Structural Analysis to Detailing to BBS

The architect decides where columns can go. The structural engineer decides how big they must be and how much steel goes inside. That decision passes through five steps.

Step 1: Loads

The engineer lists every load the building must carry:

• Dead load: the weight of the structure, walls and finishes.
• Live load: people, furniture, stored goods.
• Wind and earthquake loads: depend on location, height and soil.

Indian codes set these values. IS 875 covers dead, live and wind loads. IS 1893 covers earthquake loads, and the seismic zone of your city matters a lot.

Step 2: Analysis

The engineer builds an analysis model: a frame of columns, beams and slabs with supports and loads. Software calculates forces and bending moments in every member under many load combinations.

• Common tools: ETABS and SAFE (for frames and slabs or foundations), STAAD.Pro, SAP2000.
• Files: .e2k or .edb for ETABS, .std for STAAD.Pro.

An analysis model is not a drawing. It is a calculation. Nobody builds from it directly.

Step 3: Design

For each member, the engineer checks the size and works out the steel needed, following IS 456 for reinforced concrete and IS 13920 for ductile detailing in seismic zones. Many offices use design tools that read the analysis results and propose reinforcement. In India, one tool long known as RCDC is commonly seen for this; Bentley now sells it within its STAAD range.

Step 4: Detailing

Design results become drawings the site can use: general arrangement (GA) drawings, column schedules, beam and slab reinforcement details, footing details. Most Indian offices draw these in AutoCAD. Larger steel and precast jobs often use Tekla Structures, which models every bar or steel member in 3D.

Good detailing answers site questions before they are asked: lap positions, cover, bar bends at beam-column joints, and where to stagger laps.

Step 5: BBS

The bar bending schedule (BBS) lists every bar: its mark, diameter, number, shape, cut length and weight. The bar bender cuts from it. The purchase team orders steel from it. The QS bills steel from it.

The weight of a bar per metre is d² / 162 kg, where d is the diameter in mm.

A worked example: one BBS row

Beam B1 has four 16 mm bottom bars, each with a cut length of 5.20 m.

• Weight per metre: 16 x 16 / 162 = 1.58 kg/m
• Total length: 4 x 5.20 = 20.80 m
• Weight: 20.80 x 1.58 = 32.9 kg
• At an illustrative ₹60 per kg: about ₹1,970

A G+2 house may have a few hundred such rows. A tower can have tens of thousands. A 3% error in cut lengths across 25 tonnes of steel is 750 kg, about ₹45,000 at the same rate.

Where it goes wrong

• The BBS is made from an old revision. Always check the structural sheet revision first.
• Cut lengths ignore bend deductions or laps. The bar comes out short or long.
• Site changes are not fed back. A beam is deepened on site, the BBS is not updated, and billing goes wrong.

What comes next

Structure is one discipline. Services are another, and they must fit through the same beams and slabs. That is the next lesson.