DESIGN CRITERIA FOR BUILDINGS

To analyze or design a structure, it is necessary to establish criteria for determining whether a given structure is acceptable for use in a specified circumstance or for use directly as a design objective that must be met. The most important criteria are;

Serviceability. Efficiency, Construction, Costs, Other

The structure must be able to carry the design load safely without excessive material distress and with deformations within an acceptable range. The ability of a structure to carry loads safely and without material distress is achieved by using safety factors in the design of the element. By altering the size, shape, and choice of material, stresses in a structure can be maintained at safe levels and such that material distress(e.g. cracking)does not occur. This is basically a strength criterion and is of fundamental importance.

Analysis and design process

The overall structural analysis and design process typically consists of several steps

Geometry definition: – the basic geometry of the structure is defined first, with particular attention paid to member hierarchies (which member supports which other members) and spanning directions.

Load assessment: – the loads acting on the structure are determined next. Typically, this involves determining loadings associated with the so-called live loads on the structure resulting from its occupancy (e.g. loadings do to wind and earthquake forces) and the so-called dead loads associated with the self wait of building elements.

Modeling of the structure and boundary conditions:-the structure and its constituent elements are modeled. Typically, such modeling includes characterizing complex real-world construction connections consisting of items such as anchor plates, bolts, etc as one or another an idealized set of support conditions (e.g. pins, rollers, or rigid joints)

Design methods

Permissible stress method (working load method)

In this method, permissible stress in the members are restricted by a factor of safety

to be sufficiently below the ultimate stress of the material to be well within the limit of proportionality of steel reinforcement.

Load factor method

It is the ratio of ultimate load to the working load. This theory asses ultimate bending to mad use of plastic action of concert and pick stress are calculated by elastic theory and relieved by plastic action. Thus, it is based on plastic theory.

Limit state design method

It includes serviceability limit states and ultimate limit states.

A serviceability limit state includes deflation, cracking, vibration, robustness, etc to be controlled and checked.

The ultimate limit state requires that the strength of the stricture should be adequate to withstand the design load s with due consideration being governed where appropriate to buckling and the general overall stability.

Slab types analysis and design

There are many types of slabs such as solid slabs, ribbed slabs, flat slabs, flat plate slabs, etc.

All of the above types of slabs can be analyzed and designed by using elastic or plastic theories.

Elastic methods are a coefficient method and finite element method.

Plastic methods are yield line method and hillerborg, strip methods of analysis and design.

1.7. Usually used slab analysis and design methods [ref 2&3]

The coefficient method is based upon elastic theory but the reinforcement for slabs is calculated by strength methods that account for the actual inelastic behavior of structural members at the factored load stage. This method limits the structural use of the material up to ascertain load at which the maximum stress in extreme fibers reaches the yield stress of the material in bending.

The exactness of this method is restricted to square or rectangular slabs with symmetric supports and for uniform distribution loads.

1.8. Alternative methods of analysis and design [ref.1, 2&3]

Plastic theories like yield line and Hillerborg strip methods are the alternative to the usual coefficient and finite element method of analysis and design method.

Plastic analysis methods are derived from the general theory of structural plasticity, which states that the collapse load of a structure lies between two limits, an upper and a lower bound of the true collapse load (.Yield line method which is one of the plastic theories can be applied to the slabs of any shape, any support condition, uniformly or partially and none uniformly distributed loads, slabs with holes of any size, and so on.

Yield line is an upper bound theorem which states that the structure is in equilibrium and the load factor (’.calculate for any assumed collapse mechanism is greater than or equal to the true collapse load(. i.e. ( ≤ (’

Hillerborg strip method is a lower bound theorem that states that a structure is in equilibrium and the applied moment does not exceed the plastic moment of resistance anywhere,i.e. (’ ≤ (

Most engineers are scared to use the yield line theorem for design since even valid yield line patterns give results that are either correct or theoretically unsafe which is the upper bound solution. It can deter some designers.

1.9. Basic structural components [ref.5]

All buildings have two zones a structural above ground level is called Super- Structure the underground part is Sub-Structure. The building’s superstructure & sub-structure are defined by geometry, i.e., points, lines, surfaces, spaces, and bodies (solids)

In an ordinary building, the superstructural part is composed of the nodes or the point elements of joints (connections), the linear members of beams &columns, the surface elements of slabs and walls.

Joints /connections

At the intersections of every building, a component occurs a joint. Joints range from the large scale of a building joining the ground to the beam-column connection and to the small scale of mortar joints bonding brick together in certain patterns in a masonry wall.

The joint type is dependent on the location and position of adjacent members the main joint types are:

structural joints (e.g. load-bearing &energy dissipating connections)

movement joints which include soft joints (e.g. expansion joints, isolation joints)

control joints (e.g. slip joints, shrinkage strips)

Beams

Beams are linear members and they are distinguished in shape, cross-sections material, Support conditions.

The effect of load action (eccentric versus concentric) on beam behavior in response to member shape and profile may be in

simple bending

biaxial bending, or

unsymmetrical bending

Beams, in general, must be checked for the primary structural determinant of bending, shear, deflection, possible load effect of bearing, and lateral stability.

Usually,

– Short beams are governed by shear,

– Medium-span beams by flexure, and

– Long-span beams by deflection.

Column

Columns support loads in compression. Whereas hangers, ties, and thin braces do so in tension. They carried bending and axial compression where the bending can be more important than the axial load. Columns are the primary components of skeleton structures. They may carry an entire building. It can be

– Short or long

-slender or stocky

The cross-section of columns may be varied (Sloped, curved, twisted, stepped, etc) and may have its thinnest part at mid-height or top and /or bottom in X or Y directions.

Columns range from one-story to multistory, from a single column to groups (e.g. clustered pipe columns), from exterior to interior, from vertical to inclined columns, and so on. They may be separated from beams so that mainly axial forces are transferred, or they may be continuous with beams to form beam columns.

Columns normally may not solely serve their function as support, according to engineering requirements, but also may define space and may provide visual order to a wall. They may also help to refine proportion and compositional considerations, or they may be more ornamental, expressing other layers of meaning than just function.

Slab

Slabs are plate elements forming floors & roofs in buildings that normally carry uniformly distributed loads. Slabs may be simply supported or continuous over one or more supports and are classified according to the method of support as follows.

Spanning one –way between beams or walls
spanning two-way between the support beams or walls
Flat slabs carried on columns and edge beams or walls with no interior beams.

Slabs may be solid of uniform thickness or ribbed with ribs running in one or two directions.

There are two types of slabs based on the load transferring mechanisms. These are one-way and two-way slabs. One-way slabs transmit their load in one direction while two-way slabs resist being applied in two directions. These types of slabs are composed of rectangular panels supported at all four edges by walls or beams stiff enough to be treated as unyielding.

Foundation

It is the part of the structure that is usually placed below the surface of the ground and transmits the load to the underlying soil or rock. All soils compress noticeably when loaded and cause the supporting structure to settle. The two essential requirements in the design of foundations are that the total settlement of the structure is limited to a tolerably small amount and the differential settlement of the various parts of the structure be eliminated as nearly.