Monthly Archives: June 2010

Why are RCC plinth beams required?

Protecting the building starts with the foundation and plinth. In fact, over-protecting also begins here…

Building with nature is often termed as temporary, ridiculed as kutcha (raw) and warned as one that would fall apart if anything small goes wrong.

The typical rural housing made up of small foundations, no good plinth, soft mud walls and thatch roof has led professionals to believe so. If nature is not temporary, the natural building too need not be temporary. If protecting the nature becomes important, protecting the building too becomes important.

Of course, protecting the building starts with the foundation and plinth. Incidentally, over-protecting also begins here!

The plinth needs to be strong, but need not be made up of reinforced cement concrete (RCC) all the time. However, irrespective of the specific contexts, RCC plinths are increasingly being cast in most constructions today.

Normal justification

The normal justification is to strengthen the building, where the owners dare take a stand against it, yet end up paying for it!

If argued against it, the team at site would mention how small an addition it is to the budget; not mentioning that many such small additions finally escalate the overall cost.

Most soil conditions and building loads can be managed with normal 1:2:4 plain concrete (PCC), which is good in taking compression. Steel, specially reinforced, is best to tackle tension, which does not prevail much at the wall bottom.

It’s the sheer wall load that needs to be transferred to the foundation; as such, 4” thick plinth concrete is adequate. As a cost-effective measure, reducing plinth concrete width to the wall thickness, say 9” only instead of it being twice the width at 1’6” is feasible, if appropriate conditions prevail.

Uneven settlement

If so, why are the RCC plinth beams required? In soft and water-logged soils, if the earth settles downwards, the plinth may face uneven settlement.

In RCC frame structure, plinth acts as a tie, as a beam at ground level itself. More recently, after few disasters we have faced, RCC plinth beams are being recommended for their earthquake-resistant properties.  In case of boulder-packed foundation in loose soil, RCC plinth is advised to ensure proper load transfers. It is obvious then that we should not question RCC plinths, but question their wasteful application where not necessary.

Before we start with the wall, at the ground level itself, we need to think about anti-termite treatment, borewell, open well, basement room and such others.

Let us look at their cost and climatic implications next week.

The importance of plinth foundation

Is it a foundation above ground, or a basement wall? Read on to know more about this interesting aspect… 

Essential: In waterlogged areas, the higher the plinth, the better

How does one explain what is ‘plinth’? My comical statement if a new client enquiries is “the foundation above ground”! Of course, no technical person would accept this definition, while the construction teams at Bangalore sites call it a basement wall. Actually, plinth is the part of a building above ground up to the ground floor.

If we walk around a typical village, many houses could be found without any noticeable plinth. Raised floor levels seem associated with the rich and important people. As such, the common image we get is that of a landlord sitting high on verandah platform and ordering his labourers! However, it is important to realise that plinth is also an ecological necessity, even though it costs dearly!

The elderly in the community may say that a high plinth avoids crawling creatures from entering the building, if there are open areas around with snakes, lizards and such others. From construction experiences, there are deeper reasons. Surface water tends to flow all over the top soil, especially during the monsoon, before it percolates at the softer parts. If it flows at sub-floor levels, under the building, it can lead to part of the structure sinking and walls cracking, termed as unequal settlements. Plinth provides stability to the building and the consolidated earth filling inside the plinth provide a stable sub-floor for the floor finish.

Other important roles

Among the more important roles of plinth is ensuring adequate height for the toilets and bath-water traps, ensuring their smooth gravity flow into the sewage systems or drains. By misfortune, if the roadside chambers get clogged, houses with high plinth do not face back flow of sewage into the toilets.

The subsoil water tends to flow up the building walls, which needs to be stopped before the superstructure begins. Accordingly, a layer of concrete called damp-proof course or traditionally a stone slab is laid just before the wall starts. This course, in short called DPC, laid on top of the plinth, distances the floor from the ground water. Incidentally, it also acts as a levelling course for the wall such that all of the first row bricks are at same level.

