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Bolts and Welds: Bolts Connection

PrEN1993-1-2 states that net-section failure at fastener holes need not be considered, provided that there is a fastener in each hole, because the steel temperature is normally lower at connections due to the presence of additional material.

Based on limited amount of tests, the code assigns the same strength reduction factor kb,θ for bolts in shear and tension, regardless of bolt types. For friction grip bolts, it is assumed that the bolts slip in fire and the fire resistance of a single bolt may be designed as if the bolt is in bearing.

Basically, the fire design resistance for bolts should be determined from:

(1)

where

Fb,Rd is the design bearing resistance according to EN1993-1.8;
Ft,Rd is the design tension resistance according to EN1993-1.8;
Fv,Rd is the design shear resistance of the bolt according to EN 1993-1-8;
kb,θ is the reduction factor for appropriate bolt temperature from Table 1;
γM2 is the partial safety factor at normal temperature;
γM,fi is the partial safety factor for fire conditions.

The variation of strength reduction factor is illustrated in Figure 1. As can be seen, bolts are considered as more vulnerable to fire attacks than welds.

Figure 1: Strength reduction factors for bolts and welds at elevated temperatures

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