Estimate Tributary Area for a beam from the shear at support.

[hotmailbox]

Context: I am developing (hired a CS student to be exact) Wood Design Addin for Etabs (curved or straight beams, posts, studs, braces, joists, or whatever being covered in NDS). Since all newly created woood members in Etabs can't be assigned to steel or concrete for design, the Auto Live load Reduction won't work for these members and you can't get member design forces (which includes Live Load Reduction). That means I have to do it and below is what I am trying to do to estimate tributary area for a beam. For columns or posts, it's easy and straight forward. I'd love to hear everyone's feedback.

Assumptions:
1. Beams shall be broken into pieces ends of which are either free end (truly free or supporting a secondary beam) or support (column or connected to a stiffer beam). Beams shall be modeled this way.
2. So each design member shall be considered as either end supported one or a cantilever one.
3. Live load to be 40 psf. It can be whatever but for the simplicity, assume it's 40psf.
4. Floor loads shall be assigned to one-way floor plate which distributes load to joists and beams based on joists spacing and point loads onto beams.

For beam pieces "supported" at each end: Total live load supported by the beam = Sum of Absolute shear value at each support due to live load. The tributary area = Total live load/40psf.
For cantilevered beam: Total live load supported by the beam = Maximum shear in the beam due to live load. Tributary area = total live load/40psf.

From ATT above, use KLL = 2 to calculate the live load reduction factor per Code Equation.

Below are my questions:
1. Is this too conservative for continuous multi-support beam because we may not see KLL*ATT ever greater than minimum? I have not figured out how to estimate the total live load based on the shear diagram of the beam? (Maybe = Absolute shear at each end + shear jump at each support but I have to think how to code to differentiate shear jump at supports and shear drop at concentrated loads).
2. How do Etabs or other software estimate live load reduction? I would assume it uses the same approach to estimate the tributary area for each member.

 

[KootK]

1) A wood module for ETABS would be great.

2) I'd be tempted to leave LLr as a user input thing until you've got the market share to justify the effort of sorting it out programmatically. It can be pretty tough.

3) For a cantilevered beam, I'd be working with the sum of three shears: the pinned end, the backspan side of the cant support, and the cantilever side of the cant support. I don't believe that it would be just the maximum beam shear/load. Similar strategy for continuous beams although many engineers will ignore continuity for LLr.

4) You'll need to track reducible live load separate from unreducible.

5) I find it much simpler to drop the KLL factor and just work directly with the influence area concept.

6) KLL=2 probably will work for many trabeate framing situations, particularly when all of the load originates from the floor in which the member resides. That said, KLL = 2.0 won't be appropriate for all situations, particularly when some of the load comes in from floors above.

7) Even by hand, not all engineers agree about how LLr should be handled for cantilevers and continuous beams.

8) Investigate how RISA floor handles this. I believe that it's based on a separate FEM analysis of sorts whose job it is to track the loaded area increments through the structure and add them up. That should give you an indication of how complicated this can get.

 

[JoshPlumSE]

I too would love to see someone add a wood module into ETABS. I've used it for minor wood projects here and there. But, get a little frustrated at having to check the members by hand (or using EnerCalc or such).

I remember this as being a lot more complicated than it first sounded.
1) The area that comes directly to the member from the deck. And, how that gets attributed to the supports.
a) Relatively easy for interior members / support.
b) Much trickier for cantilevers because if you just look at the reactions, you will end up reducing the trib area at the far support because of the uplift caused by the moment in the cantilever. You should be able to correct for it. But, it's tricky and whatever scheme you come up with should be tested thoroughly.

2) The tributary area that comes from other members into the member. Whether that load came from the same floor or from a different floor level.

3) I can't remember what the process should be when you have multiple types of live load areas. Roof Live, Floor Live, Unreducible versions of each and a "special" live load related to different treatment by ASCE for LL greater than 100psf (i.e. Heavy Loads) or for live load ini Passenger Vehicle Garages. Specifically, what tributary area do you use.... the sum of all of those tributary areas or the sum of only the reducible areas or such. But, that was something that I remembered putting a lot of thought into a while back. RISAFloor (at least the version from 2017 or so) should reflect the conclusions I came to.

