1. Effect of Location of Axial Restraint on Beam-Columns behavior in Fire
Mahmud M.S. Dwaikat
Civil & Env. Engineering
Michigan State University

2. Outline Background: Beam-Columns Modeling Location of Axial Restraint
Finite Element Model
Idealization
Validation
Parametric Studies
Effect of Fire Scenario + Beam Length
Effect of Beam Slenderness Ratio
Conclusions

3. Background Steel Beam-Columns L
At room temperature steel beams are designed for flexure
Axial Restraint
Under fire, steel expands non-uniformly due to thermal expansion
The restrained member will develop axial force and bending moment because of the restraints
Expansion = Δℓ
The member will no longer behave
like a beam, but like a beam-column
Axial force
Bending moment
Beam-column

4. Modeling Location of Restraint
In real situations, location of axial restraint is variable

5. Finite Element Model Idealization
ANSYS software – SUR151 and PLANE55 elements for thermal analysis – SHELL93 element for structural analysis
Steel temperature is independent of its stress state.
Non-uniform temperature over the cross-section, and uniform along the length of the member.
ANSYS Creep Model 11: Generalized high-temperature creep - Time hardening, including primary and secondary creep strains
L/3 P ks
1D mesh using SURF151 element
2D mesh using PLANE55 element 1 I J x y K
SURF151 1D-thermal
Surface-effect element 4 1 2 3 I J K L
SHELL93
element
PLANE55 2D-thermal solid element
Thermal Discretization
Structural Model

6. Model Validation Fire Test Data (Li and Guo 2007)
Restrained beams tested under design fires
No local buckling, no lateral torsional buckling
Unprotected steel beams
H250X250X8X12
Fire exposure
H300X300X12X20
H300X300X16X25
Ceramic fibre blanket
3 mm thickness
4.5 m
150
300
450
600
Temperature ˚C
750
Time min 4 12 20 28 36 42
900
H250X250X8X12

7. Model Validation Thermal Response
H250X250X8X12
Ceramic fibre blanket
3 mm thickness
Test data
900
Fire
Bottom flange
750
Web middle
Top flange
600
450
Temperature ˚C
300
150
Model predictions 4 12 20 28 36 42
Time min

8. Model Validation Structural Response

9. Parametric Studies Fire Scenarios – Beam Lengthks
W24x76

10. Parametric Studies Fire Scenarios – Beam Length
W24x76
L/3 P ks

11. Parametric Studies Beam Slenderness Ratio
Exposed
L/3 P ks

12. Conclusions
The location of the axial restraint in a beam exposed to fire has a significant influence on the fire performance of the beam.
Shifting the axial restraint in the beam to the bottom flange greatly enhances the ultimate fire resistance for beams with low to moderate slenderness ratio (L/r ≈ 20).
For beams with high slenderness ratio, the location of the axial restraint has little influence on the ultimate fire resistance of the beam.
Fire scenario does not affect the influence of shifting the axial restraint on the fire resistance of the beams. This because fire scenarios affect the heating rate, and has little effect on the thermal gradient developed in the section

13. Acknowledgment
Thank you!