ASCE 113 Deflection

[cajeng]

I was wondering how anyone familiar with substation steel design has been handling deflection due to rotation per ASCE 113. Specifically, I frequently have columns with rotation, causing deflection in horizontal members.

For instance, if I have a Class B structure, the vertical member (cantilever column) has a limit of l/100, while the horizontal (cantilever) has l/200. If the column has some rotation, even less than 1 degree, that may cause a total displacement of the horizontal member greater than l/200, even though the horizontal member locally has very little deflection.

113 does not specifically speak to rotation of members, so how are you typically handling this? Should we disregard the displacement due to rotation or consider total displacement regardless of cause?

Thanks in advance.

 

[transmissiontowers]

I don't have my copy here at home , but IIRC our intent ( I was on the original ASCE 113 committee) was to limit the deflection of the horizontal member to L/XXX with respect to its connection at the column. So I believe it was a local deflection. We are currently revising the document ( I Chair the revision committee) and the new figures may clear up the concept.

Now, you do realize the load case for the deflection is a reduced wind speed (5 year MRI) instead of the maximum wind on the structure. Off the top of my balding head, there was a 0.78 factor to convert the 50 year MRI to the 5 year MRI in one of the tables and I believe the intent was to apply the factor to the wind velocity and not the pressure so the effect was squared.

My personal opinion is the deflection limits are over rated. If you cable connect all the electrical stuff and allow the structure to move around (within reason) the whole reason for deflection limits goes away. The biggest problem is the "A" connection from the high bus to the low bus where it turns a corner. If you hit the bus with a big Short Circuit, the A frame will shake apart. We have eliminated the "A" connection for our bus runs on new stations.

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I have been called "A storehouse of worthless information" many times.

 

[cajeng]

Thanks transmissiontowers! It has been my interpretation as well this was a local deflection.

I am using the squared value in the deflection checks.

Do you know when you anticipate having the newest version of 113 out? How frequently do committee positions become available?

 

[transmissiontowers]

It's hard to predict when publication will be. We have gone through about 900 committee comments and the first round of revising for the committee comments is getting done. We still need to go through Peer Review and address those comments. We are trying to have a "Pre-Standard" appendix that will be a first look at what we think an ASCE Standard would look like for substation structure design.

The committee is not accepting new members since we are a few years into the revision, but after it is published and out in use for a few years, there will be another committee started to create a Standard if ASCE feels that there is interest in going beyond a Guide.

The original committee started in the late 1980's to publish the first edition because it was felt that there was no Guide available for the unique structures in a Substation and we wanted to pass along the knowledge that we had gained to the new Engineers coming out of school. It was a long process to get that first edition published and we hope that it was on target.

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I have been called "A storehouse of worthless information" many times.