Issue #5

1. Code & Standards Watch

Stay informed of newly released codes and standards; keep current on updates and errata; and let your voice be heard through working sessions, public comments, and balloting on standards under development.

2. Research Snapshot

Design and performance of precast, prestressed concrete beams made with fly-ash-based alkali-activated concrete
You’ve probably heard about how much carbon is produced in processing ordinary Portland cement for the sorts of concrete we use today, and likely some about using pozzolans like fly ash as admixtures to help reduce the required Portland cement for structural members.  There’s also been research underway to pursue different cements with no Portland cement at all.  One major contender in this field is “alkali-activated” concrete (AAC), which uses strongly basic activators coupled with some of those same pozzolans to produce a different type of concrete entirely, but one that has demonstrated similar strength to Portland Cement Concrete (PCC).

Until this particular paper, most studies have focused on the performance of AAC as it’s produced under lab conditions, but these researchers pivoted the focus to establish mixes that actually work at precast plants.  The first tinkered in the lab and discussed with a local precaster to come up with a safe (strong industrial basic solutions to contend with!) & implementable method, then cast several prestressed beams and tested them for shear and flexural strength.  While some glitches did arise, functional beams were produced that demonstrated a pretty close fit to ACI 318 equations for flexural strength, and slightly lower-than-predicted shear strength and stiffness.

Key Takeaways: Alkali-activated Concrete is starting to make the transition from a green construction lab curiosity to actual commercialization.  This study produced functional beams, though it also highlighted the need for more work on process controls, cure retarders, and possible honing of some of the ACI 318 equations.  While this doesn’t clear AAC for takeoff, it makes meaningful strides in the right direction!

3. Tools & Workflow

CSiBridge 27.1.0 Released

4. Case Study of the Week

Collapse of 2000 Commonwealth Avenue: Punching Shear Case Study
In January 1971, a Boston apartment building nearing completion suffered a progressive collapse that took out ⅔ of the building and killed four workers, injuring another 30.  The incident initiated when column E5 suffered a punching shear failure at the roof, where construction loads were greatly exceeded, and shoring plans were not followed appropriately.  Approximately ⅓ of the roof slab collapsed to the floor below, and then the action paused for about 10-20 minutes, allowing most workers to evacuate.  Then the top floor gave way, and a progressive collapse of one floor crushing the next took out about ⅔ of the building.  

Initial reports suggested possible design failures were to blame, or that building code requirements were not robust enough.  After a detailed study by a mayoral commission, no shortage of contributing factors was found: low-strength concrete from poor mix design and lack of protection from the cold (Boston in January, much of this concrete was trying to “cure” in 25° F), improper detailing, many changes of ownership, poor construction supervision, lack of inspections, and no properly-licensed builder in charge, to name a few.

Key takeaways: Punching shear is a major and disastrous failure mechanism for flat-plate concrete structures, and must be carefully considered, as it can often lead to progressive collapse if one occurrence presents.  Proper code enforcement and supervision/inspection by responsible individuals is critical to ensure designs make it from paper to the field.  And be wary of any project that’s changing owners / responsible professionals too often, those are big red flags that should put any professional on their guard!

5. Upcoming Free Live PDH

6. Quick Hits

👋 From the Editor

I’m Eric, the engineer behind StructEd Brief. I dig through stacks of journal articles and software patch notes to find useful information for practicing engineers and keep an eye on the scattered code updates & errata for you. I’m just getting started, so if you find this useful, the best way to support the newsletter is to share it with a colleague or post it on LinkedIn. It helps more than you’d think!

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