Common Sources of Errors in Structural Design

Reading time: 1 minute

There are several sources of errors in structural design and drawings. It is important to specify and explain these sources to avoid such mistakes in the design of buildings.

Fig.1: Structural Designs of Structures

Common Sources of Errors in Structural Design

• Failure to understand behavior of structure which are being designed
• Errors in looking up and writing down numbers
• Errors due to conversion of units

Design Error Due to Failure to Understand Behavior of Structures

Inappropriate or false understanding of the behavior of structure is an extreme design error which can compromise the purpose of the design. It is obvious that the structural designer would not be able to adequately understand the response and behavior of the designed structure without an extensive and wide understanding of principles of statics and structural behavior. Therefore, such background knowledge is a prerequisite for producing a successful and safe structural design. If the designer does not have such prerequisite, then any efforts and trials to design a building would be meaningless and the safety of the structure would be in a severe danger. If the analysis of the structure is performed using computer program, then it is recommended that the designer check samples of results to ensure that the values generated by the program is realistic and correct. So, it is discouraged to blindly use the result of structural analysis conducted by computer program since such process could lead to a fatal mistake in the design. The sinking of the Sleipner Offshore Platform was caused by errors made in the computers program (finite element program NASTRAN were employed). The investigation conducted to determine the reason of the failure found out that the program underestimated the shear stress by 47 percent.

Fig.2: Failure of Sleipner Offshore Platform

Errors in Looking Up and Writing Down Numbers

These types of errors are made when the designs do computations quickly to meet the deadline set for the design to be finished. The designer needs to invest adequate time for examining the design and guarantee that all values are written correctly. When numerical calculations are conducted, it is advised to write results with three important figures. The final result would have two significant figures provided that numerical computation is done with three significant figures. As far as rounding of values are concerned, there are recommendations which can be used to carry out rounding numbers and prevent errors in the design. Table 1 provides recommendations regarding rounding results in design calculations. Table-1: Recommendations for Rounding Off Design Computations

Types of calculation	Record/ round to the nearest	Type of element designed
Loading	0.1 kPa	Slabs
	1 KN/m	Beams and girders
	0.1 KN	Point loads
	5 KN	Column and footing loads
Dimensions	10 mm	Span length and location of load
	5 mm	Effective beam and slab depth
Computations	10 KN.m	Bending moments
	10 mm2	Reinforcement areas
Design selection	5 mm	Slab thickness
	25 mm	Beam depth and width
	25 mm	Column cross sectional dimensions
	25 mm	Bar spacing in slabs and walls

It is far more significant for the designer to understand the behavior and response of structure under applied loads, location of reinforcements and arrangement of structural members rather than conducting design computations with unnecessary accuracies.

Errors in Structural Design Due to Conversion of Units

This type of errors is not common but it is possible to occur especially by students or graduates during performing numerical operations. Usually, conversion mistakes generate considerably unrealistic results whether it is small or large values. There are number of measures which can be taken to prevent such sources of errors. For example, the engineer should familiarize himself or herself about unit conversions and their relationships. It is recommended to show units in the final result of calculations and finally use common sense to examine the result of calculations, specifically when very small or large values are obtained. There have been examples of unit conversion errors that cause the failure of the project. For instance, Mars Climate Orbiter spacecraft was destroyed due to errors made while metric units converted to English units. This incident proves that errors in design of structures are possible to occur and need to be taken into consideration. Read More: Types of Design and Detailing Errors in Construction and their Prevention Construction Errors During Concreting at Site Roles and Responsibilities of Structural Design Engineers in Construction