I-beams Strengthened with CFRP

The main goal of this study is to investigate the structural behavior of steel I-beams strengthened with CFRP and their failure modes.

Understanding CFRP’s failure modes is essential; not only will you be able to find solutions for preventing or deferring these failures, but you may also consider such failures in the design process.

I-beams Strengthened with CFRP

Figure 1- I-beam strengthened with CFRP

Traditional technique of strengthening aging I-beams

Welding/bolting heavy steel plates to the tension side of the steel I-beam [1].

Disadvantages of the traditional technique

This technique is only a short-term solution since the repair material is not durable and the repaired member is still at risk of environmental, corrosion and fatigue damage. Other disadvantages are:

  1. High costs
  2. Changing the shape of the existing structure by its thickness
  3. Maintenance against corrosion is required
  4. Poor fatigue performance
  5. Reduced I-beam strength because of drilling holes through the steel flange. There is stress concentration around the edges of the holes, and it will make the I-beam weaker and there is no way to strengthen these weak points
  6. Difficulties in construction sites due to the high weight of steel plates
  7. Increasing the dead load of the steel structure

Strengthening I-beams by externally bonding CFRP to the bottom (tensile) flanges has demonstrated significant potential as an alternative to steel.

Advantages of the CFRP-strengthening technique

  1. High strength to weight ratio
  2. Increased stiffness as well as strength
  3. Increased fatigue life of the I-beam
  4. Lightweight
  5. Easy and fast installation
  6. Corrosion-resistant
  7. Reduced costs of transportation, installation, repairing and strengthening of I-beams
  8. Decreased vertical deflection of I-beams
  9. Improved load-carrying capacity of steel beams
  10. Enhanced flexural stiffness of the repaired beams

CFRP failure modes of flexural-strengthened steel I-beams

CFRP-strengthened I-beams usually fail by either debonding or rupture of the CFRP. CFRP failure modes of flexural-strengthened steel I-beams include [2]:

  1. End debonding
  2. End delamination
  3. Below point load debonding
  4. Below point load delamination
I-beams Strengthened with CFRP

Figure 2- CFRP Failure modes

Failure modes of a CFRP-strengthened I-beam

The purpose of strengthening I-beams with CFRP includes increasing the following items [2]:

  1. Load capacity

  2. Ductility

  3. Stiffness

  4. Fatigue life of the beam

  5. Resistance against environmental factors

Failure modes of a CFRP- strengthened I-beam include [3]:

  1. Buckling of the web in shear
  2. Buckling of top flange in compression
  3. CFRP debonding
  4. CFRP rupture

Each one of these failure modes shall be considered in the structural design process to achieve improved resistance.

I-beams Strengthened with CFRP

Figure 3- Failure modes of an CFRP strengthened I-beam

How to avoid CFRP failure in I-beams

  • Using shorter CFRP straps causes premature end-debonding, and using longer CFRP straps increases the beam’s resistance against end-debonding ²
  • Point load splitting can be reduced by using thicker CFRP and the load-bearing capacity of the CFRP will increase considerably ²
  • Load-bearing capacity of the strengthened I-beam improves by increasing the thickness and length of CFRP (not more than the effective length of CFRP)


Parastoo Azad and Dr. Mehrtash Soltani (March 17, 2021)


  1. Siwowski TW, S. P. (2018). Experimental study on CFRP-strengthened steel. Composites Part B

  2. Kambiz Narmashiri, N. R. (2012). Failure analysis and structural behaviour of CFRP strengthened steel I-beams. Construction and Building Materials.

  3. Oral Buyukozturk*, O. G. (2004). Progress on understanding debonding problems in reinforced concrete and steel members strengthened using FRP composites. Construction and Building Materials.