Load Combinations in Reinforced Concrete Design.

Load combination.

The load combination for reinforced concrete design is 1.4Gk + 1.6Qk where Gk and Qk represent dead and imposed loads respectively. 1.4 and 1.6 is the factor of safety for dead and 1.6 is the factor of safety for imposed loads.

Other local combinations include; 1.2 Gk+1.2Qk+1.2Wk where Wk represents wind load.

For full information on load combinations see table 2.1 page 9 of BS 8110 part 1, 1997.

In reality however, various load combinations of dead, imposed and wind loads should be considered. This is to ensure that the structure is designed for the worst possible case.

Load cases:

While it is ok to load all parts of the structure using 1.4Gk + 1.6Q, it is also to be appreciated that theoretically, these loads can vary i.e from 1.0 Gk to 1.4 Gk and 0.0 Qk to 1.6 Qk, in the light of these, various load cases need to be considered to achieve an accurate design.

For example, assuming we are working on a 3 span beam with Gk of 25kN/m and Qk of 10kN/m. It is good practice to consider the following load cases;

Case 1: loaded even spans with 1.4Gk + 1.6Qand odd spans with 1.0Gk.

Even Span Loaded 1
even span loaded 2

Case 2: loaded odd spans with 1.4Gk + 1.6Qand even spans with 1.0Gk.

odd spans loaded 1
odd spans loaded 2

Case 3: All spans with 1.4Gk + 1.6Q 

All spans loaded 1
All spans loaded 2

Each load case is analyzed for bending moments and shear forces.

Bending moment diagrams for each load case. (all values in kNm);

CASE 1. (Even spans loaded):

Moment diagram for even span loaded

CASE 2. (Odd spans loaded):

moment diagram for odd spans loaded

CASE 3. (All spans loaded)

moment diagram for all spans loaded

The results of load cases are then compared. So for any beam span, the highest value of bending moment and shear forces of the 3 load cases is taken for design.

Final Result.

Final result


Mosley & Bungey, Reinforced concrete design. 5th Ed. Page 30.

BS 8110 part 1, 1997.

Design of One Way Slabs with Excel

Hello, today I am introducing to you an excel spreadsheet program specially created to help design one way continuous reinforced concrete slabs. A one way slab is a slab in which its length divided by its breadth is greater than or equal to 2. However, a slab with length/ breadth ratio of not less than 1.5  can still be designed as one way. The spreadsheet named RCC31 designs to BS 8110 and it is free. It was developed by the Reinforced concrete council and its been around for a while.

RCC 31

You can download this spreadsheet by clicking the link below.


How to use Excel Spreadsheet Program for One way slab design.

You can watch the video by clicking the link below to see how its works. https://www.youtube.com/watch?v=0tQFdL0LwqA

At the top of the spreadsheet is the title section. This is where you input title data for the slabs being designed. You can input names of the project and client as well as other title information.

Below it is the materials section where you can input material strength properties of concrete and steel. Including factors of safety.

In the span section you can set slab span and thickness, note the units.

Under loading pattern you set factor of safety values for dead and imposed load.

At the supports section, you can set 3 types of supports, including, kinife, cantilever and encastre.

A look at the loading section, and you will realize the spreadsheet can handle up to six slabs at once.You can input dead imposed and point loads n this section. Again note the units in which the loads should be provided.

Now all what I have explained so far is within the MAIN tab of the spreadsheet. To see the calculated span and support moments, click the ACTIONS tab.

To see calculated and design steel reinforcements, click the SPANS tab.

In the SPANS tab notice that top and bottom reinforcements are provided in the left right and center for each span designed. However in your detailing you can rearrange reinforcement for example, in each span you can pick the highest designed bottom reinforcement occurring either in the left, centre or right end and use as general for the whole span.

If design fails in maximum spacing ( max S), reduce size of reinforcement bar.

Basic Subject Areas in Civil Engineering.