One final clarification: how high should the plinth be? There is no fixed answer, but keep it at least 1′ 6” above natural ground. In waterlogged areas, the higher the better. After all, it adds to the safety of a house.

The challenge before super structures

After going against nature in your building, don’t blame architecture or construction methods

Ecologically sensitive structures are much easier to build than we tend to think. Until a few centuries ago, what human beings built for shelter was a direct derivative from nature.

The early human beings built conical and pyramidal forms in thatch, after observing hills and ant hills, designed with the sloping edge along the angle of repose. The thicket in the forest had numerous tree trunks upholding large foliage above. No wonder people thought of vertical supports, holding up horizontal supports and roof material — the post and beam construction.

Earth forms such as caves led to discovering arches, vaults and domes. Sea and other shells suggested round huts and curved forms. Two tree branches inclined against each other stood strong, so people realised sloping roof was among the easiest that anyone could build. Materials of construction were visible to all, for they were left exposed. In all these examples begin the history of architecture and human settlements.

Knowing all the above, with much difficulty, we struggle to build against nature. Then we suffer heat and cold; rain and dust; cost and waste; cracks and repairs; looks and leaks, finally blaming architecture and construction.

We need not repeat primitive forms or design but may at least attempt applying the theory behind the preceding examples. Building wall over wall allows the weight to move down vertically, termed as designing for compression. With column and beam, negligible load gets transferred straight down, with most weights moving sideways to reach a column and then get transferred to ground. The load factor at unsupported parts create bending, hence demanding designing for tension.

Traditionally, people designed for compression, and such structures have been standing for centuries.

Both from cost and climate considerations, the walls and roofs, together termed as superstructure, play a major role. Fortunately, most research and application of alternative ideas have also happened in this direction. Let us look at them in the coming weeks.

RCC is a new phenomenon

The time has come to rethink what we question and accept in construction practices

Current practice: Most foundations have stepped formation

Nearly two dozen e-mails during the one-month-old Green Sense essays is gradually turning this weekly column on eco-friendly and cost-effective building ideas into a reader’s column.

One of the intentions, of sharing experiences, is being slowly fulfilled. Incidentally, the unexpected is also happening — readers are reacting and enquiring. As a reaction and clarification to recent mails, let us look at the idea of foundations more critically.

No pillars

If we inspect the houses of our elders, we realise there hardly were concrete or stone pillars, but the structure stood on load-bearing walls.

Stone or wood pillars were restricted to large areas such as living room or dining hall. Even today in hard soils, ground and two-storeyed buildings with nominal room sizes are possible without any RCC pillar beam system.

Yet, if we look around the cities today, we hardly find any new construction without RCC all over them. Naturally, the wall foundations we have been discussing may appear questionable.

Care needed

To allay this fear, let me only state that among all the standing houses in India, those with RCC columns are still a small and recent minority. It is a beneficial technology, but should be used judiciously and appropriately.

Most foundations we see have stepped formation of stones or the local material. This practice of stepped method is a British introduction, later formalised through the PWD all over.

Surely, it has advantages, but it is not always necessary to spend money and materials in dressing of stones or stepped construction.

Filling the remaining part of trench, plinth protection at ground, precision wall centre line and such others then become inevitable. Bangalore-based architects, Kanade Brothers, have tried to build with simple stone slabs laid over each other as foundation.

Local ideas

In the Mangalore region, traditionally, laterite was used for foundation, with main foundation for external walls and nominal ones for internal ones.

In different regions, varied localised ideas like brick footings, wooden piles, river and hill side stones have been used to consolidate the ground below walls.

The claim that mainstream ideas like RCC pillars or stepped footings are informed and studied not only justify the current practices, but also dilute the common sense of ages.

If an ecological alternative evolves from scientific research, we tend to agree with it. But if it is rooted in local realities, we question them. It is time to rethink what we question and what we accept.