Web openings in steel W-shapes 1

[Santino314]

I apologize for the long winded explanation but it's a problem that's been nagging me for some time.

I'm in the process of analyzing over 300 beam penetrations for a composite steel building. I've modeled the structure using RAM Steel for this purpose. I have also developed a spreadsheet to calculate the V-M interaction based on AISC Design Guide Chapter 2. Doing so has raised a number of questions.

1. For a number of my openings, RAM Steel gives an error, saying that the openings are too close to a support or point load. I understand that further analysis is necessary for point loads over an opening. However, the point loads generating this error are due to beams framing into girders. In other words, the point loads are located approximately at the girders' centerlines. Should this be a cause for concern? Additionally, once this check fails in RAM, it neglects to calculate the V-M interaction. Therefore, even if I were to ignore this error, I still lack a pass/fail from a strength/deflection perspective.

2. Concerning deflection, for those beams where no error messages are returned, RAM provides a report that includes "deflection multipliers for effects of web openings". I have yet to see a multiplier greater than 1.0 and am worried that deflections are not being accurately reflected. Do I have a cause for concern?

3. This isn't so much a question, more of an observation. It appears that RAM uses a phi of 0.90 for both composite and non-composite V-M interaction checks when AISC Chapter 2 says to use 0.85 for composite.

In short, I'm looking for a magic bullet that automates the process like RAM but without these issues. I have the spreadsheet I made to check openings but I lack the time or desire to churn through 300+ openings. Even if I wanted to, I can't find a way to obtain pre-composite shears and moments at a given location from RAM to plug into my spreadsheet.

Any help or guidance would be appreciated. Thanks.

 

[MiketheEngineer]

How big and how many "openings" are we talking about?? What size beams??

 

[Santino314]

There are over 300 openings ranging from 8" diameter circles to 12x24" rectangles. Beam sizes are typically W24s or W30s in the 55 to 108 pound range though there are a couple W30x235s in the mix, along with other larger sizes.

 

[SethGuthrie]

One thing to note is that the Design Guide 2 has different Po limits for bare beams and composite beams. Most composite construction is unshored however, so the more restrictive Po limits of bare beams are imposed during the pre-composite design check of composite beams in RAM Steel. One way to get around this, or force the program to accept the less restrictive limits, is to make the decking shored.

Regarding the deflection amplification, here is an excerpt from the RAM Steel beam manual that clarifies:

“The effects of web openings on the beam deflection may be significant. However, the approximate procedures given in the design guide are not general enough to universally apply to the opening configurations possible in the program, so no procedures were implemented. Thus the deflections reported by the program do not consider the additional effects of the openings. The engineer may consider applying a more stringent deflection criteria to these beams to account for the fact that the beams will deflect more than reported.”


Regarding the phi factor, here is an excerpt from the manual to clarify:

“Although Design Guide #2 was written prior to the publication of AISC 360, the program designs web openings in conformance with AISC 360 when that is the selected code. Some modifications were made to the implementation of the requirements in Design Guide #2 to align those requirements with those of AISC 360. For example:

• For AISC 360 LRFD the φ factor used for composite design is 0.9 rather than 0.85.”

 

[Santino314]

Seth, thanks for your clarification on the deflection and phi factors. That's unfortunate to hear about the deflection. What is common practice to calculate these deflections? According to AISC Design Guide Chapter 2, the only available method involves matrix manipulation, which I'd rather not do 300 times. The last section discusses a ratio of the midspan deflections for beams with and without openings (Donahey 1987). This might be a more easily adopted process, albeit with slightly less confidence. Additionally, for cantilevers, can tip deflection be substituted into this ratio instead of midspan deflection?

Can you expand upon the Po limits? I don't see any reference in the design guide about Po. Are you talking about the compression limits in the tees above openings? Why is it acceptable to tell RAM the decking is shored when in fact it is not?

 

[SethGuthrie]

The Po term I was speaking of was the "opening parameter" as defined in Design Guide 2, section 3.7.a.3, equation 3-24. Looking at the guide now, I see that this affects the size of the opening, not the location relative to a point load, so you can disregard those comments for your issue (but maybe someone else will benefit from that tip in the future).

Regarding deflections, you're out of my league. My inclination would be to put a more restrictive deflection criteria on the member with web openings, maybe something based on the ratio of the effective moment of inertia relative to the original section Ixx. That would be conservative at least.