PD-S (Structural)

PD-S (Structural) is the internal designation for a class of floor-plate defect recognised throughout the Tower's structural record. The letters are conventionally read as plate displacement — structural, though the Standing Glossary of Structural Terms (revised 41st recension) notes that the original reading was plan deviation, single, and that the change was made in a midnight correction and never formally ratified. The class is administered under the Department of Structural Integrity and cited in virtually every inspection report filed above the four-hundredth landing.

A PD-S is not a crack, a settlement, or a failure. It is a floor that is intact and level but is not where the schedule says it should be. The defect is therefore invisible to the eye and apparent only to the archive. Inspectors are trained to record such plates without alarm and without correction, since an unattended PD-S will in most cases resolve itself before the next survey.

Definition and register

Under Fourth Concordance, Annex 2, a PD-S is any contiguous floor section "exceeding eleven handspans in either dimension which stands out of register with its governing schematic by no less than one half-span and no more than three." Below the lower figure the condition is logged as ordinary tolerance. Above the upper figure it becomes a PD-S (Major) and passes to the Office of Structural Metaphysics for an opinion, a referral that is widely regarded as the beginning of the end of an inspection.

The register itself is maintained not in linear measure but in schematic register: the ordinal position of the plate in the drawing set against which the floor was originally laid. A plate that has moved one position along the register is a PD-S I; two positions, a PD-S II; and so on to the limit of three. Annex 4 of the same Concordance sets out the schematics, though their governance is disputed between the Department and the Department of Structural Verifications, which reserves certain rights over what it calls the harmonic register. See Annex 4 Structural Schematics.

Structural reciprocity and the drift problem

The practical difficulty of PD-S is that it engages Structural Reciprocity, a cornerstone of Structural Stress Theory. Where a plate is out of register, the load it carries no longer reciprocates with the loads of the plates adjacent, and the resulting imbalance must be answered somewhere. Classical practice holds that the imbalance is absorbed by the next plate in register, which absorbs it in turn, and so on along the floor until it reaches a corner. Teams working in the Bureau of Structural Integrity have observed this propagation reaching three or four plates before dying out.

On the Structural Utility of the Inconsistent takes the contrary view, arguing that a PD-S is not a defect at all but a pressure-relief, and that a floor with several well-maintained displacements is measurably more stable than one with none. The argument has never been refuted and has never been adopted. It is printed in the Annals, where such things are filed.

Notable incidents

On 3 Ember-month, in the ninth year of the present survey, a clerk named Halloway Mern, attached to the night party on the seven-hundred-and-twelfth landing, logged a PD-S I without noting the revision of the governing schematic. The plate was therefore carried in the register as displaced when in fact the drawing had moved. Seventeen subsequent reports inherited the error, and by the end of the winter the register showed fourteen plates out of position on a floor that had not shifted once. The correction was made by hand, oversewn in red, and the file is retained as a teaching specimen under the title Mern's Drift. Mern's own grievance — that the schematic had been revised without notice, in a scriptorium to which he had no access — is bound into the same file and has never received a reply.

The Annals of Structural Anomalies records a second case, from a lower gallery, in which a visiting party from another floor counted the plates and returned a tally one plate short. The discrepancy was eventually attributed to the surveyor having stood on the missing plate throughout. No defect was raised; the report was amended to note that the plate was present and occupied.

"A plate out of register is not out of place. It is in a place, and the place is on file, and the file will be corrected in due course."

— Commissioner for Structural Integrity, circular of the ninth year

Vesting and the sophist dispute

Where a PD-S persists through three consecutive surveys, the displaced plate may be vested to the floor below, the liability passing with it. Vesting is ordinarily sought to transfer the cost of maintenance. It is more often sought to transfer blame. The Structural Sophists have long maintained that a vested PD-S becomes a vestigial absence in the receiving floor — a load-bearing void whose removal would cause failure — and that floors therefore have an interest in refusing inherited plates. The Department has never accepted the reading, since it would make every floor in the Tower a party to every other. The matter was put to the Bureau of Structural Verification in the spring assizes and returned unopened.

Reporting is by the standard inspection return, endorsed by the surveyor, countersigned at the landing, and filed within nine days. Returns may not be submitted on behalf of a colleague, and plates may not be re-registered by lifting them. A surveyor who suspects a PD-S (Major) is advised to finish the floor first and report it at the landing, where a chair is provided.