Structural design is a process whereby civil, mechanical, and aerospace engineers create safe, efficient, and cost-effective structures capable of withstanding loads and any environmental conditions.
Furthermore, structural design is where an engineer translates the architectural drawings to produce a safe and economical structure.
A reinforced concrete structure consists of beams, columns, slabs, and walls rigidly connected to form a monolithic frame.
Each individual member must be capable of resisting the forces acting on it, so the determination of these forces is an essential part of the design process.
From this point of view, the suitable member sizes are determined to carry the load safely and economically.
As the name of this blog post suggests, the literature review is a critical, analytical summary of research on a specific topic. Our topic today is Structural Design.
So, today, I will briefly walk you through how or what areas you should review and know if you are going to conduct research related to structural design.
Table of Contents
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Structural Design
Structural design aims to ensure that with an acceptably high probability, a structure will remain fully functional during its intended life.
The expected lifetime of a structure is formally known as its design life. During its design life, a structure must be capable of safely sustaining all applied loads and other stress–inducing actions that might reasonably be expected to occur.
Thus, it is necessary to identify the various types of load that will act on its members. The condition of structure when it becomes unfit for use or unserviceable is called a limit state.
Limits – state theory
Limit state theory is a philosophy of design under which structures are designed to fulfill several basic functions or conditions.
A limit state is a situation where the structure ceases to fulfill one of the specific functions or conditions for which it was originally designed.
For concrete structures, two main groups of limit states exist:
1. Ultimate Limit State:
To satisfy the Ultimate Limit State, the structure must not collapse when subjected to the peak design load for which it was designed.
A structure is deemed to satisfy the Ultimate Limit State criteria if all factored bending, shear, and tensile or compressive stresses are below the factored resistance calculated for the section under consideration.
Types of Ultimate Limit State are;
- Ultimate Limit State due to bending,
- Ultimate Limit State due to shear,
- Ultimate Limit State due to direct compression or tension and
- Ultimate Limit State due to overturning.
2. Serviceability Limit State:
Are those for deflection and cracking? It requires that the appearance, durability and performance of the structure must not be affected by deflection and cracking.
There may be several other Serviceability Limit States such as durability, fire resistance, excessive vibration, and fatigue.
The relative importance of each limit state will depend upon the nature of the structure and its intended purpose.
The design of the structure should be based upon the crucial limit state, and checks should be made to ensure that all limit states are satisfied.
Also Read: 16 Different Types of Concrete You Should Know Right Now
Loads
The Loads on a structure can be as follows:
Characteristic Dead Load
Dead loads are permanent or stationary loads that are transferred to the structure throughout the life span.
The dead load is primarily due to the self-weight of structural members, permanent partition wall fixed permanent equipment, and weight of different materials.
Characteristic Imposed loads or live loads
Live loads are either movable or moving loads without any acceleration or impact. These are assumed to be produced by the intended use or occupancy of the building, including weights of movable partitions or furniture, etc.
The floor slabs have to be designed to carry either uniformly distributed loads or concentrated loads, whichever produces greater stresses in the part under consideration.
Since it is unlikely that at any one particular time not all floors will be simultaneously carrying maximum loading, the code permits some reduction in imposed loads in designing columns, load-bearing walls, piers supports, and foundations.
Characteristic Wind loads
The dead and imposed loads should be increased by 20% to allow vertical movement.
(Source: Reinforced Concrete Designer’s Handbook by Charles E. Reynod, James C. Steelman.)
Partial safety factors for load
In practice, the applied load may be greater than the characteristic load for any of the following reasons:
- Calculation errors,
- Constructional inaccuracies,
- Unforeseen increases in load.
To account for these, the respective characteristic loads are multiplied by a partial safety factor to give the ultimate design load appropriate to the limit state being considered.
Values of fy for various load combinations are given in Table 2.1: BS 8110: Part 1:1985 reproduced here:
The table below shows the Load combinations and values of fy for the ultimate limit state (BS 8110 Part 1: 1985 Table 2.1)
Load Combinations for the Ultimate State
Various combinations of the characteristic values of Dead load (Gk), imposed load (Qk), wind load (Wk) and their partial factors of safety must be considered for the loading of the structure.
The partial factors of safety are specified by BS 8110 Part 1:1985: Clause 2.4.3, and for the ultimate limit state, the loading combinations are as follows:
- Dead and imposed load
1.4Gk + 1.6Qk
- Dead and wind load
1.0Gk + 1.4Wk
- Dead, imposed, and wind load
1.2Gk + 1.2Qk + 1.2Wk
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Material properties
Characteristic strength of materials
Unless otherwise stated in the code of standard, BS 8110: Part 1: 1985, the term Characteristic strength means that the value of cube strength of reinforcement, fcu, the yield of proof strength of reinforcement, fy, or the ultimate strength of a prestressing tendon, fpc, bellow which 5% of all possible test results would be expected to fall.
Table 9 of BS 5328 ‘Concrete’ Part 1, ‘Guide to specifying concrete’, as reproduced here, lists the characteristic strengths for various grades of concrete.
These are, in fact, the cube strengths of the concrete at 28 days. The yield strength of reinforcement is given in BS 8110 Table 3.1, reproduced here.
