English abstract
Fire has always been a serious threat to buildings. The September 11 incident at the World
Trade Center in 2001 has encouraged significant interest in understanding the behavior of
structures under fire. The reduced material strength and additional thermal stresses due to
elevated temperature are leading to the failure of the member. This study is an attempt to
understand the effect of continuity in steel frames under different levels of elevated
temperature. A 3-story steel building with a footprint of 25 m x 22.5 m is modeled using
SAP2000 using a non-linear static analysis. Three models were used with different spans
length in the load-bearing direction to study the effect of the continuity aspect on the
behavior of the structure under elevated temperatures. Spans in the first model were
equally spaced by 7.5 m for each span, and in the second model, the middle span was
shorter than the equaled edge spans, where the middle span was 4.5 m and edge spans
were 9 m, and finally, the middle span was longer in the third model with a middle span
of 9 m and edge spans of 6,75 m. Each model was studied for three temperatures which
are 250, 500, 750 °C. Thermal loading was applied considering three scenarios based on
the location of application: Outer span, inner span, and the whole span. In addition, results
were compared for fixed base and pinned base conditions. The results showed that as the
temperature increased, the internal forces and deformation on the affected beams
(Moments at spans and supports, axial forces, and deflections) increased. Furthermore,
the bending moments in both spans and supports as well as the axial forces in spans are
affected by the location of the thermal loading. It was observed that the members, which
are near the location of thermal loading, experienced higher internal forces. Generally,
results illustrated that with the pinned base condition, bending moments at both supports
and spans are more than the values with the fixed base condition, which is happening due
to the contribution of the fixed support to carry moments. The maximum axial force was
more in magnitude with fixed support than with pinned support, this is because the pinned
base will allow for expansion of the beam and release part of the axial force. On the other
hand, the axial force transferred to the next span is more when it is pinned at the base
which is again due to the flexibility that pinned support provides. There was no
significance of changing the base support on the plastic hinge formation mechanism, as
the hinges developed approximately at the same levels of temperatures and showed the
same overall behavior. Additionally, number of plastic hinges were increased as the
temperature increased because as the temperature increases the loading on members will
increase and hence the member will undergo plastic behavior at different locations.
However, in the cases of heating the outer or inner spans, only heated spans had plastic
hinges formed, whereas in the case of heating the whole spans, there were hinges
developed at the three spans.
