体外FRP筋的极限应力分析

为获得体外预应力混凝土梁的正截面抗弯强度、必须首先确定体外预应力筋的极限应力。在过去的半个多世纪中,许多学者对体外或体内无粘结预应力筋的极限应力进行了试验及分析研究,但这些研究主要以钢材、很少以纤维复合材料(FRP)作为研究对象。本文根据相关的试验研究数据,分析了以FRP为体外筋和以钢材为体外筋的情况下,等效塑性区长度与破坏截面中性轴比值的差异及变化。分析表明,对FRP体外预应力混凝土梁,这一比值比较稳定,并可取为常数。但该常数与以钢材为体外预应力筋的构件的有所不同。本文建立了分别以FRP和钢材为体外预应力时,构件的等效塑性区长度与破坏截面中性轴比值的转换关系式。在此基础上,建议了既适用于简支梁,亦适用于连续梁;既适用于常规的体外预应力钢筋,亦适用于非常规的体外预应力FRP筋的体外预应力筋极限应力计算方法。

Ultimate stress analysis of FRP bars in vitro

In order to obtain the flexural strength of external prestressed concrete beams, the ultimate stress of external prestressed tendons must be determined first. In the past half century, many scholars have conducted experiments and analysis on the ultimate stress of unbonded prestressed tendons in vitro or in vivo, but these researches mainly focus on steel and rarely on fiber composite materials (FRP). Based on the relevant experimental data, the difference and change of the ratio between the length of equivalent plastic zone and the neutral axis of the failure section are analyzed in the case of FRP and steel reinforcement. The analysis shows that for external prestressed concrete beams with FRP, this ratio is stable and can be a constant. However, this constant is different from that of the components with steel as external prestressing tendons. In this paper, the conversion relation between the length of equivalent plastic zone and the ratio of neutral axis of failure section is established when FRP and steel are prestressed in vitro respectively. On this basis, it is suggested that it can be applied to both simple supported beams and continuous beams. The calculation method of the ultimate stress of external prestressed FRP bars is suitable for both conventional external prestressed steel bars and non-conventional external prestressed FRP bars.