4) It can be problematic to use LLReduction with load combinations that include lateral loads.... But, only when you cannot use superposition. So, for structures that have a lot of non-linearity. RISAFloor does (I believe) does not do this. But, I proposed a solution at one point where you track the reduction force at every spot in a member. That way, you can manipulate the design forces in the member based on your tracked forces / moments, your "reduction force/moment" and the factor on the LL in your load combination. But, you always apply the WHOLE load in your analysis.

Caveat:
I worked for RISA for 16 years ending in November of 2017 (so, don't blame me for any recent changes / decisions) and was heavily involved in the RISAFloor treatment of LL Reduction.

 

[hotmailbox]

Thank you for your valued input. I have completed wood stud, beam, post design modules and been thinking how to implement auto live load reduction for beams. It's not an easy task to get an elegant solution.

A wood module for ETABS would be great.

I too would love to see someone add a wood module into ETABS.

At least I feel I am not doing something crazy here. It does help. Architects keep doing crazy things with open space and keep changing floor plans every meeting and need answers instantly. The hardest part for this is to how to systematically consolidate the entire NDS Supplement Design Values and Adjustment Factors into a database that can be used for value lookup. They are all over the place with different cases and conditions. And add those values into an Etabs template file. This will take a while.

I'd be tempted to leave LLr as a user input thing until you've got the market share to justify the effort of sorting it out programmatically. It can be pretty tough.

That's the idea. The program should just suggest the reduction factor and that can be overwritten by user based on further check.

You'll need to track reducible live load separate from unreducible.

To track the tributary areas for each type of loading (different surface load values, reducible or non-reducible), I create different live load patterns/cases for this purpose and for skip loading.

3) For a cantilevered beam, I'd be working with the sum of three shears: the pinned end, the backspan side of the cant support, and the cantilever side of the cant support. I don't believe that it would be just the maximum beam shear/load. Similar strategy for continuous beams although many engineers will ignore continuity for LLr.

For now I leave the multi-span beams alone and estimate reduction factor for loads on single span (conservative).

I find it much simpler to drop the KLL factor and just work directly with the influence area concept.

This works nicely with straight forward framing plan. However when you have transfer or discontinuity in gravity load path in multi-story structure, it's not easy.

6) KLL=2 probably will work for many trabeate framing situations, particularly when all of the load originates from the floor in which the member resides. That said, KLL = 2.0 won't be appropriate for all situations, particularly when some of the load comes in from floors above.

I am still thinking loads from above don't make any different. Say 25k live load (from 40psf floor live load) from column supporting floor above would be equivalent to 625sf of tributary area going to the beam supporting the column. KLL for the column can be 4 but we use only 2 for the beam.

Relatively easy for interior members / support.

Not sure if you talked about the interior columns but for all columns/posts I use the same approach. Tributary area = Total axial live load (from same type, same psf)/surface live in psf for that type even for columns/posts in multi-story structures. I wish CSI will provide API to get deflection values along the beams for different load cases. Now I still have to check manually each beam for deflection criteria by right-click on member's moment diagram. Why doesn't CSI create a wood design module? I don't see it too complicated and it does have market.

 

[Celt83]

I've only got some back of napkin stuff for automated live load reduction.

From above sounds like pulling loading diagrams for the members is not currently possible.

Can you get the shear and moments at stations along the beam?

For Simple Beams: (really for simple beams and a consistent uniform reducible live load)
- Check for end moments - This means there is an attached cantilever
- if end moments apply counter moments to the end of the beam to negate them
- Look for shear discontinuities - This means there is a point load (assume this is a beam or column transfer)
- If shear discontinuities apply a counter point load equal to the delta at the discontinuity to negate the applied concentrated load
- the resulting shear diagram should now be continuous, compute the derivative of the shear diagram to get the a sudo line load
- from the sudo line load divide by your uniform Live Load pressure to get trib area at several stations on the beam
- Sum the various trib areas to get total trib area on the beam and store it for tracking purposes
- add to the above area an areas that track into the beam via another beam or column.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[hotmailbox]

For simple beam, I just need to get the sum of the absolute of the shear due to live load at each end. That sum is the total live load supported by the beam. Tributary area = That total load/live load pressure.