Concrete compressive strength (BS 5328 Part 1 1990 Table 9)
Strength of reinforcement (BS 8110 Part 1 1985 Table 3.1)
Partial factors of safety
Other possible variations, such as constructional tolerations, are allowed for by partial factors of safety applied to the strength of the materials and the loadings.
It should theoretically be to derive values for these from a mathematical assessment of the probability of reaching each limit state.
Lack of adequate data, however, makes this unrealistic, and in practice, the values adopted are based on experience and simplified calculations.
Partial factors of safety for materials, my
Design strength = Characteristic strength (fk)/ Partial factors of safety ( ym )
The following factors are considered when selecting a suitable value of ym
- The strength of the material in the actual member. This strength will differ from that measured in a carefully prepared test specimen, and it is particularly true for concrete where placing, compaction, and curing are so important to the strength. Steel, on the other hand, is a relatively consistent material requiring as small partial factor of safety.
- The severity of the limit state being considered. Thus, higher values are taken for the ultimate limit state than for the serviceability limit state.
Values of ym for the ULS are given in BS 8110 Table 2.2, which is reproduced here below.
Values of ym for the ultimate limit state (BS 8110 Part 1 1985 Table 2.2)
Ultimate design strength of materials
The ultimate design strength of a material is obtained by dividing its characteristic strength by the appropriate partial safety factor.
Ultimate design strength of concrete = (fcu/1.5)=0.67fcu
Ultimate design strength of reinforcement = (fy/1.5)=0.87fcu
It is important to appreciate that the formulae and design charts in BS 8110 have been derived with the relevant partial safety factors for strength included.
Therefore, it is only necessary for the designer to insert the relevant characteristic strength values fcu or fy to use the formulae and charts.
Structural Element Parts
A reinforced concrete structural element is a combination of beams, columns, slabs, foundation, and walls rigidly connected to form a monolithic frame.
Each member must be capable of resisting the forces acting on it, so the determination of these forces is an essential part of the design process.
Slabs
Reinforced concrete slabs are used in floors, roofs and walls of buildings and as the decks of bridges.
The floor system of a structure can take many forms, such as in situ solid slabs, ribbed slabs, or precast units.
Slabs may span in one direction or two directions, and they may be supported on monolithic concrete beams, steel beams, walls, or directly by the structure’s columns.
Beams
Beams are horizontal members used to carry loads from the floor to columns or walls.
A reinforced concrete beam design consists primarily of producing member details that will adequately resist the ultimate bending moments, shear forces, and torsion moments.
At the same time, serviceability requirements must be considered to ensure that the member will behave satisfactorily under working loads.
All calculations should be based on the effective span of a beam that is given as follows:
- A simply supported beam: the smaller of the distances between the canters of bearings or the clear distance between supports plus the effective depth.
- A continuous beam: the distance between centers of supports.
- A cantilever beam: the length to the face of the support plus half the effective depth or the distance to the centre of the support if the beam is continuous.
Columns
Columns carry loads from the beams and slabs down to the foundations. Furthermore, they are primarily compression members, although they may also have to resist bending forces due to the continuity of the structure.
The design of the column is governed by the ultimate limit state; deflections and cracking during service conditions are not usually a problem, but correct detailing of the reinforcement and adequate cover are important.
Class of Columns
Columns can be classified as;
- Braced column: where walls resist the lateral loads or some form of bracing.
- Unbraced column: where the lateral loads are resisted by the bending action of the columns.
Columns are further classified in BS 8110 as either short or slender. A column is short if both lex/h and ley/b are;
- less than 15 for a braced column and
- less than 10 for an unbraced column.
Where:
lex effective height in respect of column major axis,
ley effective height in respect of column minor axis,
h depth in respect of the major axis,
b width concerning the minor axis.
While lex and ley = βlo
A column can also be classified as slender if the slenderness ratio about either axis is;
- >15 for a braced column
- >10 for an unbraced column
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Foundations
A building is generally composed of a superstructure above the ground and a substructure that forms the foundations below the ground.
The foundations transfer and spread the loads from a structure’s columns and walls into the ground.
The safe bearing capacity of soil must not be exceeded otherwise, excessive settlement may occur, resulting in damage to the building and its service facilities, such as the water or gas mains.
Foundation failure can also affect the overall stability of a structure so that it is liable to slide, lift vertically, or even overturn.
It is important to have an engineering survey made of the soil under a proposed structure so that variations in the strata and the soil properties can be determined.
In the design of foundations, the areas of the bases in contact with the ground should be such that the safe bearing pressures will not be exceeded.
Settlement takes place during the working life of the structure, therefore, the design loading to be considered when calculating the base areas should be those that apply to the serviceability limit and typical values that can be taken as:
- Dead plus imposed load = 1.0Gk + 1.0Qk
- Dead plus imposed load = 1.0Gk + 1.0Wk
- Dead plus imposed load plus imposed load =1.0Gk + 0.8Qk + 0.8Wk
These partial factors of safety are suggested as it is highly unlikely that the maximum imposed load and the worst wind occur simultaneously.
Wrapping Up
As explained above, Structural design is the methodical investigation of the stability, strength, and rigidity of structures.
The basic objective of structural analysis and design is to produce a safe structure capable of resisting all applied loads without failure during its intended life.
The primary purpose of a structure is to transmit or support loads safely. If the structure is improperly designed, it will probably fail to perform its intended function.
Hopefully, some tips and recommendations in this blog post have been helpful for you. I would love to hear any feedback below!
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