Why Prestressing Jacking Elongation Deviates from Theory / 为什么张拉伸长值会偏离理论值
A practical checklist for friction, duct deviation, strand slip, equipment calibration, tendon profile and reading errors. This is one of the most common site problems for PT contractors. 从摩阻、孔道偏差、滑丝、设备标定、束形和读数误差快速判断伸长值偏差，这是预应力现场最常见的问题之一。
Engineering Meaning / 工程意义
Elongation deviation is not only a calculation issue. It may indicate friction change, duct misalignment, strand slip, wire breakage, wrong jacking sequence or equipment-reading problems. 伸长值偏差不只是计算问题，可能反映摩阻变化、孔道偏位、滑丝、断丝、张拉顺序错误或设备读数问题。
- Small isolated deviation may be a reading or seating issue; repeated deviation across similar tendons usually means the assumption or construction condition has changed. 单束小偏差可能是读数或锚固回缩问题；同类钢束连续偏差通常说明计算假定或现场条件变化。
- A high elongation value and a low elongation value have different risk meanings and should not be treated with one standard comment. 偏大和偏小的风险含义不同，不能用同一句意见处理。
Typical Red Flags / 常见危险信号
The first screen should separate normal tolerance discussion from conditions that require immediate technical review. 首先要把一般允许偏差和需要立即复核的情况区分开。
- Several tendons in the same beam show the same deviation trend. 同一根梁多束钢绞线呈现同方向偏差。
- Measured elongation changes suddenly between stressing stages. 分级张拉过程中伸长值突然变化。
- Oil pressure is normal but elongation is clearly abnormal. 油压正常但伸长值明显异常。
- There are signs of strand slip, anchor seating abnormality, wire breakage, duct blockage or unusual sound. 出现滑丝、锚固异常、断丝、堵管或异常声响。
Diagnosis Logic / 诊断逻辑
A reliable review compares calculation assumptions, equipment condition and site execution records before giving a conclusion. 可靠诊断必须同时比较计算假定、设备状态和现场施工记录，不能只看一张表。
- Step 1: Check tendon length, profile, curvature, friction coefficient and anchorage-loss assumptions. 第一步复核钢束长度、束形、曲率、摩阻系数和锚具损失假定。
- Step 2: Check jack and gauge calibration, oil pressure conversion and reading sequence. 第二步复核千斤顶和压力表标定、油压换算和读数顺序。
- Step 3: Compare tendons by beam, axis, stage and stressing end. 第三步按梁号、轴线、张拉阶段和张拉端进行横向比较。
- Step 4: Decide whether the issue is calculation, equipment, construction, material or structural risk. 第四步判断属于计算、设备、施工、材料还是结构风险。
First Evidence / 首先收集的证据
We need jacking records, theoretical elongation, jack calibration report, tendon profile, duct layout, site photos, actual stressing sequence and any notes on slip or abnormal sound. 需要张拉记录、理论伸长值、千斤顶标定、束形图、孔道布置、现场照片、实际张拉顺序，以及滑丝或异常声音记录。
- Jacking record sheet with oil pressure, initial reading, final reading and elongation. 张拉记录表：油压、初读数、终读数、伸长值。
- Theoretical elongation calculation and tendon profile drawing. 理论伸长值计算书和钢束线形图。
- Jack calibration certificate and pressure gauge information. 千斤顶标定报告和压力表信息。
- Photos of anchorage zone, stressing end, tendon number marks and any abnormal site condition. 锚固区、张拉端、束号标记和异常现场情况照片。
Review Output / 复核成果
A troubleshooting report should separate calculation assumptions, equipment factors and site construction factors, then give corrective action: continue stressing, recheck equipment, monitor, release and re-stress, or stop for structural review. 诊断报告应区分计算假定、设备因素和施工因素，并给出继续张拉、复核设备、监测、卸载重张或暂停结构复核等建议。
- Deviation summary by tendon and beam. 按梁号和束号汇总偏差。
- Likely cause ranking with evidence. 带证据的原因排序。
- Recommended site action and information gaps. 现场处理建议和需补充资料。
- Clear boundary: this is technical review support and does not replace the local engineer's statutory approval. 明确边界：这是技术复核支持，不替代当地工程师法定审批。