Eccentricity of single angle tension members. 1

[Berniedog]

We are designing bents for a large coke conveyor system. We are using single angle, tension only bracing. The reviewer states that we must consider teh eccentricity of the connection since only on leg of the angle is bolted to the gusset plate.

Is there anything in the code or other reference that states the eccentricity does not have to be taken into account for tension members? I know section E5 states that the eccentricity can be ingnored for a compression member.

My partner has been designing bents using single angles for years and has never had to address this.

Thanks

 

[ishvaaag]

AISC 360-05

"D3. AREA DETERMINATION

3. Effective Net Area

The effective area of tension members shall be determined as follows:
Ae = AnU (D3-1)
where U, the shear lag factor, is determined as shown in Table D3.1.
Members such as single angles, double angles and WT sections shall have connections proportioned such that U is equal to or greater than 0.60. Alternatively, a lesser value of U is permitted if these tension members are designed for the effect of eccentricity in accordance with H1.2 or H2."

So, what the code states is that you can use the single angle members without any consideration of eccentricity as long as U equal or bigger than 0.60 (2, 3 or 4 in-line bolts' connections satisfy this per case 8 in table D3.1) and of course at such evaluated effective area you have the required section for the forces. You can get even larger U value from case 2 at table D3.1. And in all these cases you are allowed to not need the eccentricity account.

It might turn in your case that the required strength is not met by the above standard, in which case (and in general) the code allows the use of the consideration of tensile stress plus eccentricity. In short, in concept you are right, it is not obliged in general. Other thing is that you might need to use the allowance in the code to prove the particular competence of the angles.

 

[ishvaaag]

My last paragraph lets a bit to wish... in any case it is clear that account of eccentricity for pure tensile members (as per the model, which is a designer's prerrogative ... to be followed in the connection detail) is not mandatory in general and there is an alternative proof using eccentricity that is permitted when the stated requirements are not met.

 

[Berniedog]

Thank you. Very helpful. The reviewer is a pain

 

[Stillerz]

Some notes on shear lag ...might help

 

[nutte]

Specification section J1.7 says you don't need to consider the eccentricity in the connection.

Groups of welds or bolts at the ends of any member which transmit axial force into that member shall be sized so that the center of gravity of the group coincides with the center of gravity of the member, unless provision is made for the eccentricity. The foregoing provision is not applicable to end connections of statically loaded single angle, double angle, and similar members.

 

[GalileoG]

I can see this being an important thing to consider for bridges where fatigue is a concern.

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[ishvaaag]

nutte, my understanding of your quote of section J1.7 is other: for members other than angle or double angle (or whatever implied like such) either the connection is concentrical or must account for the standing eccentricity. On the contrary, for L or double angle and similar, the connections need not to be concentrical, but the appropriate method of design should be used, that may or not need to require account of eccentricity.

 

[nutte]

Technically speaking, I can see your point. However, the commentary to this section confirms my interpretation of the intent. The commentary states that these eccentricities have long been ignored for statically loaded members as having negligible effects on the strength.

 

[msquared48]

Nutte:

The section pay still apply due to the term "statically loaded" if the member sees lateral induced forces.

That being said, by patterning the weld, if welding is used, it is possible to have the center of force coincide with the center of gravity to avoid any eccentricity.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[nutte]

If the member is dynamically loaded, then yes, the eccentricity needs to be accounted for.

However, lateral wind and seismic forces are considered to be static forces, since the frequency of their occurrence is so small.