by a joint cast with the slab: the moment the joint is asked to carry to hold their ends together, sagging positive, over thirty years, with the joint cast when the beams are 28, 90 and 365 days old; dashed, the joint's cracking moment either way, 140 kN·m. Cast at 28 days the joint is sagged to 211 kN·m; at 90 days it ends at 40; at a year it is hogged to −214.

The joint cast over the pier

Two pretensioned beams made continuous over a pier by a joint cast with the slab have ends that are still turning when the joint goes in, and the joint stops them. What it is then asked to carry depends on the day it is cast. At four weeks the beams' creep is still lifting their ends and the joint is sagged to half as much again as its cracking moment. At a year the beams have nearly stopped and the slab's shrinkage hogs the joint by as much the other way. Cast at about sixteen weeks, it is asked for almost nothing.

deflection · End rotation

A joint poured twice ends near where the slab's day puts it. For two 12 m pretensioned beams, 300 mm wide and 700 mm deep, with a 160 mm in-situ slab 1.2 m wide, in 60 per cent humidity, made continuous over their pier: the moment in the joint, sagging positive, over thirty years, with its first stage — the diaphragm and the beams' bottom connection — cast at 28 days and the slab at 42, 90 and 365 (solid), against the joint cast once, with each slab (dashed). With the slab at 42 days it ends at 139 kN·m against 160 poured once; with the slab at 90 days it ends at 41 kN·m against 40 poured once; with the slab at 365 days it ends at −165 kN·m against −214 poured once. The first stage starts each history early and the slab's weight kicks it into hog on the day it is cast; after that each solid line runs beside its dashed one.

The joint poured twice over the pier

A continuity joint over a pier can be cast in two stages — the diaphragm and the beams' bottom connection first, the slab weeks later — in the hope that the early pour catches the beams' creep and the late one leaves the slab's shrinkage little to do. It does neither. From the slab's day on the composite section holds exactly what a single pour on that day would have held, and the first stage adds only its own small account, of either sign. It trims a year-old slab's hog by a fifth, and it asks a joint with half the cracking moment to carry the beams through the months between.

deflection · End rotation

Named alongside it

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

Continuity Creep End rotation Prestress Restraint moment Shrinkage

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