twenty three continuous beams no beams concrete construction: - PowerPoint PPT Presentation

A RCHITECTURAL S TRUCTURES : Systems F ORM, B EHAVIOR, AND D ESIGN ARCH 331 • beams separate from slab D R. A NNE N ICHOLS • beams integral with slab S PRING 2019 – close spaced lecture twenty three • continuous beams • no beams concrete construction: T-beams & slabs Concrete Slabs 1 Architectural Structures F2009abn Concrete Slabs 2 Foundations Structures F2008abn Lecture 23 ARCH 331 Lecture 23 ARCH 331 T sections T sections • negative bending: min A s , larger of: • two areas of compression in moment   6 f 3 f possible   c c A ( b d ) A ( b d ) s w s f • one-way joists f f y y • effective width (interior) • effective flange width – L/4 – b w + 16t – center-to- C f center of C w beams T Concrete Slabs 4 Foundations Structures F2008abn Lecture 23 ARCH 331 Concrete Slabs 3 Foundations Structures F2008abn Lecture 23 ARCH 331 1

One-Way T sections • • Joists usual analysis steps – standard 1. assume no compression in web stems 2. design like a 0.85 f’ c – 2.5 ” to 4.5 ” rectangular beam a/2 a=  1 c C slab 3. needs reinforcement – ~30 ” widths in slab too T – reusable 4. also analyze for negative forms moment, if any Concrete Slabs 5 Foundations Structures F2008abn Concrete Slabs 6 Foundations Structures F2008abn Lecture 23 ARCH 331 Lecture 23 ARCH 331 One-Way Compression Reinforcement • Joists • doubly reinforced – wide pans • negative bending – 5 ’ , 6 ’ up • two compression forces – light loads & • bigger M n long spans • control deflection – one-leg • increase ductility stirrups • needs ties because of buckling Concrete Slabs 7 Foundations Structures F2008abn Concrete Slabs 8 Foundations Structures F2008abn Lecture 23 ARCH 331 Lecture 23 ARCH 331 2

Compression Reinforcement Slabs • analysis • one way behavior – like beams – A s & A s • two way behavior – more complex – T = C c + C s – T = A s f y – C s = A s ( f s - 0.85 f c)   – C c = 0.85 f c ba with a c 1 – f s not known, so solve for c (n.a.) – f s < f y ? – M n = T(d-a/2)+C s (d-d ) Concrete Slabs 9 Foundations Structures F2008abn Concrete Slabs 10 Foundations Structures F2008abn Lecture 23 ARCH 331 Lecture 23 ARCH 331 Slab Design Slab Design • min thickness by code • one unit wide “ strip ” • reinforcement • with uniform loads – bars, welded wire mesh – like “ wide ” beams – cover – moment / unit width – minimum by steel grade – uniform curvature • 40-50: • with point loads A s    0 . 002 – resisted by stiffness bt • 60: A s of adjacent strips    0 . 0018 – more curvature in middle bt Concrete Slabs 12 Foundations Structures F2008abn Concrete Slabs 11 Foundations Structures F2008abn Lecture 23 ARCH 331 Lecture 23 ARCH 331 3

One-Way Slabs Precast • A s tables • prestressed • max spacing – PCI Design Handbook –  3(t) and 18 ” – double T ’ s –  5(t) and 18 ” – temp & shrinkage steel – hollow core • no room for stirrups – L ’ s • topping • load tables Concrete Slabs 13 Foundations Structures F2008abn Concrete Slabs 14 Foundations Structures F2008abn Lecture 23 ARCH 331 Lecture 23 ARCH 331 4