ECS 478

REINFORCED CONCRETE DESIGN

Chapter 3:

REINFORCED CONCRETE SLAB

ECS 478

REINFORCED CONCRETE DESIGN

Chapter 3:

REINFORCED CONCRETE SLAB

HILLERBORG STRIP METHOD

SLAB

HILLERBORG STRIP METHOD

SLAB

LEARNING OUTCOME (WEEK 8):

1.0 Explain the objective of the Hillerborg’s analysis

2.0 Calculate the moment and design in the slab

3.0 Draw the detailing of the slab

By completing this chapter, students shall be able to:

ECS 478 – Chapter 2 :

Continuous Beam Design & Detailing – continuous beam design

Introduction

• This method based on the Lower Bound Theorem of Plasticity.

• The basic idea of the method is to find a distribution of moments, which fulfil the equilibrium equations and designing the slab for these moments.

HILLERBORG STRIP METHOD

Simply Supported Rectangular Slab

Rectangular Slab with a Free Edge

Strong Bands

• The strong bands act as beams and more heavily reinforced compared to the rest of the slab.

• It is often convenient to increase the thickness in order to accommodate steel reinforcement.

• Distributed load on the rest slab can be distributed between the edge supports and strong bands.

Slab with a hole

• By providing strong bands around the hole, edge beams are created and the loads can be distributed between the edge supports and strong bands

Slab with re-entrant corner

• By providing a strong band, the slab is conveniently divided into two rectangular slabs which can be effectively designed separately.

• The strong band acts as an additional support to the two slabs.

FLAT SLABFLAT SLAB

LEARNING OUTCOME (WEEK 8):

1.0 Determine effective column head, Lh & effective diameter of column head, hc.

2.0 Analyse design moment of a flat slab panel by simplified method stated in clause 3.7.2.7 BS8110: Part 1.

3.0 Detail ultimate design a flat slab (flexural & shear resistance) & apply durability and serviceability checks.

By completing this chapter, students shall be able to:

ECS 478 – Chapter 2 :

Continuous Beam Design & Detailing – continuous beam design

Introduction

Defined as a slab with or without drops, supported

generally without beams by columns with or without column heads.

The slab is thicker than that required in T-beam floor slab construction but the omission of beams gives a larger clear height and simplification in construction and formwork.

FLAT SLAB (CL 3.7)

http://nisee.berkeley.edu/elibrary/Image/GoddenF79

http://www.cityu.edu.hk/CIVCAL/book/misc_advanced.html

c. (i) Slab without drop panel; (ii) slab with drop panel and flared column head.`

a. Flat slab Floor Plan b. Sectional view of flat slab structure

(i) (ii)

DropColumn Head

General Design Requirement (cl 3.7.1) l. Ly/Lx < 2. (cl 3.7.1.2)

2. Design moments may be obtained by(a) equivalent frame method(b) simplified method (cl3.7.2.7)(c) finite element analysis

3. The panel thickness is generally controlled by deflection. The slab thickness must be>125 mm. (cl 3.7.1.6)

4. Drop panels only influence the distribution of moments if the smaller dimension of the drop ≥ 1/3Lx. However, smaller drops provide resistance to punching shear. (cl 3.7.1.5)

General Design Requirement (cl 3.7.1)-con’t 5. The effective dimension of the column head, Lh is the

lesser of:(a) the actual dimension lha or(b) lhmax = lc + 2(dh - 40) (cl 3.7.1.3)

where lc = column dimension measured in the same direction as L; For a flared head lha is measured 40 mm below the slab or drop.

6. The effective diameter of a column or column head is the diameter of a circle whose area equals the cross-sectional area of the column or, if column heads are used, the area of the column head based on the effective dimensions as defined in 3.7.1.3.

In no case should hc > Lx/4. (cl 3.7.1.4)

Analysis (cl 3.7.2.7)1. The code states that normally it is sufficient to consider only

the single load case of maximum design load, (1.4gk + 1.6qk) on all spans.

2. There are at least three rows of panels of approximately equal span in the direction being considered;

3. Moments at supports taken from table 3.12 may be reduced by O.15Fhc.

4. The limitation of 3.7.2.6 need not be checked.Allowance has been made to the coefficients of table 3.12 for 20 %

redistribution in accordance with 3.5.2.3.

Design ProcedureDesign Reinforcement For Column Strip & Middle Strip.

Shear Resistance cl 3.7.7

Maximum shear stress at the face of the column

Shear stress on a failure zone 1.5d from face of the column

1.5d

1.5da

1.5

d1.5

db

Uo = 2a+2bU = 2(3d+a) + 2(3d+b)

Deflection cl 3.7.8

Cracking cl 3.7.9

Detailing cl3.12.10.3.1

Cont…

(c) Check maximum shear stress at the face of the column.(d) Check shear stress on a failure zone 1.5d from face of the column.