"Precast or in-situ?" is rarely a whole-building answer. It is tempting to frame it as a single decision - a frame is precast or it is cast in place - but the honest answer on most real projects is "both", split element by element. On a recent hospital, we made the call separately for each part of the frame, and the logic behind those splits is more useful than any blanket rule.
Why it is an element-by-element decision
Precast and in-situ are not better or worse in the abstract; they are better or worse for a given element, under that element's particular mix of repetition, tolerance, access and programme. A choice that is obviously right for a floor plank can be obviously wrong for a transfer beam. Treating the whole building as one decision forces a compromise that suits nothing; splitting it lets each element play to the method's strengths.
What went precast
The repetitive, tolerance-tolerant elements went precast:
- Floor planks, stair flights and façade panels - made many times over, in a factory, to a controlled finish and on a predictable schedule.
Repetition is the friend of precast; one-offs are its enemy.
That single sentence is most of the decision. Precast pays back its mould and setup cost through repetition, delivers factory quality away from the weather, and speeds erection because the components arrive finished. Where an element is made dozens or hundreds of times and can tolerate the joints and connections precast implies, it is hard to beat.
What stayed in-situ
The elements where continuity and coordination mattered more than speed stayed cast in place:
- The transfer structure - heavily loaded, often one-of-a-kind in geometry, and benefiting from the monolithic continuity that in-situ concrete gives for free.
- The heavily serviced cores - where last-minute changes, dense reinforcement and the need to coordinate around penetrations and embedded items make the flexibility of in-situ work worth its slower pace.
In-situ wins exactly where precast struggles: complex one-off geometry, full continuity, and tolerance for late coordination.
The deciding factors
Underneath the two lists is a consistent set of levers, weighed per element rather than per project:
- Programme - precast can compress the build, but only if the design is frozen early enough to manufacture ahead.
- Crane reach and capacity - a precast element you cannot lift into place is no use; the site logistics are part of the structural decision.
- Tolerance stack-up - precast components must fit together within achievable tolerances; the more interfaces, the more this bites.
- Connection complexity - every precast joint is a designed, detailed connection, where in-situ continuity is effectively free.
The takeaway
The framework is simple to state and harder to apply: do not ask whether the building should be precast or in-situ. Ask it of each element, weigh repetition, tolerance, crane and programme, and let the answer differ across the structure. The best frames are usually hybrids, with each method used where it is genuinely strongest. The related retrofit case study and timber-roof case study show the same element-by-element, whole-life thinking applied to other material decisions.