For
ease of design, any structure is visualized and broken down into smaller units
(structural elements). Each of these elements is designed separately while
continuously integrating them using appropriate load transfer method to come up
with a structural system. A building can be divided into the following
structural elements:
· Roof:
of importance to a structural engineer is the design of the rafters, purlins
and trusses.
· Slabs: slabs
are horizontal plate
elements carrying lateral loads. There are basically 3 types of slabs depending on where they are used:
o
Ground floor slab
o
Suspended floor slab
o
Suspended roof slab
The
design of these slabs defers and therefore before designing any slab, a
structural engineer should consider where the slab is being used.
· Walls:
vertical plate elements resisting vertical, lateral or in-plane loads. The
walls in a structure are vertically and can either add to the structural
strength of the building or not. The walls are constructed as either masonry
walls, timber walls, or reinforced concrete walls. Though other wall
construction materials exist, the most widely used material for wall
construction in Kenya is masonry. The masonry walls can either be load bearing
or non-load bearing.
o
Load bearing walls are
walls that carry vertical and/or lateral loads in addition to its self weight
and transfer these loads to the structural elements below it. These walls are
mostly 200mm thick and built of strong masonry blocks with strength not less
than 3.5 N/mm2.
o
Non load bearing walls don
carry any structural loads. They have no structural significance except that
their self weight is considered in the design of structural elements below
them. They are mainly used as partitions and should be between 100 and 150mm
thick.
· Beams: are
structural elements mostly rectangular in cross-section and spans horizontally.
They take up flexural loading from the slabs, roofs and walls above it and
transfer them as point load to the columns. The beams can either be of timber,
steel or reinforced concrete. During my attachment and also in design of low
rise building, the use of reinforced concrete beams was considered.
The four types of
beams according to their uses are:
o
Ring beam:
are always put on top of load bearing masonry walls to uniformly distribute the
load and also to brace the building. Usually 450mm deep.
o
Lintel:
Lintels are 300mm deep and are short beams placed above openings such as doors
and windows to protect their frames and carry the loads due to weight of
materials above them.
o
Deep beams: can
either be downstand or upstand beam. They are usually deep, at least 4500mm
deep. They provide structural strength and resistance to bending.
o
Strip beams: mainly
provided to stiffen long slabs. Their depths equal that of the slab. They do
not carry any structural loading.
o
Ground beams: are
used at the ground level to tie and brace the columns together, especially
where there is no ground floor slabs as is with some buildings where the ground
floor is used for parking.
· Columns: are usually vertical or diagonal structural
members in cooperated to carry and transfer mainly compression loads. However,
some columns also transfer flexural loads as well as moments. Columns are
classified as either braced or un-braced.
o
Braced columns: are
those prevented from swaying by walls.
o
Un-braced: are
susceptible to sway since they are not restrained by walls.
Columns
can also be classified as short (stocky) or slender, depending on the ratio of
its height to the smallest cross-sectional dimension.
o
Short/stocky columns: clause 3.8.1.3
of BS 8110 classifies a column as being short if lex/h < 15 and ley/b< 15 otherwise if any of these 2
ratios is greater than 15 it is considered slender.
Where,
Bex
effective height of the column in respect of the major axis (i.e. x–x axis)
Bey
effective height of the column in respect of the minor axis
b width of the
column cross-section
h depth of the
column cross-section
· Foundations: Foundations
are required primarily to carry the dead and imposed loads due to the
structure’s floors, beams, walls, columns, etc. and transmit and distribute the
loads safely to the ground. The purpose of distributing the load is to avoid
the safe bearing capacity of the soil being exceeded otherwise excessive
settlement of the structure may occur.
There are many types
of foundations which are commonly used, namely strip, pad and raft. The foundations
may bear directly on the ground or be supported on piles. The choice of
foundation type will largely depend upon:
i.
ground conditions (i.e. strength and
type of soil) and
ii.
type of structure (i.e. layout and
level of loading).
o
Pad footings
are usually square or rectangular slabs and used to support a single column.
The pad may be constructed using mass concrete or reinforced concrete depending
on the relative size of the loading.
o
Continuous strip footings
are used to support loadbearing walls or under a line of closely spaced
columns. Strip footings are designed as pad footings in the transverse
direction and in the beam subject to the ground bearing pressure.
o
Where the ground conditions are
relatively poor, a raft foundation may be necessary in order to distribute the
loads from the walls and columns over a large area. In its simplest form this
may consist of a flat slab, possibly strengthened by upstand or downstand beams
for the more heavily loaded structures.
Where the ground
conditions are so poor that it is not practical to use strip or pad footings
but better quality soil is present at lower depths, the use of pile
foundations should be considered. The piles may be made of precast
reinforced concrete, prestressed concrete or in-situ reinforced concrete. Loads
are transmitted from the piles to the surrounding strata by end 