CFRP strengthening of slender SHS steel columns



Introduction

Due to a recent increase in the cost of building new structures, repairing and strengthening existing structures is in high demand. Rehabilitation of steel structures is typically required due to cross-section damage from corrosion. Increasing applied loads on structures and fatigue cracking are other reasons why structures would require strengthening. It is worth mentioning that by strengthening the structure it is possible to increase its load-bearing capacity.
Conventional methods of strengthening steel columns include welding or bolting additional steel plates to the columns. The shortcomings of these techniques are an increase in the dead load and long installation time. Welding due to fatigue problems is not suitable in many cases due to additional fatigue problems arising from weld defects. ¹ Also, bolted connections are expensive and time consuming.
CFRP is a great solution for structural strengthening and significantly enhances the behavior of steel columns.
The advantages of CFRP include the following:
  1. High strength to weight ratio
  2. Increases stiffness as well as strength of the section
  3. Lightweight
  4. Easy and fast installation
  5. Corrosion-resistant (durability)
  6. Reduces costs of transportation and installation
  7. Prevents buckling of the column
  8. Improves the load-carrying capacity of columns
  9. Enhances the flexural stiffness of the repaired section
  10. Excellent fatigue characteristics
  11. Flexible CFRP sheets are applicable to complex or curved surfaces
Retrofitting steel columns by wrapping CFRP sheets in a longitudinal direction increases their ultimate load-bearing capacity by 10.06–37.74%. ²
It is worth noting that an increase in the column’s strength as a result of CFRP retrofitting depends on the following factors: modulus of elasticity, number of layers, thickness, type of adhesive material and surface preparation.
Advantages of using SHS (square hollow section) steel columns
  1. Ideal for all compression members
  2. Less weight than I-section columns which results in a lighter building
  3. Cheaper sections making it more economical
  4. SHS columns can easily be filled with concrete which is advantageous for composite strength design and fire protection
  5. SHS columns have less surface area than I-sections therefore less surface protection is needed (painting, coating)
Figure 1: Square hollow section

CFRP strengthening of SHS (square hollow section) steel columns

By increasing the slenderness ratio of a SHS steel column, the axial strength of the section reduces significantly, which generally agrees quite well with Euler’s concept.
The application of CFRP to slender SHS sections delays local buckling and results in a significant increase in axial capacity. ³

Figure 2: CFRP-strengthened SHS steel column

Facts about SHS (square hollow section) steel columns strengthened with CFRP

  • The load-bearing capacity of slender SHS steel columns is affected by the coverage percentage of CFRP sheets as well as the position of wrapped CFRP sheets on the steel columns. 
  • The support conditions of slender SHS steel columns affects the CFRP’s effectiveness. 
  • CFRP sheets are most effective on SHS steel columns with pinned-pinned end conditions. 
  • In SHS steel columns with fixed-fixed end conditions, by increasing the coverage percentage of CFRP sheets, the critical load carrying capacity of the columns does not change much due to the columns’ failure mode type. 
  • CFRP increases the axial load capacity of SHS steel columns up to two times that of un-strengthened columns. ⁴ 
  • By increasing the slenderness ratio (KL/r) of columns, the effectiveness of the CFRP system in increasing the axial strength of the columns increases. For example, the axial load capacity of SHS steel columns with a slenderness ratio of 46, 70 and 93 increased about 6, 35 and 71% respectively. ⁵ 
  • By increasing the number of CFRP layers, the load-bearing capacity of the columns increases. The effective thickness of steel SHS columns increases through installation of CFRP strips which reduces the slenderness value of the columns, ultimately delaying local buckling as well as increasing their overall buckling strength. ⁴ 
  • By increasing the number of layers of CFRP, the energy absorption of the section will increase; although it has been observed that the majority of the increase occurs by adding the first two layers of CFRP. ⁶
Figure 3: SHS steel column strengthened with CFRP ⁷

You can use Rhino Carbon Fiber™ CFRP to meet or exceed the needs of your projects while staying on schedule and budget.

Authors 
Parastoo Azad and Dr. Mehrtash Soltani (May 20, 2021)
References 
  • Kulak, G. L. (2002). Limit states design in structural steel. Canadian Institute of Steel Construction,. 
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  • Amr Shaat and Amir Z. Fam, M. (2009). Slender Steel Columns Strengthened Using High-Modulus CFRP Plates for Buckling Control. Journal of Structural Engineering.
  • R. Bambachi, M. E. (2007). Plastic mechanism analysis of steel SHS strengthened with CFRP under large axial deformation. Thin-Walled Structures.
  • Sundarraja, M. & Sriram (2014). Investigation on Corroded Hss Tubular Members under Compression Strengthened with CFRP Composites. Archives of Civil Engineering