What methods are people using to quantify rebar for WBLCAs?

Hello everyone.

We are finding ourselves doing more LCA’s on structural systems and more WBLCA’s for whole building systems and are looking to formalize our method for quantifying rebar. Since we do not typically model rebar or have access to the rebar shop drawings at the time of the LCA calculation, we need to come up with the rebar quantities in a different manner. Up to now, we’ve tried several different methods, including basic psf of steel/area sf metric, coming up with a spreadsheet to calculate the rebar in different structural elements, and doing hand takeoffs based on the drawings.

What all methods does everyone out there use to quantify rebar that is not modeled in the structural Revit model? Ideally, we’re looking for an efficient method that is also reasonably accurate. Counting every bar is not always reasonable, unfortunately.

Thanks in advance for any guidance provided!

1 Like

Here’s some data from OneClick:

The data seem comprehensive, but the website listed as a source lacks documentation of assumptions, modeling or data collection parameters, etc. (Reinforcement Estimates).

In some peer-reviewed wbLCAs, researchers assume factors around 0.1 kg reinforcing steel per kg concrete. They won’t say this directly, but the assumption becomes obvious when you back-calculate.

Perhaps a useful starting point?

2 Likes

We typically ask our structural engineer for the weight of rebar is per sf (slabs) or volume of concrete for each mix/use. They seem to have that information.

-Kjell

1 Like

Thanks, jackrusk! I forgot to mention, but had seen this before. Thanks for linking it and reminding me!

Definitely useful as a starting point, and your comment about the peer-reviewed wbLCAs is appreciate as well.

1 Like

Kjell_Anderson, that makes sense! I may simply need to reach out to the engineer doing the structural design for this information. I guess I was looking for maybe a more automated way to do this that wouldn’t involve sending another email, ha!

Your input is much appreciated. Thank you!

Laura,
Until WBLCA becomes more ingrained into what we do regulalry as designers, the email to the structural engineer is likely going to be necessary! A few thoughts about this:

  • The rebar quantities may or may not be something the structural engineer can easily provide. The reasons for this may be related to the individual engineer’s experience level, the stage of design, the company’s or individual’s workflow, or how typical vs unique the building is. An experienced engineer working on a project type they are familiar with might be able to rattle off reasonably accurate rebar estimates off the top of their head. It might take the same engineer several hours to calculate a similarly accurate (and still rough) estimate of rebar for a complex/unique project.

  • Since it’s likely going to take at least some effort by the engineer, it will be appreciated if you explain why you are asking and what level of precision you are interested in.

  • Likely some structural elements will be easier to calculate a typical value for than others. And some are more important than others in an WBLCA considering the volume. If you get a decent estimate for a typical slab in multistory builidng, it will likely be fine to take a wild guess for the one small retaining wall on site.

  • Be sure to clarify that the estimate should be inclusive of hooks, laps, etc (and ideally “carry bars” or support bars - these are usually outside of the engineer’s design scope, but they have embodied carbon just the same). Depending on the type of element, these extra allowances may add 20%-50% to the weight of rebar just calculated from the bar size and spacing. The structural engineer will be best positioned to choose an appropriate factor for each type of element (unless this is a retroactive WBLCA of a completed buildng, in which case, see if you can get weights from the contractor!)

  • Be clear about what units are needed. Total tons of rebar for the project or total tons by element type? Pounds per square foot (of a wall or slab)? Pounds per cubic yard? Rebar % by volume? The units you requested will depend on the LCA software you are using, and more sophisticated programs may have multiple input options. Share this with the engineer so they can select the easiest way to calculate what you need.

I know you were looking for a short way to the answer, but there usually is no automated way to a good answer (see note below)! Discussing these kinds of things up front with your engineer will lead to a more appropriate answer faster, with less frustration for all parties.

– Note: some software packages do have built-in values of rebar % for different types of elements. I’ll just speak about Tally since I’m most familiar with that personally. If you are using Tally and are trying to perform a very rough WBLCA with little or no input from the structural engineer, the built-in values (“low” column rebar, “medium” column rebar, “high” column rebar, etc for various types of elements) are certainly a reasonable starting point. The project structural engineer, with a bit of effort, could likely come up with more accurate rebar values to use, or at least help steer you toward low/med/high value. I find that typical, efficient concrete elements in low seismic regions usually land in the “low” or “medium” built-in Tally rebar values, but there are various reasons (e.g. higher seismicity, longer than usual spans or cantilevers, geometric constraints forcing suboptimal section sizes) that rebar may hit the “high” value or even higher.

—Final note: if your structural engineer is familiar with WBLCA, invite them to participate more directly if they choose! They may be able to make LCA material assignments direclty in their model that you link in (again, that’s a Tally example – other programs may have other good ways to collaborate). You may find that you both save time, and you may both feel more incentivized to make lower carbon design choices in your own scope of work when those decisions are going to be accurately and consistently reflected in the WBLCA!

4 Likes

This is a great topic, and I’m curious to hear others responses as well. @ewborchers response covers it very well based on what I’ve seen, so I’ll only add to/elaborate on a couple of the things mentioned.

  • Early on in the project, the structural engineers should be able to provide allowances by types (not Revit ‘Types’, but more broad groupings). For example: Slabs-On-Grade: ## PSF, Pile Caps: ## PCY, Retaining Walls: ## PCY, etc. These numbers can change quite a bit from project to project, and as mentioned, the units will vary based on what’s being quantified. These groupings can be assigned to the Revit elements as additional metadata to make it easier to extract the quantities in a way that is useful for applying rebar allowances.

  • Tools like Tally and EC3 do provide areas to input rebar allowances. This can be a bit tedious when you only know the rough allowances, but is certainly one route if you want something more accurate - shy of an LOD 400 model from Tekla or some other software that is more appropriate for modeling rebar.

To all:
KL&A, the structural engineering firm I worked for prior to my retirement, developed our own standards to supplement those used in Tally, which we did not always agree with. The advice to discuss this with your own structural engineer is spot on. Keep in mind that the accuracy of reinforcing steel amounts are not super critical to the accuracy of the final LCA, but the amounts should be within +/- 10% in my opinion. Our standards included the following categories: Stem Walls, Core Walls, Exterio Basement Walls, Grade Beams, Columns, Caissons + Caps, Footings, Mat Foundations, Slab-on-Grade, Mild Structural Slab, PT Structural Slab, Slab on Metal Deck, CMU Walls and Beams & Girders. We followed the Light, Medium, Heavy approach, and provided the type and amount of reinforcing for each category so the user could know what they are choosing.

Please ask your structural engineer to get involved in LCAs!

Bob Redwine

1 Like