Advanced Structural Design - Lecture Note 06

8/10/2019 Advanced Structural Design - Lecture Note 06

1/16

10/30/2014

Prepared by: Eng. Chamil Duminda Mahagamage

B.Sc.Eng (Hons), C Eng, MIE(SL)1International College of Business and Technology

6102BEUG- Lecture Note: 06

DESIGN OF MASONRY STRUCTURES

EC6 applies to the design of buildings and civil engineering works in, predominantly,

unreinforced and reinforced masonry. It is based on limit state principles and ispublished in four parts as named below

Part 1.1 General rules for reinforced and unreinforced masonry

Part 1.2 Structural fire design

Part 2 Design consideration, selection of materials and execution of masonry

Part 3 Simplified calculation methods for unreinforced masonry structures.Part 1.1 of EC6 provides the basic information necessary for the design of masonry

structure and deals with the material properties and gives detailed rules which are

mainly applicable to ordinary buildings.

Part 1.2 deals with the accidental event of fire and addresses, amongst other aspects,

fire protection of load bearing members to prevent premature collapse of the structure

and the measures required to limit the spread of fire in masonry structures.

Part 2 provides guidance on, amongst other aspects, the selection of mortars and

masonry units for various exposure conditions and applications.

Part 3 contains simple rules for the design of various types of unreinforced masonry

walls including panel, shear and basement walls.


2/16

10/30/2014

Prepared by: Eng. Chamil Duminda MahagamageB.Sc.Eng (Hons), C Eng, MIE(SL)

2International College of Business and Technology


List of Symbols

Refer Cl. 1.6 of EC6 for full list of symbols.


3/16

10/30/2014




Design Basis

EC6 is a limit state code. The two principal categories of limit states relevant to the

design of masonry structures are durability and strength. Design for durability is

discussed in chapter 4 of EC6 and largely relates to the selection of masonry units and

mortars for particular structure types and exposure classes. The design rules dealing

with ultimate limit states are given in chapter 6 of EC6. Generally, in order to assess the

effect of these loading conditions on masonry structures the designer will need to

estimate

- the design values of actions

- the design strength of materialsDesign values of actions


4/16

10/30/2014




Design compressive strength

The design compressive strength fd= fk/M

where,

fk= characteristic compressive strength of masonry

M= partial factor for materials

The characteristic compressive strength of masonry is a function of the following

material characteristics:

- Group number of masonry unit

- Normalised mean compressive strength of masonry unit- Compressive strength of mortar

The partial factor for materials is a function of the following aspects:

- Category of (masonry unit) manufacturing control

- Class of execution control

Some of the above information is provided by the manufacturer of the masonry units

whereas the others are determined using the guidance given in EC6.


5/16

10/30/2014





6/16

10/30/2014




Normalised compressive strength

The normalised compressive strength, fb, is the compressive strength converted to the

air dried compressive strength of an equivalent 100mm wide x 100mm high unit of the

same material.

fb= conditioning factor x shape factor x declared mean compressive strength

Compressive strength of Mortar


7/16

10/30/2014




Category of unit manufacturing control

Units manufactured in accordance with European specifications can be further classified

as belonging to Category I or Category II depending on the manufacturing control.

Category I units are those where the manufacturer operates a quality control scheme

and the probability of the units not reaching the declared compressive strength is less

than 5%. Masonry units not intended to comply with the Category I level of confidence

are classified as Category II. It is the manufacturers responsibility to declare the

category of the masonry unit supplied.

Class of execution controlEC6 allows for up to five classes of execution control but, as in BS 5628, only two classes

are used in the UK National Annex, namely, 1 and 2. Class 1 or Class 2 execution

control should be assumed whenever the work is carried out following the

recommendations for workmanship in EC6-2. Class 1 execution control corresponds to

the specialcategory of construction control used in BS 5628 and Class 2 to the normal

category.


8/16

10/30/2014




Characteristic compressive strength of masonry

The characteristic compressive strength of unreinforced masonry, fk, built with general

purpose mortar can be determined,

fk

= K*fb

0.7*fm

0.3

where,

fm= the compressive strength of general purpose mortar but not exceeding

20N/mm2or 2fb, whichever is the smaller

fb= the normalised compressive strength of the masonry units

K= a constant obtained from following Table

Partial factor for materials M


9/16

10/30/2014




Design of unreinforced masonry walls subjected to vertical loading

Design value of the vertical load NEdDesign vertical resistance of the wall NRd

Cl. 6.1.2.1

= ,

where,

,= the capacity reduction factor, at the top or bottom of the wall, or , in the

middle of the wall, as appropriate, allowing for the effects of slenderness and

eccentricity of loading, obtained from 6.1.2.2.t= thickness of the wall

fd= design compressive strength of the masonry.

= 1 2/ =

+ 0.05

= the eccentricity at the top or bottom of the wall

= the design bending moment at the top or bottom of the wall

= the design vertical load at the top or bottom of the wall

= the eccentricity at the top or bottom of the wall, if any, resulting from horizontal

loads

= the accidental eccentricity resulting from construction in accuracies and can be

taken as hef

/450


10/16

10/30/2014





11/16

10/30/2014




Note-: A simplified sub-frame analysis procedure described in Annex C of EC6 can be

used to calculate the values of the bending moments and then the corresponding values

of ei and emk. The smaller of and is used to estimate the vertical design

resistance of the wall.

Clause 6.1.2.1(3) notes that where the cross sectional area of a wall is less than 0.1m2,

the characteristic compressive strength of masonry should be multiplied by (0.7+3A),

where A is the loaded horizontal cross sectional area of the member, expressed in m2.

Effective Height, hef(Cl. 5.5.1.2, EC6)

The effective wall height is a function of the actual wall height, h, and end/edge

restraints.

hef= nh


12/16

10/30/2014




Effective thickness, tef(Cl. 5.5.1.3, EC6)

The effective thickness tef, of a single-leaf wall, a double-leaf wall, a faced wall, a shell

bedded wall and a cavity wall, as defined in 1.5.10, should be taken as the actual

thickness of the wall, t.

For cavity walls in which the leaves are connected by suitable wall ties


13/16

10/30/2014




For walls stiffened by piers


14/16

10/30/2014




Example 01

The internal load-bearing brick wall shown in Figure below supports an ultimate axial

load of 140kN/m including self-weight of the wall. The wall is 102.5mm thick and 4m

long. Assuming that the manufacturing control of the units is category II and the

execution control is class 2, design the wall.


15/16

10/30/2014




Example 02

Re-design the wall in Example 01 assuming that it is only 0.9m long.

Example 03

A 3.5m high wall shown in cross section Figure below is constructed from clay bricks of

standard format size having a declared air dried mean compressive strength of

30N/mm2 laid in a 1:1:6 mortar. Calculate the ultimate load bearing capacity of the wall

assuming the partial safety factor for materials is 3.0 and the resistance to lateral

loading is (A) enhancedand (B) simple.


16/16

10/30/2014




Example 04

A cavity wall of length 6m supports the loads shown in Figure below. The inner leaf is

built using solid concrete block of 440mm (L) x 100mm (W) x 215mm (H) , faced with

plaster, and the outer leaf from standard format clay bricks. Design the wall assumingthat the execution control is Class 1 and that the manufacturing control of the concrete

blocks is Category II. The self weight of the blocks and plaster can be taken as 2.4kN/m2.

Advanced Structural Design - Lecture Note 06

Documents

Transcript of Advanced Structural Design - Lecture Note 06