Prestressed rebar (wire strand) quantity calculation: pre-tensioning vs post-tensioning calibers, curved segments by true length not projection, anchorage adjustments, and railway/highway/municipal differences

Topic Positioning

Rules for computing prestressed rebar (low-alloy bars, carbon wire, wire strands, strand) mass (tonnage): length calibers for pre- vs post-tensioning, the "true length, not projection" rule for curves, post-tensioning anchorage adjustments, and differences among municipal, railway, building, and highway norms (BOQs) — for unified calibers in budgeting, BOQ preparation, and takeoff review.

Core Conclusions

1. Basic formula: mass = design length x theoretical unit mass, in t. Railway: "the mass of prestressed rebar (wire, strand) is design cutting length x theoretical unit mass. Anchors, ducts, anchor plates and tie plates, end sealing, binding, and welding materials shall not be counted" (TB/T 10823-2024 Vol. 2 volume notes).
2. Pre-tensioning: "pre-tensioned rebar by member-dimension length x unit theoretical mass in t" (SJG 181.2-2024, 5.2.5).
3. Post-tensioning: by design duct length per anchorage type (SJG 181.2-2024, 5.2.6): (1) low-alloy bars with screw anchors both ends: duct length - 0.35m, screws separate; (2) one end upset-plate, other screw anchor: duct length, screws separate; (3) one end upset-plate, other bearing-plate anchor: +0.15m; bearing-plate both ends: +0.3m; (4) concrete self-anchoring: duct length + 0.35m; (5) carbon wire with conical anchors: duct <= 20m +1m; > 20m +1.8m; (6) carbon strand with upset anchors: +0.35m; (7) low-alloy strand with JM/XM/OVM/QM anchors: <= 20m +1m; > 20m +1.8m. Railway buildings share these values (TB/T 10831-2024 Vol. 10 volume notes).
4. Curved segments — true length, not projection: "the computed length of prestressing tendons should be the actual length, not the projected length" (ZJQ08-SGJB 018-2018 Annex I, I.0.3, note 1 — the general caliber). Post-tensioned curves are measured along the developed duct arc, not by horizontal projection or straight distance, nor by field cut length.
5. No stock-length laps: tendons are supplied whole; the ordinary-rebar "one lap per 9m/12m stock" rule does not apply; per GB/T 50854-2024 Annex E.7.9(11), only design- (or code-) specified laps count.
6. Itemization: prestressed rebar, wire, and strand are coded separately by material type and spec (E.7.9(7)).
7. Sector differences (never mix):
   • Railway (TB/T 10823-2024, TB/T 10831-2024): design cutting length x unit mass; anchorage adjustments as above;
   • Municipal (SJG 181.2-2024): pre-tension by member length; post-tension by duct length with anchorage adjustments;
   • Highway estimates (JTG/T 3831-2018 Vol. 2): "the quantities of prestressed strand and rolled thread bars include the mass of working lengths"; "pre-tensioned strand mass is the drawing mass; the norm already includes strand waste and the working/tensioning lengths between precast yard members".
8. Norm inclusions (prevent double counting): highway prestress norms already include strand making/inserting/tensioning, corrugated-duct or sleeve-hole forming, and grouting; bearing plates and spiral bars are in the anchor price — "these shall not be computed again"; purchased corrugated ducts may be substituted. Railway prestress items include duct grouting. Check volume notes before applying.

Applicable Boundaries

• Applies to prestressed rebar/wire/strand mass (t); ordinary (non-prestressed) rebar laps and merges follow the general rebar calibers.
• Each sector applies its own: municipal SJG 181.2-2024, railway TB/T 10823-2024 and TB/T 10831-2024, building BOQ GB/T 50854-2024, highway JTG/T 3831-2018; the executed sector, local current norms, and contract govern.
• The 20m boundary and anchorage adjustments follow the norm text; design-specified anchor forms and cutting lengths govern.
• "True length, not projection" is quoted from a bridge standard annex (context: segmented elongation calc); tensioning-elongation details are out of scope.
• Not applicable: ordinary-rebar stock laps, non-prestressed takeoff, tensioning operations (control, elongation checks).

Further Reading

• SJG 181.2-2024, 5.2.5 (pre-tension), 5.2.6 (post-tension adjustments).
• TB/T 10823-2024 Vol. 2 volume notes (design cutting length; no anchors/ducts).
• TB/T 10831-2024 Vol. 10 volume notes (same adjustments; laps and splices per drawings and codes).
• GB/T 50854-2024 Annex E.7.9 (items 1, 7, 11).
• JTG/T 3831-2018 Vol. 2 rebar notes (working lengths; pre-tension by drawing mass; no re-computation).
• ZJQ08-SGJB 018-2018 Annex I, I.0.3 (true length; segmentation and friction).