Civil engineering is a vast discipline with various subject areas. Common subject areas which a Civil engineer is required to have general knowledge of, is as follows;  


Surveying is the science of map making. To start any development activity, the relative positions of various objects in the area with respect to horizontal and vertical axes through a reference point is required. This is achieved by surveying the area. Earlier, the conventional instruments like chain, tape and levelling instruments were used. In this electronic era, modern electronic equipments like electronic distance meters (EDM) and total stations are used, to get more accurate results easily.

Construction Engineering.

Construction engineering is a professional discipline that deals with the designing, planning, construction and management of infrastructure such as buildings, highways, bridges, airports, dams, railroads etc.

Structural Engineering.

Structural engineering involves the application of maths and science to determine the appropriate type and sizes of component members required to build a structure. It involves structural analysis and Design. Structural analysis involves finding the internal stresses in components of a structure. It enables engineers understand the effects of load/stresses caused by gravity, the users of the structure, and the widely varying climatic conditions and ground conditions. Choosing appropriate materials for the structure as well as finding suitable sizes of structural components is known as structural design.

Structural engineering also involves analysis of various structures like buildings, water tanks, chimneys, bridges etc. And designing them using suitable materials like masonry, wood concrete, reinforced concrete, prestressed concrete or steel. A structural engineer should design safe and economic structures using mathematical optimization techniques.

Earthquake Engineering.

Earthquake engineering involves the study of occurrence, magnitude and behaviour of earthquakes forces and the design of structures that can resist them. It is also the study of behaviour of structures and geo-structures subject to seismic loading as well as protecting the society (the natural and man-made environment) from earthquakes by limiting the seismic risk to socio-economically acceptable levels.

Foundation engineering.

Foundation engineering involves the determination of suitable soil type and safe bearing capacities of soil suitable for foundation of buildings. It also involves the various studies required for design of pavements, tunnels, earthen dams, and canals as well as ground improvement techniques.

Quantity Surveying.

Quantity surveying deals with estimating what a construction project will cost. The determination of total cost of a construction project includes knowledge of material cost, labour cost, contract cost, contingency cost etc.

Quantity surveying also involves managing all costs relating to building and civil engineering projects, from initial calculations to final figures.

Fluid Mechanics.

Fluid mechanics involves the study of mechanics of fluids and flow characteristics in order to design and construct efficient hydraulic structures. Civil engineers are more involved in the mechanics and flow characteristics of water.

Irrigation Engineering.

Water is to be supplied to agricultural field. Hence suitable water resources are to be identified and water retaining structures are to be built. Identifying, planning and building water retaining structures like tanks and dams and carrying stored water to fields is known as water resources and irrigation engineering. Constructing canals, aqueducts and regulators form part of irrigation engineering.

Transportation Engineering.

Design of good roads involves the design of base courses, surface finishes, cross drainage works, road intersections, culverts, bridges and tunnels. Roads need suitable design of horizontal and vertical curves also. Railway is another important long way transport facility. Design construction and maintenance of railway lines and signal systems are part of transportation engineering. Design, construction and maintenance of harbours and airports are also the need of globalization era. For proper planning of these transport facilities traffic survey is to be carried out. All these activities constitute the transportation engineering.

Environmental Engineering.

 Environmental engineering is the application of engineering principles to improve the natural environment, to provide healthy water, air and land for human habitation and for other organisms, and to clean up pollution sites. It involves study of sources, causes, effects and remedial measures associated with air pollution, water pollution, land pollution and noise pollution forms, as well as supply of potable water to rural areas, towns and cities as well as disposal of waste water and solid waste.

Town Planning.

Town planning involves the design and regulation of the use of space that focus on the physical form, economic functions, and social impacts of the environment and the location of different activities within it.

New towns, cities and extension areas of existing cities are to be planned properly so that suitable communication systems, educational facilities medical facilities, shopping centres are provided along with residential areas.

Project Management.

Project management is the application of knowledge, skills and techniques to execute projects effectively and efficiently. Civil engineers are involved in managing construction projects from start to finish.

In addition to acquisition of general knowledge of all subject areas described above. The civil engineer by education and experience will go on to specialize in one or 2 of them. So it is not enough for one to say he is a civil engineer, he or she should also state subject areas of specialization in the field.