Loads and Design of Steel Structures

For designing a steel structure, first the dimensional requirement of the structure should be defined. After this, possible combinations of loads acting on the structure must be defined. Loads provide an important understanding on the design of structure and type of steel section to use.

Design process begins with elements subjected to primary loads, proceeding to its supporting members until the foundation is reached. For an example, first a building floor slab is designed, followed by its supporting beams, columns and lastly the foundation footings. For this, we need to specify the loads acting on them. In brief, we will discuss various types of loads below:

Dead Load

Dead loads is basically the weight of the structure. It consist of weight of members of the structure. Dead load estimation of a structure is done by experienced engineers. For a building, dead loads include the weight of the columns, beams, girders, floor slab, walls, plumbing, interior fixtures and other permanent objects attached to structure. To get a rough idea, average weight for steel building is 2.9-3.6 kN/m.sq., for RCC building it is 5.3-6.2 kN/m.sq. compared to timber buildings with 1.9-2.4 kN/m.sq.. There can be error of 15-20% in estimation of dead load.

Live Load

Live loads are loads that can be caused by temporary weights on a structure, moving vehicles, natural forces, etc. They may change in position and magnitude. Following are the various live loads:

a) Building Loads: These are loads for which the building is designed. Load of furniture, accessories, people, basic objects, etc. These loads are determined by history of data on buildings of similar category. Various local, state and national codes are available for the same.

b) Wind Loads: Exposed structures are all subjected to wind loads. Intensity of it depends on the location, direction of wind, velocity of wind, height of structure, density of air, etc. Intensity of the wind load is increased if the building has an opening.

c) Snow Loads: In some regions, snow load needs to be taken care. It depends upon the shape of the roof and the roofing material.

d) Impact Loads: Impact loads are caused by moving vehicles. For example, a moving vehicle may bounce or go sideways over a bridge, imparting an impact.

e) Earthquake Loads: Includes the load effect caused by ground motions in an earthquake. During an earthquake, ground vibrates both horizontally and vertically. For high rise buildings, its analysis becomes important.

f) Bridge Loads: Loads of truck, vehicles on highway; train on railroad etc.

g) Hydrostatic and Soil Pressure: Pressure caused by water and soil. For example in dams, ships, basements, etc.

Structure Design

Allowable Stress Design: In this method, both the material and load uncertainties are included into a single factor of safety. As loads occur in combinations and it is very much possible that all loads will not occur at the same time. For example earth quake load and peak wind load may not occur at same time. In this method, stress in the material should not exceed the allowable stress for different load combinations. Typical combinations:

• dead load
• 0.6 x (dead load) + 0.6 x (wind load)
• 0.6 x (dead load) + 0.6 x (earthquake load)

Limit State Method: Also know as Load and Resistance Factor Design (LFRD). In this method, most critical limit states of strength and serviceability are considered. It takes care of the maximum and realistic load on the structure. Some of the combinations are below:

• 1.4 x (dead load)
• 1.2 x (dead load) + 1.6 x (live load) + 0.5 x (snow load)
• 0.9 x (dead load) + 1.0 x (wind load)
• 0.9 x (dead load) + 1.0 x (earthquake load)

Design of connections and loads on them should also be considered. A factor of safety should be used when designing. Nowadays, computer analysis is used for steel structure design and analysis.