ME31B: CHAPTERTHREE

INTRODUCTION TO STRUCTURAL DESIGN

3.1 DESIGN

The process by which dimensions of structural members are arrived at.

It is a trial and error (iterative)

process. 

3.2 STEPS IN DESIGN

a) Calculation of loads which the structure/structural member shall carry.

b) Analyze to obtain the distribution of stress among the various members.

c) Arrangement of structural elements eg. beams, columns etc.

d) Design: Selection of the materials and dimensions required for each member and for the necessary connections between members.

3.3 LOADS

Loads can be: a) Dead load: Load that is not

movable or is permanently set eg. self weight of all permanent construction eg. roofs, walls, floors etc.

b) Live loads: Load that is movable e.g. persons, furniture, animals, products, wt. of stored products, equipment, vehicles etc. 

Loads: Wind Loads

c) Wind load: Live loads but usually treated separately due to their temporal nature and their complexity.

It is often the most critical load imposed on agricultural buildings.

3.4 ESTIMATION OF LOADS

a) Dead loads: See Table 5.3 (FAO Farm structures book) for estimation of loads of building materials.

b) Wind loads: Wind load depends on wind speed, location, size, shape, height and construction of a building.

Estimation of Dead Loads

Estimation of Wind Loads Contd. Where wind velocities have been

recorded, estimate expected pressures on building walls using: q = 0.0127 V 2 K

where q is the basis velocity pressure(Pa); V is wind velocity(m/s);

K = (h/6.1)2/7 where h is the design height of building (m) - eave height for low and medium roof pitches.

6.1 is the height at which wind velocities are often recorded.

Estimation of Wind Loads Contd. In places where wind velocities have not

been measured, estimate wind velocity using the Beaufort scale of winds (Table 5.1, FAO book).

Wind loads depend on whether the building is open or completely closed. It also depends on whether the roof is low or high pitched.

The following table (Table 5.2, FAO book), gives coefficients used to determine expected pressures for low pitched and high pitched gable roofs and open and closed buildings.

c) Estimation of Live or imposed loads: See Table 5.5 for mass of farm

products. Also see Table 5.4 for loads on suspended floors.

3.5 CLASSIFICATION OF LOADS BASED ON LOCATION

The classification of loads (dead, imposed and wind) are based on the duration of load. Loads can also be classified according to location:

a) Concentrated load: Acts at a point e.g. weight hanging from a ceiling.

b) Uniformly distributed load (udl): Acts along the length of a structural member eg. roof loads, wind loads etc.

SolutionSolution: Beam A carries the floor loads contributed by half the area between beams A

ands B i.e. the shaded area L. Beam C carries the loads contributed by the shaded

area M.

Load carried by beam A = 1 m x 4 m x 10 kN/m2 = 40 kN or 40 kN/4 = 10 kN/m

Load carried by beam C = 2.5 m x 4 m x 10 kN/m2 = 100 kN or 100 kN/4 = 25 kN/m

This loading per m can be used to calculate the cross sectional area of beams

required(i.e. design the beams). See later in the next chapter.

10 kN/m

25 kN/m

4 m 4 m

Loading on Beam A Loading on Beam C

Loads Classification Based on Location Contd.

c) Distributed loads with linear variation: The load is triangular eg. pressure of

grains on bins or of water on retaining walls or dams.

Stress Ultimate

stress

Dam yield stress

Strain

3.6 FACTOR OF SAFETY

Most designs are based on the ultimate stress and a factor of safety normally given as:

Factor of safety = Ultimate stress

Working stress Design stress = Ultimate or yield stress

Factor of safety

Reasons For Factor of Safety Factor of safety is used since we lack

the knowledge of all properties of the material e.g. its weakness under use.

There is also lack of knowledge of all the loadings.

Also equations to calculate stresses use simple assumptions.

Factor of Safety Contd.

The safety value is chosen based on the accuracy in the loading assumptions,

The permanency of the loads, The probability of casualties or big

economic loss in case of failure, the purpose of the building,

The uniformity of the building material, the workmanship expected from the builder and the building cost.

Factor of Safety Concluded Note: For materials like concrete that do

not have a well defined yield point or brittle materials which behave in a linear manner up to failure, the factor of safety is related to the ultimate stress(maximum stress before breakage).

In this case, factor of safety is taken as 3 to 5. For other materials like steel with well defined yield point, yield stress is used. In this case, factor of safety is 1.4 to 2.4.

For farm houses, lower values are assumed e.g. 1.5 to 2

3.7 STRUCTURAL ELEMENTS

a) Cable: Cables, cords, strings, ropes, and wires are flexible because of their lateral dimensions in relation to their length and therefore have limited resistance to bending.

b) Rods: Rods, bars and poles resist tensile or compressive loads.

c) Column

Rods and bars under compression. They are named after the material of construction e.g. stanchions (steel), piers (brickwork or masonry) and pillars (wood materials e.g. timber).

Columns are used to transfer load effects from beams, slabs and roof trusses to the foundations. They can be loaded axially or may be designed to resist bending when the load is eccentric.