Partially Composite Beam Questions 2

[starcasm]

Hello everyone. I was wondering if I can get some guidance on a composite beam analysis task that I have. I have an existing steel beam (c. 1924 construction) that had shear connectors and a 5” concrete slab added to it during a renovation in 1981. I followed some design examples and referenced AISC and easily found that the beam is only partially composite. I will give more parameters if you need them. But I am really looking for some basic guidance.

One of my goals is to find the partially composite beam moment capacity. Problem is that the examples that I have seen simply tell the reader to go into the AISC tables. Those tables assume 36ksi or 50ksi steel which I don’t have. Since I have not done this in awhile, I am getting hung up on the basics.

1.) How do I calculate the moment capacity without the AISC tables? I know I need to calculate the section properties but which one?
2.) Is it based on the transformed section properties?
3.) Don’t I need to locate the PNA (plastic section modulus)?

Assuming I need to locate the PNA to calculate the moment capacity, I haven’t been able to find a good example to calculate the PNA.

4.) How do I calculate the location of the PNA?
5.) Is it as easy as working with the transformed section properties?

Any help or guidance would be much appreciated and I will try answering any other questions you have.

 

[dik]

The CISC in one of their earlier Steel Handbooks had the methodology and well explained... I'll see if I can dig up a copy tomorrow... They had a min 25% composite for deflection and 50% for strength, back then...

Dik

 

[dik]

Catch the following:

 

[dik]

and...

 

[starcasm]

Thank you, dik! I have some CISC references at work and didn't think to check them. Your references do shed more light on the partially composite beam analysis than I had before.

 

[WillisV]

Since you mentioned AISC: They (AISC) updated the design examples for the 14th Ed. Manual (

http://www.aisc.org/designexamples)

to include a "by hand" example of a partially composite girder as illustrated in Example I.2 that will walk you step by step through the process and appears to address all the questions you have asked.

To briefly address a few of your points, the PNA is the line at which the compressive/tensile capacities (sum of forces) above and below the line are equal, and the moment capacity is the sum of each of these forces times their moment arms about a datum (does not matter what datum, though the PNA or base of the beam are convenient locations to choose). The only difference between a fully composite and partially composite beam is that in a partially composite beam the compressive force in the concrete is limited to the sum of shear capacities of the shear studs between the end of the beam and the point of max moment instead of the maximum concrete crushing strength or overall steel tensile strength. Again this is addressed in the aforementioned design example.

 

[dcarr82775]

Just about any steel textbook worth a damn will go through how to do it.

PNA is just the centroid of the Tension and Compression forces at ultimate conditions. Since it is only partially composite, the PNA will be down in the beam itself. The studs will give you the concrete compression force, then using the concrete force, you can easily calculate how much of the steel you need to have the compression force equal the tension force. That gives you the location of the PNA. I find it easiest to break the beam up into flange and web segments.

 

[starcasm]

Thank you WillisV and dcarr!

@ WillisV - the AISC Examples v14 is exactly what I was looking for. It makes much more sense now.