Innovation in hybrid building design

The reinforced concrete (R.C.) structure is the accepted standard, for the design and construction of residential and commercial buildings in Bangladesh. All types of structures from 2-storey homes to 40-storey commercial, multi-purpose buildings have been built on R.C. design theory and construction principles.
The R.C. construction principle relies on three or four simple building elements: columns, beams and floors. A floor is the functional element of a building which supports the load, of people, furniture etc. The floor is supported on its edge by four horizontal elements known as beams. The beams' ends are connected to the columns. Columns are vertical elements which transmit the self-weight of the floors and beams, along with the loads of the occupants and furniture to the foundations of the buildings. A multi-storey building will also have a central 'core', consisting of the elevator shaft and stair cases connecting the floors. The core in multi-storey buildings enhances the rigidity of the building in resisting seismic and wind forces.
A brief analysis of the total weights or mass of different building elements of a 20-storey building of 4540 square feet in each floor is presented in Table - 1.     
It is apparent that floor slabs account for 31.5 per cent of the total weight of the building. The beams, columns and the foundation have to be designed to safely bear the weight of the slab.
Innovation to reduce the dead weight of the floor slabs was always at the forefront of considerable engineering research. A key breakthrough came with concrete floors supported on steel trusses. A truss is one of the oldest engineering structures, but its use to support concrete floors was a relatively recent event - dating back 50 years.
The weight distribution of different building elements in same 20-storey building with steel truss supported concrete slab is shown in Table - 2.

The total weight of a 20-floor slabs in a traditional concrete building, in Table - 1 is 3450 Tons while the weight of 20 truss supported floors, in Table - 2 is only 1588 tons. This weight saving favourably impacts the overall weight of the building. The lighter building weight is of immense importance for earthquake safety. Earthquake-induced ground movements have greater impact on massive objects as an object with greater mass has greater inertia.
The saving in the total building weight is dramatic, from a total of 10,958 tons in a traditional R.C. slab-building to 8458 tons for a truss-supported-slab building. This is a saving of 2500 tons or 23.12  per cent. In both the building types, the weight of only the reinforcing steel and concrete, the structural component, has been considered and shown for easy and convenient comparison.
The comparative analysis of the two different floor systems, on a 20-storey building was done by generating a finite element model on ETABS (Extended Three Dimensional Analysis of Building Systems) software by a leading engineering firm in Dhaka and reviewed by two leading civil engineering professors in BUET (Bangladesh University of Engineering and Technology). The ETABS software is used by civil engineers in 160 countries of the world and is universally accepted as the construction industry's most reliable software. The building was designed for a location in Dhaka and the Bangladesh National Building Code (BNBC) guidelines were followed in the design.
The total cost, inclusive of construction materials and direct labour for the traditional 20-storey concrete building stands at Tk. 265,334,212. The estimated cost of a steel truss-supported floor of 20-storey building of the same area and layout is Tk.215,646,232. This means a saving of 18.72 per cent. The time to construct a truss-supported floor is reduced by 33 per cent as no shoring is required. If the time saving is factored as a cost element then the total savings will be even higher, over 20 per cent.
In Europe and North America and also in the Middle East truss-supported floors have become the norm and accepted method of building construction. In the last five years, truss-supported floors have made inroads and gained popularity in neighbouring India as well.
The impact of the huge cost savings in housing construction by using truss-supported floors will have a favourable impact on the country's economy. It has been estimated, by 2050, the country's urban population will be 100 million. In 2014 it was 48 million. This implies another 52 million people will migrate to multi-storey concrete houses from thatched rural houses in 35 years. If the average family size of the 52 million new urban dwellers is assumed to be five persons, the number of new housing units will be 10.4 million. If the living area for each family is estimated to be only 700 square feet, this will mean 7.280 billion square feet of new housing space. If the building cost is Tk.1640 by conventional R.C. method the total cost comes to Tk.11.94 trillion. Saving 18.72 per cent of this colossal amount, by steel truss-supported floors, translates into Tk. 2.235 trillion or Tk. 223,500 crores over 35 years. Thus the annual national savings will be Tk. 6385 crores (63.85 billion). The urbanisation figures were compiled from research conducted by the Centre for Urban Studies, Dhaka University and various United Nations agencies. The studies do not cover the projected huge growth in multi-storey garment factories and other industrial buildings.
The key factor in the ready acceptance and growth of the truss-supported floor industry depends on the production and ready availability of high-quality, high-strength structural steel shapes such as angles. These structural angle shapes have to conform to the demanding standard ASTM 572 Grade 50, which is internationally accepted as the raw material for the manufacture of floor trusses.  [BSRM has successfully started the manufacture of ASTM 572 Grade 50 angles in its modern state-of-the-art rolling mill in Nasirabad, Chittagong, with a capacity of 450,000 tons per annum.]
Engr. M. Firoze, P. E. is
Head of Product Development,
BSRM Group.