Monthly Archives: April 2014

Practice makes perfect

It is time arches get revived, before the skills vanish completely.

archIt goes unnoticed that a large number of people actually love arches and may like to see them in their homes and offices, but end up without one. The blame may lie upon the architects and engineers who hesitate to build them for fear of defects and mistakes. The mistakes could be blamed upon the masons, who had no chance to learn about arches with no proper instruction, ending up spoiling it all. While the trend of senior masons teaching the juniors has diluted, there are not many centres like Auroville, where one gets exposed to constructing arches.

However by studied observation, basic knowledge of arch action and sincere willingness, anyone can learn to build a simple arch, which can then be perfected across time. The critical point about an arch is not in the final product, but in the design process and in the making of the arch. Firstly, one needs to finalise the shape, be it semi-circular or segmental, depending upon the clear width, the rise and the strength of the side supports possible. This shape should fit well within the springing point at the bottom and key stone at the top.

The arch profile should be marked on the ground on actual scale using a thread as a compass, from the centre point of the arch, and double checked for accuracy. The central line should be drawn and the key stone marked, such that rest of blocks can be fitted equally on the either sides of the key stone. With the same thread, each stone or brick should also be marked, keeping the size of the said masonry block in mind. Now, each individual block should come in a tapering shape, all blocks should be in full size and the first block near the springing point clearly sized.

Dummy

Now a dummy arch with open joints can be placed on the ground for final verification. This done, each block can be lifted piece by piece, building the arch on the wall, upon the temporary centering support provided. Unskilled masons also use a template of the curve, made from plywood, holding it across the arch under construction to ensure the profile is maintained. Over the time, skilled masons may do arches without such elaborate preparatory procedure.

Though all arches are curved, the specific type of arch may vary – with one or more layers, bricks projecting out, different placement patterns, mix of materials, pointed arches, twisted profiles, arches overlapping on each other and such others.

Providing just one arch in a building may appear out of place, hence using arches as a repetitive design element may be more pleasing, but it is important that the owners should appreciate arch as an idea, before trying them out. A flat RCC lintel comes with no choice, but arches come with dozens of variations, often at lower prices and different aesthetics. It is time arches get revived, before the skill levels vanish completely.

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Building an arch

The earliest brick arches were built possibly around the Mesopotamian civilisation, more than 3,000 years ago. Both abroad and in India, we still see arches standing tall, often bereft of the roofs, open to rain and sun, yet lasting for centuries.

19HAB-greensens_19_1851562gIt is strange to see how the horizontal and straight line has come to dominate building construction, especially in lintels, beams and roofs. Most of us may not know that early humans did not create shelters only with the horizontal, instead built sloping roofs, conical forms, domed profiles, arched windows, corbelled projections, vaulted shelters and a range of such forms where there were ideas beyond the horizontal and this continued until a few hundred years ago. Of course there are region-specific exceptions where long timber members or stone lengths were available; however the idea of horizontal lintel and beam has been gaining universal application only for a few centuries.

The simple example to discuss the effectiveness of a curved lintel is to hold a book horizontal and place a weight in the centre – of course it will bend down in the centre, or deflect, as a technical person may explain. If it is held in a curved profile, the same book will take the weight.

Depending upon the type of curve, masonry material, construction skills, strengths of the side walls, columns holding the arch and such others, the clear width of the arch gets decided by expert masons and engineers. The load gets transferred to the sides, enabling a support-free space under the arch.

Free standing

The earliest brick arches were built possibly around the Mesopotamian civilization, more than 3,000 years ago. Both abroad and in India, we still see arches standing tall, often bereft of the roofs, open to rain and sun, yet lasting for centuries. Arch is among the few forms around us that can be free standing unlike a vertical post and horizontal beam which may fall sideways. Hence many gates, bridges and openings in forts were arch shaped.

Unbelievable but true – many hundreds of years ago, many masonry bridges in stone and bricks were built with many hundreds of feet clear width!

Arch performs under a concept termed as load transfer by compressive method, where the weight of the building comes on the arch, trying to push it down, but ends up moving along the curved profile to the edges, creating a side thrust there. Thicker the arch and stronger the side wall, the wider can be the arch with greater load. Accordingly, people in the past used very thick walls; however, with the advent of modern technologies today an arch can be built with steel and cement spanning across thousands of feet.

Most Indian buildings predominantly use masonry materials even today, though concrete and steel buildings are making inroads mainly for larger complexes. With the load bearing wall anyway in place, it makes tremendous sense to support the roof over an arch. Even in a house with RCC framed system, arches can be judiciously introduced at appropriate places to reduce the cost. We only need to re-consider it as a viable option.

