11/30/2016

How to Calculate the Design Strength (LRFD) and Allowable Strength (ASD) for Double Angle in Tension

In this post, I will discuss the third steel design example in our steel design course covering the analysis and design of tension members. The goal of this example is to calculate the design strength (LRFD) and allowable strength (ASD) for double angle in tension. We are using the 13th edition of the AISC Steel Construction Manual. For this problem, we will first consider a single angle and calculate the governing strengths for this angle. Once the governing strength is determined, it will be doubled to account for the double angle shape.

 

 

The first step is to calculate the nominal strength based on the gross area.

 

The second step is to calculate the net area. The net area is equal to the gross sectional area minus the area of the holes.

 

The third step is to calculate the effective area. The effective area is equal to the net area multiplied by a reduction factor. The reduction factor in this problem is equal to 0.75.

 

The fourth step is to calculate the nominal strength based on the effective net area.

 

The fifth step is to calculate the design or allowable strength based on yielding of the gross area. The design strength based on yielding is equal to the nominal strength multiplied by a resistance factor of 0.9. The allowable strength based on yielding is equal to the nominal strength divided by a safety factor of 1.67.

 

The sixth step is to calculate the design or allowable strength based on fracture of the net area. The design strength based on fracture is equal to the nominal strength multiplied by a resistance factor of 0.75. The allowable strength based on fracture is equal to the nominal strength divided by a safety factor of 2.

 

The seventh step is to compare both design or allowable strengths and choose the smaller value, and this will be the governing strength.

 

The last step is to double the governing strength because we need to account for the double angle shape, and this will be the final governing strength for the double angle shape.