Simple guide for manual design of structural elements of building

Architectural plans of buildings come with different internal spaces representing rooms and other service spaces. During the preparation of general arrangement (GA) for the manual structural design of the building, one or combination of these spaces form panels. The panels come in different sizes depending on the size of the different rooms or spaces. The panels can also be two-way slab panel or one-way slab panel depending on the ratio of the longer span (L_y) to the shorter span (L_x).

Generally,                   

If          L_y/L_x ≤ 2, the slab panel is two-way spanning

If         L_y/L_x ˃ 2, the slab is one-way spanning

Students are usually thought in school how to design different structural elements (slabs, beams, columns etc) of a building such that on graduation, most of them may find it difficult to carry out a full structural design of building because they do not understand how the load transfer process flows in a complete structure. Figure 1 shows a diagrammatic illustration of this.

Structural designs can often be described as a reverse process of construction. Whereas construction work starts from the foundation and goes upwards to the roof, design starts from the roof and moves to the foundation. Due to the fact that loads are subject to gravitational force (the force that acts on every member thrown upwards and brings it down), the loads on the uppermost component of building is usually analysed first and transferred to adjoining lower member.

The load transfer process can be summarized as follows (2 storey building);

1. Roof beams carry roof loads, concrete facia load and its own weight and transfer them to first floor columns.
2. First floor slab carry loads from humans and household properties including finishes and its own weight and transfer it to the first floor beams.
3. First floor beams carries the loads of first floor slabs and its own weight and transfer them to ground floor columns.
4. The ground floor columns carry the loads of first floor beams, first floor columns and its own weight and transfer to the foundation.
5. The foundation transfers the column loads to the soil.
6. The stairs ensure movement from one floor to another and carries loads applicable there including its own weight. The stairs are self-supporting and are designed as such.

There are lots of software available for the structural design of buildings but none have been effective to replace manual design because manual design is more reliable. Using software to design large structures would not pose much problem but using manual design and following rigorous manual design process would pose much task for the designer. Due to the tediousness involved in this manual design and shortcomings of the use of software, it is necessary to acquaint ourselves with easier method used by experienced designers to aid in the smooth completion of design of structural elements of a building without compromising safety. The steps in achieving this are as follows:

Step 1: Having paneled the floor properly usually based on the ground floor of the house because that is where the location of columns starts, find the largest two-way slab, load it and design it. Find the next largest two way slab, load it also and design it and carry out checks for deflection, cracking and shear on these slabs. These are called critical slabs. It is assumed that if these slabs does not fail in the checks, no other slab in the system would fail. Also look out for slabs with significant point loads and design them too. This system may not be economical because smaller spans may require less quantities of reinforcement than those provided to critical slabs but for brevity and ease of computation of areas of reinforcement, it is usually adopted. Figure 2 shows a simple general arrangement (GA) showing what I consider critically loaded structural members.

Step 2: Find the largest one way slab and design it. Find also the next largest one way slab and design it and carry out necessary checks. Condition in step one also applies here.

We can see from Figure 2 that Panels 3 and 5 are the largest two-way and one-way slab respectively. These two slabs can be identified as the critically loaded slabs. The beams that would be accessed as critically loaded beams are shaded in gray colour. They are the beams bearing the loads from these slabs.

Step 3: Find out the critical beams and design them. Critical beams are those beams around the critical slabs. Carry out the necessary checks on them. If they do not fail, it is assumed that no other beam would fail in the system. Be careful here to identify beams with point loads. Typically a secondary beams transmits point load to a primary beam. Such beams should also be designed separately to take proper care of the point load.

Step 4: Having designed all the beams, load all the columns in the building. Also identify the axial loads and moments in both y-y direction (M_yy) and x-x direction (M_xx) coming on the column where appropriate. Separate all the axially loaded columns from the uniaxially loaded and biaxially loaded columns. For the axially loaded columns, identify the most critically loaded column, design it and carry out the necessary checks. Do likewise for the uniaxially loaded and biaxially loaded columns. If the critical columns do not fail in necessary checks, others would not fail.

Step 5: Finally load all the pad footings incase that is the suitable foundation. For pad footings, it is necessary to design with most of the axial loads coming on the foundation because the footing dimensions depends on the axial load and bearing capacity of the soil. In this case, one can provide for the most economical footing required for each column axial load. Other footings such as raft are designed as a common unit because all the column loads are integrated in the design.

This method would mostly be suitable for uniform and repeating floors. One would be advised to always use specialized codes (BS 8110 or EC 2) and follow the proper laid down guidelines for design. We should also know that even though this method would simplify the design process, it may not be economical as some lower-loaded sections may be over-reinforced if areas of reinforcement at higher loaded sections are provided.  I would advise the designer to always subject his designs to the scrutiny of experienced professionals.