PLANNING, DESIGN AND CONSTRUCTION OF STRUCTURAL CONCRETE BLOCK MASONRY K.VENKATESH BABU.B.E; CIVIL AND STRUCTURAL

ENGINEER

INTRODUCTION: Concrete

blocks

development concrete

were

in use as an alternative

that has taken place in raw materials,

blocks are being manufactured

absorption,

surfac~

finishes

etc ... The availability

to clay bricks since early

with control over its properties

and textures,

1970 in India. Due to the

production process and availability of Hi-Tech machines,

color, acoustical

properties,

like size, shape, strength, water thermal and insulation

properties

of such high quality blocks enables the designers to make use of the blocks efficiently

resulting in greater economy and improvement

in quality of construction.

This has led to the use of concrete

blocks as exposed load bearing masonry for high rise structures. The use of structural hollow concrete block masonry in multistoried construction, GENERAL

structures

but the requirements PROPERTIES

Exposed concrete

has led to considerable

economies compared to conventional

methods of

of this method need to be borne in mind by the Engineers and Architects.

OF ENGINEERED

CONCRETE

MASONRY UNITS:

block should be dense and water tight and of such shape that it conveniently

produces

water tight walls and that will not require water proofing or painting. Also it must liberally meet the minimum load and tensile strength requirements Additional

characteristics

for a specific use.

of a high quality blocks are

•

Strength and design to withstand abnormal stresses when laid in wall.

•

Sufficient

impermeability

to withstand

serious wetting by heavy rain and the ability to dry out

quickly afterwards. • •

Adherence to consistent quality, uniform size and accurate dimensions. Finished unit dried to minimum moisture content.

•

Freedom from discoloration,

•

Pleasing appearance

popping or staining.

eliminating the necessity of painting except for decoration.

The physical properties of 190mm thick two cored hollow structural concrete blocks are as below: (if

Size of full blocks

390mm x 190mm x 190mm

(ii)

Dimensional

3mm

(iii)

Weight of full block

17.5 Kg.

(iv)

Yard dry density

(v)

Water absorption

2100 - 2200 Kg/Cum. 3 to 5%

(vi)

Net characteristic

tolerance

crushing

Strength on flange area (vii)

Thermal resistance

(viii)

Estimates sound transmission

15 Mpa. to 50 Mpa. 0.38 to 0.33 M2KfW class

46 to 58 dB

... 2

-2PLANNING

METHODS:

The design of a concrete block masonry wall depends on its required appearance, economy, strength, insulation and acoustical characteristics. I. PLANNING LAYOUT OF WALLS AND COLUMNS: The layout of walls and columns involves important considerations such as functional requirement of the structure, internal arrangement of components, opening size and locations, requirement 'Of modular planning, idealization of structural concepts, provisions for shrinkage and cracking control and weather resistance. All deserve careful planning if the wall is to successfully serve its intended purpose. Masonry structures gain stability from the support provided by cross walls, floors, roofs and other elements. Load bearing walls are structurally most efficient when the structure is so planned that the eccentricity of the loading is minimized. When the stability of a building is entirely dependent on the masonry walls, it is essential that the wall be properly arranged to resist lateral loads from any direction. This may be achieved by using I.

Double cross wall arrangement.

II. A cellular wall arrangement. III.

A complex wall arrangement.

BEARING WALLS TRANSVERSE

Fig.

SHEAR WALLS

Some examples of two-directional bearing/shear wall layouts

... 3 ~3-

BEARING WALLS

TRANSVERSE SHEAR WALLS

Fig.

: Examples of multi-directional

bearing/shear wall layouts.

The cross wall arrangement is usually associated with one way spanning floor or roof slabs whereas the other forms permit the use of two way slabs. These latter forms lead to economics in the floor system and also improve the robustness of a building, giving greater protection against progressive collapse if a load bearing wall is accidentally damaged. II. DIMENSIONAL

PLANNING:

The modular nature of masonry units tends to impose a stricter discipline on, lengths, heights and particularly the thickness of walls and piers than is found in other conventional materials. Modular planning is a method of coordinating the dimensions of various building components to simplify the work and thus lower the cost of construction. In modular plan for concrete masonry construction all horizontal dimensions are given in multiples of half the nominal height of the block. The width of doors and window openings as well as wall length should be multiples of 8".

