## Robot Structural Analysis

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# steel connection

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Dear Sir

Kindly find attached file,

I have encountered a problem with connection design as follow:

When I take base plate connection using “new connection tool bar” in connection layout

If I have double moment applied on the base I face two conflict results

If I put Mz=0 the result is ok

If I put any value for Mz>0 “even Mz=0.01 kN.m” the result not ok

Please find the file I send to you and see the attached pictures

The first Mz=0

The Second Mz=0.01 kN.m

thanks

Solved! Go to Solution.

# Re: steel connection

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Would it be possible for you to attach your hand verification of the bi-directionally loaded fixed column base connection or indicate the example of such verification done according to EC3 in some reference book or paper please?

**Artur Kosakowski**

# Re: steel connection

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Dear sir:

We didn't use manual relation for verification the connection.We use the Robot opion to design base plate connection by userdefined forces value. So when we put the moment value for some connection=0 it become ok, when we put very neglable value such as 0.01 the connection become not ok.

# Re: steel connection

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The reason for my question is that in my opinion due to the different verification formulas there is no linear scaling of results when you compare unidirectional bending with bi-directional one. My understanding is that for the latter case you reduce the capacities for unidirectional bending for each of the directions due to the influence of axial force and then you add these values plus the resistance for axial loading. As the result of such action the capacity decreases more than the increase of the bending moment in the other direction and the result you get can be surprising yet correct. This is the reason why in the steel design module you have the option to e.g. disregard bending in the other direction if it does not exceed the limit you defined.

**Artur Kosakowski**

# Re: steel connection

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Dear Sir

I have a similar problem when considering shear loads. I have no problem with shear in one direction, but when adding a negligible load in the other direction the connection fails with a very high %. What I dont understand is how Robot evaluates the bending moment resistance for a single bolt. ¿How is the maximum axial load calculated from connection bending moments and axial loads?

Thank you.

# Re: steel connection

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In general this is based on superposition of axial load and bending in one direction plus axial load and bending in the other direction minus influence of the fact that axial force has been considered twice. The details of the verification can be found in the calculation note.

If you have any reference of the way such situation should be handled post it please.

**Artur Kosakowski**

# Re: steel connection

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Dear Artur,

Thank you for your answer. Perhaps many people is trying to understand the EN 1993-1-8 standard by means of the programme's claculation note. Some rules of this standard are a bit complex.

I see that when performing connection capacity check, axial load is taken into account in both bending moment directions. So, is it neccessary to check the combination of both?, I mean that this check is not in EN 1993-1-8:

Med,y / Mrd,y + Med,z / Mrd,z =< 1.0

On the other hand, wahat i don't see very clear is the shear with level arm capacity calculation for a single bolt. I have attached a file illustrating my problem. I have an HEB300 base plate with axial tensile load and bending moment and shear in one axis. This connection is OK 91%. When adding a negligible bending moment in the orthogonal axis connection fails 445% because of shear.

Analysing shear resistance of an anchor bolt with level arm, in the correct connection, M Rk,s (characteristic bending resistance of an anchor) is 0.25 kNm. In the failing connection this resistance is 0.01 kNm. I think that this is impossible, taking into account that axial load ratio for the worst bolt has gone from 0.91 to 0.95 and:

M Rk,s = MºRk,s (1- N Sd / N R,d)

Thank you very much.

# Re: steel connection

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I'm sorry for not answering immediately but I needed some time to study this case. In my previous answer I wrote:

*In general this is based on superposition of axial load and bending in one direction plus axial load and bending in the other direction minus influence of the fact that axial force has been considered twice.*

The investigation suggests that the reduction of 'doubled' axial force is not done entirely corrects and its influence for the situation as yours is excessive. For the tension in the anchor that is close to its resistance for unidirectional bending this results in going over the tensile strength when additional bending in the other direction is added. This in turn causes the rapid increase of the verification ratio.

Thank you for your bringing this issue forward.

**Artur Kosakowski**

# Re: steel connection

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If have understood well, a negligible bending moment introduces "again" the same tensile load with a very low eccentricity. The EN standard does not give a procedure to calculate maximum tensile load in the most loaded bol. As it is an "out of standard" calculation procedure, in my opinion, ROBOT should be here more clear. For example, giving the axile load value on the most loaded bolt.

Will some day be issued a verification manual regarding steel structure connections? I hope so, because this is a quite useful and time saving feature of the program.

Many thanks.

# Re: steel connection

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If have understood well, a negligible bending moment introduces "again" the same tensile load with a very low eccentricity.

Yes, this is what happens.

For example, giving the axile load value on the most loaded bolt.

Added to the wish list.

**Artur Kosakowski**