DESIGN GUIDELINES Partial safety factors for strengthening of metallic structures. Dr. Stuart Moy University of Southampton

Transcription

1 DESIGN GUIDELINES Partial safety factors for strengthening of metallic structures Dr. Stuart Moy University of Southampton

2 Contents 1. Some examples of strengthening 2. Metallic structures 3. Why are Design Guides needed and what is available? 4. Limit State Design 5. Partial Safety Factors 6. The recommended approach for FRP composites 7. Adhesives

3 London Underground - Shadwell Station Ultra high modulus carbon fibre composite applied to cast iron struts Reinforcement RIFTed on site. Poor substrate, high UV, temperature, no load relief

4 London Underground Acton Bridge Ultra high modulus carbon fibre composite plates glued to steel beam Plates preformed. Surface preparation, humidity, temperature, some UV.

5 London Underground - Circle Line Ultra high modulus carbon fibre composite plates glued to cast iron girders. Plates preformed. Underground, damp, possible chemical leaching, no UV, no temperature.

6 Metallic Structures? Steel Cast iron Wrought iron Aluminium? Stainless steel?

7 Why are Design Guides needed? New technology so we don t want anything to go wrong. Great interest but limited expertise Cowboys!

8 What is available? Strengthening concrete structures: The Concrete Society. Technical Report No. 55. Design guidance for strengthening concrete structures using fibre composite materials. Strengthening metallic structures: Institution of Civil Engineers. ICE design and practice guide. FRP composites. Life extension and strengthening of metallic structures.

9 What is coming? Strengthening metallic structures: CIRIA design guide, RP645. Strengthening metallic structures using externally bonded fibre reinforced polymers. Likely to be published by end 2003.

10 Limit State Design Based on the assessment of uncertainty in Loading and Materials. Statistical assessment of uncertainty leads to Characteristic loading and Characteristic material strength and stiffness. To allow for the uncertainty in the loading multiply the characteristic load by loading partial safety factor(s). To allow for material variability and degradation reduce the characteristic strength or stiffness by material partial safety factors.

11 Limit State Design The risk of failure can be assessed by the intersection of the probability distribution curves of the loading and the material strength. loading design values resistance failure region

12 Limit State Design Relevant Limit States Ultimate: Strength Bending Shear Compression Adhesion Fire Serviceability: Deflection Fatigue Durability Creep Stress rupture

13 Partial Safety Factors a) Loading Use the factors currently in British Standards (BS5950, BS5400), Eurocodes or Highways Agency documents (BD21/90) b) Metals Again use the values given in Standards. Need to be particularly careful with cast iron or wrought iron.

14 Partial Safety Factors Composites Predicted or measured properties? Laminate theory versus testing? Factor for method of fabrication? method of fabrication fibre volume fraction void content stiffness 0 o direction (kn/mm 2 ) hand lay-up 0.40 up to 5% 230 RIFT 0.54 < 1% 310 pre-preg % - 2% 360

15 Composites Characteristic Strength Usual approach: σ k = σ m 2s B-basis approach: σ k = σ m k B s Value which will be exceeded by 90% of results with 95% confidence where: σ k = characteristic strength σ m = mean strength s = standard deviation k B = tolerance limit factor k B is found from statistical tables. Value varies with number of samples tested.

16 Composites long term behaviour Long term behaviour of composite materials depends on several factors. It is better practice and more realistic to consider each effect separately and then combine them. The approach adopted uses a Degradation Factor rather than a Partial Safety Factor. This makes it easier to combine effects.

17 Composites Degradation Factors Degradation Mechanism maximum value E-glass aramid Carbon E-glass minimum value aramid carbon Moisture Chemical exposure UV exposure Fatigue * * * Creep Impact Overall degradation χ d * The factor of unity applies only when there is no fatigue loading. Lower bound value due to high amplitude, high frequency load cycles.

18 CFRP Typical degradation factor Degradation mechanism Moisture Chemical exposure UV exposure Fatigue Creep Overall degradation factor χ d Factor χ d = = 0.4

19 Stiffness degradation factor Stiffness is a fibre dominated property where the fibres are oriented in the direction of loading. Environmental degradation is very small unless the environment is permitted to attack the fibres. It is good practice to apply a stiffness degradation factor, to allow for material variability. Material Carbon FRP Aramid FRP Glass FRP Factor

20 Design allowable values σ des = σ k χ d

21 Adhesives Potential problems: Surface preparation of the substrate Humidity during application Temperature during application Variability of adhesive quality control issues Peeling. Good adhesion is vital. Recommended partial safety factor on adhesive strength 5.

22 Conclusions Design guidance for strengthening metallic structures using fibre reinforced polymer composites is still in its infancy. The recommendations in the ICE Design and Practice Guide represent current best practice. They are based on practical experience and limited research. It is the only Design Guide for strengthening metallic structures using FRP available at present.