Comprised of 12 apartments and a basement car park, Living Steel's Kolkata demonstration project in India presented numerous design challenges ranging from stringent building guidelines to high winds and seismic activity. So developing a structural design strategy that could accommodate these challenges was no easy task. Yet, by Price & Myers, the project's engineering firm, more than met these challenges while also satisfying the design criteria for a simple, economical and sustainable structure.

Recently, Kevin Williams, a structural engineer with Price & Myers who assisted with the value engineering of the project, spoke to Living Steel about some of the innovative structural solutions they were able to achieve through the use of steel.

How did the use of steel as a structural building material contribute to the success of your structural design strategy?

The floor slabs are made up of concrete metal deck slabs that are shot-fired fixed to the steel beams with shear studs at regular centres and supported on steel floor beams. Once the concrete is poured, this connection creates a composite steel and concrete slab. And the primary beams are connected to steel columns, which transfer the vertical dead and imposed loads to their base.

Perforated steel panels are distinctive and critically important to the aesthetics of the building as well as the ventilation approach

This afforded numerous advantages:

• It's an economical way to increase the spans of steel-framed floors while minimising the required structural depths, and the network of primary and secondary beams keeps the slab thickness to a minimum.
• It's a very quick form of construction, further reducing job-site time and costs.
• The lightweight form of construction saves on the cost of the foundation, while also promoting health and safety on the job site.
• There is also inherent fire resistance in both concrete slabs and steel which contributes to the safety of the structure.

A pattern of braced cores ensures all lateral loads above first floor are transferred so that the slab supports a more manageable one storey of lateral load.

We designed the structural grid for the residential areas to open up the floor plans of each flat so that we could ensure future flexibility with the apartment layout. We were able to do this by hiding the structural steel in the walls so that it didn't impose on the floor space. This way, we managed to get a regular, rational grid that suited the apartment layout.

The difficulty was that this structural grid did not work with the basement car park. The Local Building Authority in India has very stringent regulations for the number of parking spaces required as well as the turning circles. So we had to adapt some of the structural points in the basement by creating a concrete transfer slab at the ground floor. This called for shallow, wide beams that support the frame above while transferring the load to the reinforced concrete (RC) columns and walls in the basement. This is then supported on piled foundations into the clay at a depth of about 30m below ground.

You mentioned that the structural grid you designed presented some problems. What were they and how did you solve them?

How did your design accommodate the location's vulnerability to extreme weather conditions?

During monsoon season, the building is subject to large lateral loads so the lightweight structure is actually a disadvantage. We designed around this by introducing cross bracing within the walls. When subjected to lateral loads, the external panels transfer the loads into the metal deck floor slab. This then acts as a plate and transfers the loads into the braced cores and then into the foundation at the base of the columns.

Since the area is also prone to seismic activity, this was another consideration for the building's structural design. Since steel framing is light weight, it makes it much more suitable for resisting seismic forces because the connections can be designed to dampen the seismic loads.

Structural points in the basement were adapted to accommodate car park requirements by creating a concrete transfer slab at the ground floor.

What are some other challenges you faced?

The loads at the base of the braced BHS are substantial and we wanted to avoid loading the transfer slab because the pull up loads at the ground floor would have been too high. We came up with a pattern of braced cores that ensured all lateral loads above first floor are transferred where there are RC columns directly below the steel. Then, the transfer slab only has to support one storey of lateral load which is much more manageable.

In what way did the double skin, perforated steel panels contribute to the structural strategy?

Panels are very distinctive and critically important to the aesthetics of the building as well as the ventilation approach. They are as important as the frame for the structure to be a success. The key was to design the steel panels to appear simple and lightweight, while also resisting the large lateral loads. Because the panels also form an envelope for the whole building, a lot are required so we also needed to come up with an economical solution.

We designed the panels to be formed out of a single 3mm steel sheet that could be folded along the four sides. This creates a box frame around the edge which stiffens the panels and creates a rigid frame. This process can then be replicated.

These perforated panels impact how the building looks, and also contributes to the building's natural ventilation. We recently inspected a sample panel in India and it appears to be a great success. We've been advised that 50 panels a day can be manufactured. It's an elegant and economical approach to design.

How did steel help satisfy the design criteria for a sustainable building?

In addition to being made with recycled content, steel can always be recycled again and again. Steel is also light weight which minimises the impact on the foundation, while also contributing to design flexibility so that the building can easily be adapted for a long building life. And the steel panels contribute to the building's natural ventilation can help save on energy costs once it is occupied.