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Module 1: Design of Compression Members and Lacing Systems

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Design of Compression Members
 
The basic steps in designing a compression member are:
 


Assume a suitable design compression stress
Calculate the effective sectional area required
Select a section that gives the effective area required, and calculate the minimum radius of gyration
Determine effective length
Determine slenderness ratio and hence design stress and load-carrying capacity
Modify the section if calculated load-carrying capacity differs significantly from the design load

Design of Built-up Compression Members
 
Builtup members comprise two or more rolled steel sections. It is used when the compressive load that will act on the member is too high for a single rolled section to withstand. It also has an equal radius of gyration in both directions of its section.
 
he steps in designing a built-up compression member are:
 


Find the effective length from the actual length and end conditions
Assume slenderness ratio to be between 30 to 60 for the built-up section
Find design compressive stress for the given slenderness ratio
Find the required area of the compressive member
Choose a built-up section such that its total area is more than the required area calculated in step 4
Find the minimum radius of gyration and then calculate the slenderness ratio
Find the design compressive stress and strength. If the design compressive strength is more than the compressive load acting externally, then the design is okay. Otherwise, choose a higher section and repeat steps 5  to 7.

Concepts of Lacing Systems
 
Lacing systems are used to keep the built-up sections throughout its length. They are designed on the basis of the failures that can occur in the system. The failure modes in lacing systems are:
 


Buckling of the built-up member as a whole
Buckling of the main component
Distortion of section
Failure of Lacing member

Design of Lacing Systems
 
The basic steps in designing a lacing system that uses bolting connections are:

Choose a lacing system and the angle of inclination with the axis of the compression member.
Compute the distance between the bolt centers and the spacing between the sections.
Find the slenderness ratio of each component.
Find the effective length of the lacing.
Determine the thickness of lacing.
Calculate the maximum slenderness ratio of lacing and ensure it is less than 145 for flat plates.
Calculate the compressive strength of the member.
Determine transverse shear and then the force in each lacing.
Calculate the developed compressive and tensile stress in the lacing system.
 Select the bolt diameter and find the minimum width of the flat plate.
 Design the end connections for the lacing system. Check if the number of bolts is sufficient to withstand the  load in the member.