What actually goes into the structural design of a building

When a building is handed over, nobody looks at the structure. That is exactly why it deserves the most care. A good structural design is a chain of decisions, and a mistake at any link carries all the way down to the foundation.

1. Know the loads

Design starts with what the building must carry. Gravity loads are the weight of the structure itself, finishes and partitions, plus the people and furniture that use the space. Lateral loads are wind and earthquake. Bangladesh is a seismically active region, and the Bangladesh National Building Code (BNBC) sets out how these loads must be calculated for each part of the country.

2. Choose the structural system

For most low- and mid-rise buildings this means a reinforced-concrete frame: columns, beams and slabs. The column grid is decided together with the architect, because a column in the middle of a living room is a design failure even if the calculation is perfect.

3. Create a clear load path

Every load needs an unbroken route to the ground: slab to beam, beam to column, column to footing, footing to soil. Gaps in that route — a column that stops on a beam, a wall that disappears on the ground floor — are where trouble starts.

4. Analyse the structure

The engineer builds a model, applies the code load combinations, and checks that the building behaves acceptably: members are strong enough, and the building does not sway or twist too much under lateral loads.

5. Design and detail each member

Slabs, beams, columns and footings are designed for the forces they receive. The most important and most overlooked step is detailing: where the bars go, how they are lapped, how they are anchored, and how closely the ties are spaced. Many failures are detailing failures, not calculation failures.

6. Produce a drawing set the site can build from

All of the above is only useful if the contractor can read it. Clear drawings, reinforcement schedules and notes turn the design into something that can be built correctly.

What to ask your designer

• Which code and which load assumptions did you use?
• Which soil report was the foundation designed from?
• Who checks the work on site?

Good designers answer these gladly. If the answers are vague, that is information too.