What is structural loading?

The different loads such as loads due to self-weight (dead load) and external loads such as live load, wind loads, seismic load, snow load, and other loads and load combinations that influence the design of a building are called structural loading. The nature of the applied load differs from changes in the location, design, and type of materials used. The design criteria require that structures must be designed and constructed to be able to withstand all load types that are expected to act upon the structure and render its usage for the designated purpose. The structural members such as beams, slabs, columns, and foundations help the structure to withstand the applied load. There are different institutes such as ASCE and ACI that provide building design codes to assure minimum safety requirements and quality of the structures.

Different types of Structural loads

• Dead load

The dead load is the load due to the self weight of the constructed structure that includes the weight of the materials used for construction and immovable fixtures and furnishings. It is also called as static load since it does not move from one place to another.

• Live load

The loads that are dynamic (changes its point of action with due course of time) in nature are called as live loads. These live loads include occupants, movable furniture, masonry walls, temporary partition in case of buildings and other structures and vehicular traffic in case of roads, bridges and highways, live loads are temporary and will change with time.

• Wind load

It considers the movement and speed of air to the location of the structures. It can be negligible in the case of small buildings, but plays a vital role in the design of skyscrapers.

• Snow load

This is an important factor in regions where snowfall is a regular occurrence and snow can accumulate to act as a significant load. The shape of the roof is an important aspect while designing the building with the consideration of snow load.

• Seismic and Thermal load

Seismic load is created due to the movement of tectonic plates under the ground. These loads tend to shake the entire building and the buildings should be designed by considering all the horizontal forces in an area of high seismic activity. Thermal loads are the loads created due to thermal variation like freezing and thawing leading to the deterioration of the structure. Wind loads, snow load, seismic load and thermal loads fall under environmental loads as they occur due to changes in the environmental activity and weather conditions.

• Load combinations for strength design (ASCE 7-10, sec, 2.3.2)

1. 1.4 Dead load
2. 1.2 Dead load+1.6 Live load + 0.5 (Live load on roof or Snow load or Rain load)
3. 1.2 Dead load +1.6 (Live load on roof or Snow load or Rain load) + (Live load or 0.5 Wind loads)
4. 1.2 Dead load +1.0 Wind loads +Live load+0.5 (Live load on roof or Snow load or Rain load)
5. 1.2 Dead load+1.0 Seismic load + Live load +0.2 Snow load
6. 0.9 Dead load +1.0 Wind loads
7. 0.9 Dead load +1.0 Seismic load

• Load combinations for allowable stress design (ASCE 7-10, Sec. 2.4)

1. Dead load
2. Dead load +Live load
3. Dead load + (Live load on roof or Snow load or Rain load)
4. Dead load + 0.75 Live load + 0.75 (Live load on roof or Snow load or Rain load)
5. Dead load + (0.6 Wind loads or 0.7 Seismic load)
6. Dead load + 0.75 Live load + 0.75 (0.6 Wind load) + 0.75 (Live load on roof or Snow load or Rain load)
7. Dead load + 0.75 Live load +0.75 (0.6 Seismic load) + 0.75 Snow load
8. 0.6 Dead load + 0.6 Wind loads
9. 0.6 Dead load + 0.7 Seismic load

Similarly, there are different structural loads and load combinations for different structures according to the building design codes used in that country.

Structural Design Steps

The following are the different steps that are involved in structural design.

Determination of bearing capacity of soil, determining the possible loads (live load, dead load, snow load, wind loads, other loads and load combinations) that would act on the building and other structures, designing the structural members following building codes and conducting structural analysis for the applied loads to ensure safety of the structure.

Design criteria

The design of a structure is based on its strength and serviceability. The structure should be in a condition to withstand the live loads applied. In structural design, for the given load combinations, the geometry is checked, assured and approved. It is a process of finding out whether the selected section would be able to bear the applied live load without failure, such as collapse, damage, deflection, or vibration.

• Structural planning and design

A structure plan is a framework to guide the development or redevelopment of an area by defining the future development and land use patterns, areas of open space, the layout and nature of infrastructure (including transportation links), and other key features. The factors that influence the structural design are magnitude and type of loading, duration of loading, clear span, material of the structural elements, shape, and size of the structural elements.

The methods used for structural design are Working Stress Method (WSM), Limit State Method (LSM) and Ultimate Load Method.

• Working Stress Method

This method is used for the design of reinforced concrete, steel, and timber design. This method assumes that the structural material is linearly elastic and safety of the building can be ensured by limiting the stresses induced by the working loads experienced by the building. No factor of safety is considered in this design method.

• Ultimate Load Method

In this method, the stress at collapse area is analyzed and the nonlinear stress-strain curves of concrete and steel are used. The design safety depends on load factor, which is the ratio of ultimate load to the working load. But this method does not guarantee the serviceability, even though there is satisfactory performance at ultimate loads. The design may sometimes lead to deflections under service loads which is not reliable. This Ultimate load method is also called as load factor method.

• Limit State Method

In this method both safety and serviceability of the structural members are considered. This method uses safety factors which are multiplied with the service load for different load combinations to ensure that the structure is designed to take sufficient load. Limit state indicates the extreme stage where the structure will not be able to take any excess load and will become unserviceable. The two types of Limit states are, Ultimate limit states (limit states of collapse) which deals with buckling, overturning and fatigue fracture. Serviceability limit state deals with the deflection, leakage and other malfunctions which causes discomfort to the occupant.

Context and Applications

This design criteria for structural loads acts as the basis of construction of structures such as buildings, dams, bridges and other structures. It has a major role in the following fields of engineering.

• Bachelor of Technology in Civil Engineering
• Bachelor of Technology in Mechanical Engineering
• Master of Technology in Structural Engineering

Practice Problem

Q1) What is shear force in structural engineering?

1. Shear force is a force acting perpendicular to a surface
2. Shear force is a force acting parallel to surface
3. Shear force is a rotational force
4. Shear force is a turning force

Answer: Option a

Explanation: It is a force that acts perpendicular to the surface and arises when two objects traveling in the opposite direction rub against each other.

Q2) ASCE stand for American Society For Civil Engineers

1. True
2. False

Answer: Option a

Explanation: The American Society of Civil Engineers represents more than 150,000 members of the civil engineering profession in 177 countries. Founded in 1852, ASCE is the nation's oldest engineering society.

Q3) What is a girder?

1. Girder is horizontal support that supports smaller beams
2. Girder is vertical support used to carry the dead load and live load
3. Girder is an inclined support used in retaining walls
4. Girder is another name of structural loads

Answer: Option a

Explanation: It is a beam that supports the smaller beams in the structure. It is mostly used in bridge construction.

Q4) What of the following is an example of lateral forces?

1. The wind loads acting against a facade
2. Seismic load against foundation
3. Gust and other loads acting against roof
4. Both a and b

Answer: Option d

Explanation: The forces that act in the horizontal direction are termed lateral loads. Both wind loads and seismic load acts perpendicular to the surface of the structure, so it is termed lateral loads.

Q5) Which of the following is not a structural member?

1. Beam
2. Column
3. Staircase
4. Foundation

Answer: Option c

Explanation: Structural members are those that help the structure carry the loads and safely transmit it to the corresponding structure. Beam, column, and foundation help the structure to carry the loads, while the staircase does not carry any load and hence it is not a structural member.