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You'll never size a beam. But every week you'll be asked whether a duct can go through one, whether a wall can lose a door opening, or why a slab can't take a fixing. The structural engineer answers those questions. Your job is to ask them well, and that needs a picture of what the structure is doing.
Every building carries its own weight (the dead load: concrete, steel, finishes), what's put in it (the live load: people, furniture, plant) and what the weather does to it (wind, snow). These loads travel down through the frame to the ground. Each member is sized by calculation for the forces it carries: compression in columns, bending and shear in beams and slabs, tension in ties and cables.
The load path
Load follows a route: a slab passes it to its beams, the beams to the columns, the columns to the foundations. Everything on that route carries the load, so a change anywhere on it can matter far from where it happens. A heavier chiller on the roof is a question about the roof slab, the beams under it, every column below, and the foundations.
- 1.First floor slab
- 2.Beam, left bay
- 3.Beam, right bay
- 4.Column A, ground floor
- 5.Column B, ground floor
- 6.Column C, ground floor
- 7.Foundation A
- 8.Foundation B
- 9.Foundation C
Everything on the route carries this load to the ground. That is why a hole, a cut or a removed element anywhere on it goes to the engineer, and why a “small” change to column B on the ground floor affects both floors above.
An illustration of how forces flow, not a design check. Every opening, removed element or extra load is the structural engineer’s decision.
Where a beam works hardest
A beam supported at both ends bends most at mid-span, where its top is squeezed and its bottom is stretched. It carries the most shear, the force trying to slice it vertically, near its supports, and the middle of its depth (the web) does that work.
Two things follow. An opening centred in the depth at mid-span is often acceptable, because it sits where the beam is doing little. The same opening near a support takes out the part that carries the shear. Flat slabs follow the same logic around columns, where the load concentrates (punching shear): it's the worst place for a core hole.
What keeps it standing sideways
Wind pushes a building sideways. Cores (stairs and lift shafts), shear walls and bracing resist it. They look like ordinary walls or diagonal steel in the model, but they're the building's stability. A door added to a core wall, or a duct through a brace, is a stability question, not a coordination one.
The slabs you can't drill
Some slabs contain post-tensioned cables, steel tendons pulled tight after the concrete sets. Drilling into one can cut a tendon, which is dangerous and very expensive to fix. On these slabs, every core and fixing goes to the engineer, and the tendons are located (from the drawings and a scan) before anyone drills. Precast planks have their own limits too: their strands sit in the bottom, and openings usually have to be cast in at the factory.
A late change adds a cable tray under a post-tensioned slab. The installer drills anchors on a 1.2 m grid. One anchor nicks a tendon. The floor is propped, the engineer designs a repair, and the area is out of use for three weeks.
The coordinator notices the slab type in the structural model's properties and flags the change. The engineer marks no-drill zones from the tendon layout, the installer scans before drilling, and the tray goes up on schedule with anchors placed between the tendons.
Room to move
The model shows a beam unloaded and unprotected. On site it deflects (sags) under load, sometimes by tens of millimetres over a long span, and steel gets fire protection that adds thickness. Services squeezed under a beam with 10 mm to spare will clash with something real. Ask the engineer for the expected deflection and the protection thickness, and keep that zone clear.
What to send the engineer
For every opening, fixing or added load: the element (its ID or grid position), the size, the exact position (distance from the face of a support or column, height in the member), what it is for and why it can't go elsewhere, and a view from the model. Group them into a builders' work schedule so they're answered together, and track each answer to closure.
Rank these four requests from most to least likely to be accepted as they stand, and say what you'd add to each:
- A 150 mm pipe through a flat slab, 200 mm from the face of a column.
- A 250 mm duct through a 600 mm deep beam, centred in the depth, at mid-span.
- A door opening in a lift-core wall.
- Six drilled anchors into a post-tensioned slab for a cable tray.
Key takeaways
- Load travels from slabs to beams to columns to foundations; everything on that route carries it.
- A simply supported beam bends most at mid-span and carries the most shear near its supports.
- Cores, shear walls and bracing keep the building standing sideways: never route through them without the engineer.
- Post-tensioned slabs hide tensioned cables: no coring or fixing without the engineer and a scan.
- Leave room for deflection and fire protection: a beam moves and gets thicker after it is modelled.