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PCI Design Handbook Precast and Prestressed Concrete – 7th Edition



Design of Precast and Prestressed Concrete Components



CHAPTER 5



5.2 Flexure • § 18.8.2 of ACI 318M-08:



• Bending of Asymmetrical Sections M X '  M X cos   MY sin 



MY '  MY cos   M X sin 



5.3 Shear • Axial compression improves shear capacity • Under Axial tension, Vc = 0 (§ 11.2.1.3) • Horizontal shear(Fh) – composite sections



Vu  Vnh



where Vnh is nominal horizontal shear strength



– Total force in topping – Compression due to sagging – Tension due to hogging



Contd.



Contd.



5.3 Shear – Shear Friction • § 11.6 of ACI 318M-08 – An existing or potential crack – Interface between dissimilar materials or – Interface between Concrete cast at different times Vu Avf  f y 



where μ is coeff. of friction



Contd.



5.4 Torsion • Low fixity compared to cast-in-situ members • Based on the Zia and McGee Proceedures – § 11.6.7 of ACI 318-05 : when d/b > 3



Cntd.



Cntd.



5.4 Torsion - Design



1. Determine Vu and Tu at critical section 2. If Tu ≤ Tu(min) Neglect torsion Vu T u 3. Ensure: Tn (max)  and Vn (max)  







(Avoid: potential compression failure due to over-reinforcing)



4. Calculate shear and torsion carried by concrete Tc and Vc 5. Stirrups for Tu - Tc & min. web reinforcement for ductility 6. Provide longitudinal reinforcement for diagonal tension due to torsion



5.5 Beams with Ledges (eg: L beams) • • • • • •



Shear strength of Ledge: Refer PCI handbook Transverse (Cantilever) Bending of Ledge Longitudinal Bending of Ledge Attachment of Ledge to Web Out-of-Plane Bending near Beam End Pocketed Spandrels Refer PCI Handbook (Important)



Cntd.



5.6 Bearing • On Plain Concrete: when Vu ≤ φ(0.85 f’c A1) • On RC: when Vu > φVn – Additional hooks



5.6.3 Dapped-End-Bearing – Failure modes •