A building stands up by passing every load down a continuous path to the ground. Roof and floor loads go into slabs, slabs pass them to beams, beams pass them to columns or load-bearing walls, and those carry everything down into the foundations, which spread it into the soil. This chain is called the load path. If any link in it is missing, undersized or badly built, the load finds another route, usually through something that was never meant to carry it, and that is when cracks and failures start. Most small houses in Tanzania use either load-bearing blockwork with a ring beam or a reinforced concrete frame with infill walls.
What loads does a building carry?
- Dead load: the weight of the building itself: concrete, blocks, roof, finishes.
- Imposed (live) load: people, furniture, stored goods, water in tanks.
- Wind load: pushes and pulls on walls and lifts on roofs. Important for light roofs on the coast and in exposed areas.
- Other loads: earth pressure on retaining walls, water pressure on tanks, and in some regions, earthquake loads. Parts of Tanzania along the Rift Valley have real seismic risk, which affects design there.
A structural engineer adds these up, applies safety factors and sizes each member so the whole chain is strong enough.
How do loads travel from roof to ground?
| Element | Receives load from | Passes it to |
|---|---|---|
| Roof sheets or tiles | Weather, own weight | Purlins and trusses, or the roof slab |
| Trusses or roof slab | Roof covering | Ring beam, beams or walls |
| Floor slab | People, furniture, finishes | Beams, or walls on load-bearing buildings |
| Beams | Slabs and walls on top | Columns |
| Columns | Beams | Foundations |
| Load-bearing walls | Slabs, roof, ring beam | Strip foundations |
| Foundations | Columns and walls | The soil |
What does a slab do?
A slab is a flat concrete plate that forms a floor or flat roof. It carries loads across its span to the beams or walls supporting it. One-way slabs span in one direction between two supports, and two-way slabs span in both directions between four. Slab thickness depends on the span: house slabs are often 125 to 150 mm thick, more for longer spans.
Hollow-block (ribbed) slabs are common in Tanzania for upper floors. They use concrete ribs with hollow blocks between them, saving concrete and weight.
What does a beam do?
A beam spans between columns or walls and carries the slab and any walls on top. It works in bending: the bottom is in tension (where the main steel goes) and the top in compression. Near the supports, beams also carry shear, which is why stirrups are spaced closer at the ends.
Beam depth is usually around a tenth to a twelfth of its span for simple cases. A 4 m span beam might be 300 to 400 mm deep. A beam that is too shallow sags and cracks the walls on top of it.
What does a column do?
A column carries load down from the beams to the foundation, mostly in compression. In houses, columns are often 230 x 230 mm to 300 x 300 mm with four to six bars and closely spaced links. Columns are the most critical elements in a frame: when a column fails, everything it supports goes with it.
Two rules matter on site: columns must line up vertically from floor to floor, and the steel must be continuous through the joints with beams, with the correct laps and links.
Load-bearing walls or a concrete frame?
| Load-bearing blockwork | Reinforced concrete frame | |
|---|---|---|
| How it works | Walls carry the roof and floors directly | Columns and beams carry the loads; walls only fill the gaps |
| Typical use | Single-storey houses | Storey houses, commercial buildings |
| Ring beam | Essential, ties the walls together at the top | Beams at each level do this job |
| Changing walls later | Difficult, walls are structural | Easier, most walls can be moved |
| Cost | Usually lower for bungalows | Higher, but needed for multiple floors |
Many Tanzanian bungalows combine both: load-bearing walls with concrete columns at corners and junctions and a ring beam at the top. Our guide to building a storey house explains why upper floors almost always need a full frame.
What is a ring beam for?
A ring beam is a continuous reinforced concrete beam on top of the walls, running all the way round the building. It ties the walls together, spreads roof loads evenly, anchors the roof trusses against wind uplift and stops walls cracking apart. On load-bearing houses it is one of the most important elements, and one of the most often skimped on.
What are common structural mistakes on small buildings?
- Columns on upper floors that do not sit over columns below.
- Adding an extra floor to a house designed for one.
- Cutting through beams or columns for pipes and cables.
- Hanging heavy water tanks on roofs and slabs not designed for them.
- Removing props too early. See curing concrete.
- Building without structural drawings because "the fundi knows". Our guide on what a civil engineer does explains why the design matters.
Frequently asked questions
Can I add a second floor to my house later?
Only if the foundations, columns and beams were designed for it. Ask an engineer to check the existing structure before you plan an extra floor.
What is the minimum column size for a house?
Many house designs use 230 x 230 mm columns for single storeys, and larger for more floors, but the engineer's design decides. Size depends on load, height and the steel.
How do I know if a wall is load-bearing?
Check the structural drawings. Without them, an engineer can inspect what sits on the wall. Walls under beams or slabs, or below the ring beam, are usually structural in load-bearing houses.
Why do engineers insist on a ring beam?
It ties the walls together, holds the roof down in strong winds and spreads loads evenly, which prevents cracking.
Planning a building that will last?
Vika Construction designs and builds structures with engineer supervision at every stage. See our building services or contact us.
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