The complete tri-dimensional building structure was modeled including all 4-blocks, slabs simulated as diaphragm with infinite stiffness in its plane, and with zero stiffness in the perpendicular plane. All connections between columns and beams and foundations were considered fixed in both directions. (Figs. 2 and 3). The structural analysis was done using the software SAP 2000(11), with accidental combination of loads recommended by NBR 14323 (12, 13):

Where:

Ffi,d - design value of action in fire

FGi,fi,k - characteristic value of permanent action i in fire

FQj,fi,k - characteristic value of variable action j in fire

FQ,exc. - characteristic value of thermal (exceptional, accidental) action, generally equal zero in the presence of gravitational load and ISO-Fire

γgi,fi = 1,2 - partial safety factor for permanent action i in fire

γq,fi = 1,0 - partial safety factor for variable action j in fire

Ψ = 0.2 for places where there is neither predominance of weights of equipment that remains fixed for long periods of time, nor of people concentration.

ψ = 0.0 for wind loads

The dead loads were pre-casted slab concrete/ceramic 120 mm thick and wall with normal brick 100 mm thick. The considered live loads were 1.5 kN/m2 on the floor, 0.5 kN/m2 on the ceiling and 3.0 kN/m2 on the stairs. The cold formed steel was verified based on the NBR 14762(14), similar to AISI(15), adapted for the fire situation. This adaptation consisted in including the reduction factors ky,θ and kE,θ for class 4 elements from Eurocode 3 - Part 1.2(8). Besides that, other equations (p. ex. eqs. 2 and 3) recommended by NBR 14323(12, 13), based on the old Eurocode 3 - Part 1.2(16), were verified using a simplified software developed by Soares(17):

Nfi,Sd - design value of the compression in fire

Nfi,Rd - design value of the compression resistance of the gross cross-section, in fire

Mx,fi,Sd - design value of the bending moment in fire

Wel,x - plastic section modulus

Nex, fi - value of elastic buckling load

A - area

ky,θ - yield strength reduction factor

fy - characteristic value of yield strength

where:

Aeff - area effective of cross section including local buckling effects

ρθ - reduction factor for flexural buckling for a temperature θ λ0,θ - reduced slenderness in fire

The compartmentation efficiency and the heat transfer between steel and slab or wall lead to a thermal gradient in the structural elements. That gradient and, consequently, the efforts were not considered in this structural analysis. The actions from the axial deformation and the little minor inertia bending moment were also not considered. By other hand, we used the maximum temperatures from metallographic or thermal analysis as a uniform distribution.

In this paper, we studied the central column with one face protected by a wall and the beam close the window, the hottest region. The column buckling lengths were 2.92 m e 3.68 m. One adopted fy = 390 MPa, minimum value from metallographic.

The column temperature was considered below 550 °C, and it is based on the metallographic tests. As shown by SMARTFIRE and Supertempcalc, the medium temperature was below 400 °C. At this level of temperature, the columns of the 1st and 4th floor have φE and φR ≤ 1.0, then, there is structural safety based on the hypothesis of this paper. It is possible to notice in loco that no failure, global or local, occurred with the column.

The hottest beam temperature ranged 650°C to 723 °C, following metallographic observation. Based on the SMARTFIRE and Supertempcalc, the beam reached a medium temperature of 610°C. For that level of temperature, φR ≤ 1,0, i. e., the structures are in fire safety. By simplified design method no collapse would happen, confirmed by actual facts. Using the curve ISO 834(6), and determining the equivalent temperature on the beam, based on the Supertempcalc, the beam fire resistance is approximately 30 min, i. e., φR ≤1,0 for FR = 30 min.

• << Prev
• Next >>