... 4

-4DESIGN METHODS: DesiQnconcepts: The design concepts and philosophies are to be well understood before attempting design of structures. It is generally believed that masonry structures are suited primarily for low rise structures. Concrete block wall has specific advantage over load bearing brick masonry or RCC framed structure as the hollow cores of the blocks can be reinforced with steel and grouted with concrete, it enables the designer to adopt anyone of the following structural system to suit the functional and architectural requirement of the building. a. b. c.

Load bearing masonry system. Framed structure with reinforced block masonry columns and beams. A hybrid system of load bearing masonry walls and reinforced block masonry columns.

Unreinforced masonry is designed to act basically in compression and is assumed to be incapable of carrying tensile stresses. Vertical loads are transmitted by load bearing walls from storey to storey down to foundations. Each floor must be designed to carry its own dead load and live loads and possibly some bending moments arising from the assumed continuity of walls and floors. Lateral loads due to winds are transmitted directly to the load bearing walls and indirectly from the floors. These lateral loads are resisted by the load bearing walls acting as continuous vertical cantilevers from the top of the building and give rise to tensile and compressive vertical bending stresses under flexure. The concrete block masonry walls are grouted and reinforced to supplement the strength of masonry in compression, tension and shear forces. Reinforced masonry increases the vertical axial load carrying capacity and also enables to use the shear wall concepts to resist the lateral forces due to wind and earthquake loads. The distribution of lateral forces between shear walls in a load bearing structure will depend upon the in-plane flexibility of the floor system and the arrangement and relative stiffness of the shear walls. The in-plane action of a floor system or diaphragm is analogous to that of the web in a plate girder. In this analogy the floor slab constitutes the web, the floor joists and beam function as stiffeners and the walls or bond beams act as flanges. The designer must also ensure that there is a reasonable probability that catastrophic collapse of the load bearing structure will not occur under the effect of misuse or accident. Damage from over loads should not be disproportionate to the nature of the overload. Research has shown that provided accidental damage provisions are considered at the design stage, the possibility of progressive collapse can be avoided with little or no extra cost to the resulting structure. The structural design shall provide for the safety, serviceability and proper functioning of the structure as a whole and its component parts, including each masonry member under the.action of the loads and forces. It shall , a. Be in accordance with accepted principle of mechanics. b. Provide for all loads and forces to be transferred through the structures to the foundation. c. Provide all necessary compatibility between each masonry member and the structural members giving vertical and/or lateral support to that masonry member. d. Be based on the material strength properties of the masonry. e. Be consistent with the following assumptions i. Plane sections remain plane under bending actions. ii. A recognized stress - strain relationship for the masonry. iii. The tensile strength of unreinforced masonry shall be taken as zero except for stresses resulting from wind loads and similar forces of short term transient nature. ...5

-5Members composed

of more than one type or grade of unit or mortar shall be designed for the properties

pertaining

to the least favorable

composite

action taking account of the different material properties of the parts.

combination

of components

used or if appropriate

shall be designed for

DESIGN FOR COMPRESSION:

Factors affecting load carrying capacity of masonry walls are (i)

The cross sectional properties of the wall based on its bedded area.

(ii)

The characteristic

(iii)

The wall slenderness

(iv)

The effective eccentricities

compressive

strength of the masonry.

ratio. of the load.

A member shall be ,designed such that the following relationship

is satisfied

Fd:: KFo a.

For ungrouted hollow units masonry: Fo = Cm fm Ab

b.

For grouted hollow units masonry: (i)

if no testing is done:

(ii)

if testing is done: Fo = Cm fmg Ad

Fo = Cm [fm Ab + Kc (fc/1.3) Ac]

Where

Fo

=

Fd

=

Cm

=

Ksz

basic compressive

capacity of the cross section

design compressive

force.

Capacity reduction factor.


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DOOR FIXATION DETAIL