【单选题】13 / 31跨中单点加载简支试件抗弯强度公式为?Aftm=Pl/(bh2)f_{tm}=Pl/(bh^2)ftm=Pl/(bh2)Bftm=3Pl/(2bh2)f_{tm}=3Pl/(2bh^2)ftm=3Pl/(2bh2)正确Cftm=Pl/(2bh)f_{tm}=Pl/(2bh)ftm=Pl/(2bh)Dftm=3P/(bh)f_{tm}=3P/(bh)ftm=3P/(bh)参考答案B参考解析ftm=3Pl/(2bh2)f_{tm}=3Pl/(2bh^2)ftm=3Pl/(2bh2)。