 

[JoshPlumSE]

I wish CSI will provide API to get deflection values along the beams for different load cases. Now I still have to check manually each beam for deflection criteria by right-click on member's moment diagram. Why doesn't CSI create a wood design module?

I'll look into both of those items. My initial thoughts are:
1) I have to believe there is a way to do this with the API, but I'm not very good with the API at all. So, I'll take this as an opportunity to learn something about the programs that I really should know better.

2) I've wondered the same thing. If we'd had any "social get togethers" this past year, I probably would have asked some of my colleagues. I'm no longer as "new" and green as I was 2 years ago when I started. However, the company has so many programs with so many capabilities that I've focused so much of my time in learning and understanding the complexities of what they do that I haven't focused on the things that RISA did that CSI doesn't.

My guess though is that it's a conscious decision. ETABSS doesn't do either masonry or wood, right? It seems to me that the company is focusing on the "higher end" projects in the structural engineering world. High rise concrete and steel buildings, advanced analysis capabilities and such. When you do that, you attract a certain kind of customer. And, you also end up discouraging another type of customer. As such, my guess is that it has become something of a cycle where the high end customers request certain things and the they don't see as much demand for the "smaller" wood and masonry projects.

For what it's worth, I'll see if I can encourage some basic additions of wood members and masonry walls. I can't promise anything as I'm still much lower on the totem pole than I was at RISA. But, I can at least promise that I'll look into it!

 

[hotmailbox]

I can at least promise that I'll look into it

Thank you for your kind support. I have read a lot of your old posts technical supporting RISA. I was in awe by how much effort you put in there.

I see a lot of opportunities with APIs and I think we'll see a lot Pluggin or Module vendors for custom solutions like this in the future. CSI needs to add more API functions to make it happen faster.

 

[Trenno]

IMO, the expanded use of API's really just takes the onus away from the big software vendors to continually improve their product for the end user and push the boundaries. Companies are essentially writing their own software, piggy-backing off API enabled software such as ETABS, with ETABS being the engine. Soon enough these companies may even write their own engines (eg GSA) and won't have any need for these expensive software packages.

 

[thejonster]

I'm definitely interested in this addin/plugin. I'm shopping for a software to use for a new engineering firm I'm starting and am excited about etabs and how they incorporate connection design and the detailing output among other things. Wood seems to be a gaping hole in their offerings considering the growing popularity of mass timber.

What is the latest in progress of development for you? When/where would I be able to get this? I hope you get many returns of your investment on this.

 

[hotmailbox]

It's been completed but has not passed my thorough testing. I'll comeback with some demos. I am not sure how I am gonna sell this. The effort to create a lock for the software or subscription scheme is too expensive for such small development.

 

[thejonster]

Just a thought, perhaps someone who has been down this road before would be willing to lend a hand, or partner with to get this out. CSI would even be wise to buy it from you directly

Tilman is listed on the CSI webpage for plugins with a couple of their products at the bottom, and their products seem to be on the order of complexity & capability that you are looking to offer. They specialize in plug-ins of this type, and they (or someone similar) would be in a great position to help you get it to market.

https://www.csiamerica.com/developer/third-party-applications

In particular, their seismic irregularities tool looks like a real time saver for a reasonable price (~$500)

https://tilmanllc.com/1_5_0_11_Seismic-Irregularities-Tool-v3-1.html

Perhaps if you reach out they would be able to help or have some ideas for you, I would think it benefits them to make ETABS attractive to more engineers that buy plug-ins.

https://tilmanllc.com/contact

Personally, I would value it if CSI offered plug-ins for sale directly on their website a-la dlubal. This would allow people to buy specific enhancements without raising the cost of the software for others. Here's their AWC/NDS timber module (~$1,100):

https://www.dlubal.com/en-US/products/rfem-and-rstab-add-on-modules/timber-structures/rf-timber-awc