Adopt an arch

Arches can be built by any mason anywhere, yet why do we ignore this option? 

12HAB-greensens_12_1839771eLet us imagine a school organising a student contest to sketch historic buildings and let us try guessing what could be the most common design element to be seen in majority of the sketches. For those who know of traditional buildings, the answer is easy – it could as well be an arch.

Right from the foundation to the roof, the simple curved form called arch has played a major role in constructing everything, from bridges to buildings. Romans built long roads to move people, often upon tall bridges supported on arches below. They built aqueducts to carry water across miles, raised high above the ground by tall arches. While these examples may sound royal, it is interesting to see how down to earth this idea is – nomadic people and farmers even today would pick up couple of long twigs, bent them, stick the ends on ground to get an arch form. This would be followed by many more of such arches to be topped with small tree branches full of leaves, thick gunny fabric, tarpaulin or some such material to withstand rain and sun.

The Mughals and Sultanets commissioned large mosques and public halls which could not have been built without arches. Though mistaken to be an element of Islamic architecture, arches are found all over the human settlements. Majority of public congregation spaces, be it in a palace or a shrine, employed this versatile curved form to create undisturbed indoor spaces. Unlike a wooden straight beam, which may not be available everywhere or cannot be stable beyond a certain length, arches could be used to create larger, clearer spaces. Also, they could be built in most parts of the world, being a construction type using smaller masonry units like stone or bricks.

The idea of an entrance, especially gateways, is not complete unless it is in arch form. In historic towns, the nobleman’s mansions would be highlighted by windows with arch top, often with coloured glass. Incidentally, a domed roof is nothing but multiple integrated arch forms erected across the space.

Arches can be built within the foundation to hold up the whole building, a technology we discussed in Green Sense column early on. Mostly they are seen atop a window like a lintel replacing the horizontal member, across the hall supporting the roof slab replacing a RCC beam, in pavilions supporting the roof replacing walls, in verandahs creating an inviting entrance and in such other places. Structurally or cosmetically, we also see them located between a large space dividing them as entrances and lounges or between living and dining rooms.

If arches are so omnipresent with such potential, why are there fewer arches seen in the buildings today? On most occasions, arches cost less money, yet why do we discard this idea and employ costlier ideas? Arches can be built locally by any mason anywhere, yet why do we ignore this option? It’s time to think about it all and revive this green and eco-friendly idea.

Lintels and ‘overdesigning’

How to minimise the design to save money and materials?

lintelHow often do we try discussing with the elderly people at home about getting a building done? The general experience is about the difficulties in forging a dialogue, due to differing construction practices from their time to our contexts. We feel there are advances made in these years and many of them feel we are wasting money by ‘over-designing.’

One simple example could be the case of lintels, which are the horizontal monolith members above any opening to support the wall above doors and windows. In the past, these could be with wood or stone, but often we notice there were no members at all – the wall sits directly on the wood frame! The frame takes the load, so why lintels?

Traditionally, carpenters would prepare the door and window frames in advance, before the walls start, such that the requisite seasoning of the wood is complete. As the mason builds the walls, these frames are placed in the centre of wall thickness and the wall continues without any extra lintel member. Even today, we can see such houses standing for centuries. If so, why are we adding the lintel beam, that too in thick and strong concrete?

Horizontal rigidity

Lintels help in tying the building laterally and if continuously placed around all the walls provide a horizontal rigidity to the building, which is a compulsory measure in earthquake-prone zones. However, do we need them all the time in all kinds of buildings, including small houses, in places like Bangalore with least risk of earthquakes? Even expert answers may differ, but reducing the lintels as per context and upon structural engineer’s advice is possible. Every concrete lintels need support by shuttering, time for bar bending and concreting, curing with water and such others, demanding time, money and materials. Can we save on this, even if it’s partly?

Once the walls have reached the lintel level, we can place a wood plank as support, place two M.S. reinforcement rods of required diameter across the opening, pour nominal thin screed concrete, place the bricks vertically with the central three bricks forming a wedge narrowing downwards.

This method uses two concepts – that of reinforced brick beam in terms of steel and principles of flat arch in the brick work. On the top of this flat masonry, two more rods can be placed if heavy loading is expected. For opening up to 5 feet wide, this method can be applied with specifications as advised by engineers. As one sees, there are no concrete beams, delay due to formwork or curing and such others.

Today we design with very high factor of safety, a precautionary approach where nothing may go wrong.  Understandably, this method very often leads to over designing, as the example of lintels may prove. We can think appropriately, minimise the design to save money and materials. To that end, all the three stakeholders, namely the designers, owners and builders, should think alike towards a cost-effective and eco-friendly building.