Frustrating, persistent misconceptions that pervade your office 1

[bugbus]

I hope this thread would be a bit of fun, and not intended to be too much of a slag-off. I don't think anyone is above making stupid mistakes from time to time, but in any office there always seems to be a whole lot of frustrating misconceptions that everyone just seems to go along with. Things that make you facepalm or die a little inside when you hear it. 'Rules of thumb' that make absolutely no sense. Conservatism in design taken to the extreme. Just general lack of understanding of basic concepts. I could go on...

For me, it is usually something along the lines of: "This is just the way we've always done it, so why change it?"

[li]General ignorance around the relationship between a structure's stiffness and its seismic response, often leading to a lot of tail-chasing and needless over-design. For example, specifying pot or spherical bridge bearings, which lock the bridge into the abutments and cause the structure to be very stiff in a lateral direction. The bearings and substructure then need to be designed for a very large shear force. Simply substituting these with elastomeric bearings would solve the problem in many cases (due to the base isolation effect), but there is this persistent idea floating around my office that the elastomeric bearings would have to be designed for the original shear force (calculated based on the superstructure being tied to the substructure) and therefore wouldn't work.[/li]

[li]Halving the length of a lap splice by adding hooks to both bars. Somehow this has been confused with the rule for development length, where the hook accounts for 50% of the development length. But it now gets routinely applied to lap splices, despite this not being allowed in our local code (or any code I am aware of).[/li]

[li]Needless caution around the use of stainless steel fasteners connected to non-stainless steel. I understand there is a risk of bimetallic corrosion when you connect a relatively large stainless steel item to some non-stainless fasteners. But when it is the other way around, I believe this is very safe to do. This seems to have developed into a rule of thumb whereby any dissimilar metals must not, under any circumstances, come into contact with one another. I've seen this leading to frivolous and expensive over-use of stainless steel and never seems to be questioned.[/li]

Would love to hear some of yours.

 

[3DDave]

That first one almost hurts. I watched a stress guy suggesting that the muffler heat shield welded brackets needed to be stronger so they would not fail in fatigue from thermal cycling. I asked how he thought that was going to help and he suggested that the heat shield would be the same temp as the muffler, which didn't explain where the loads came from. He missed the reason for having a heat shield is to not be the same temp. After the little light came on, the welds were replaced with slots in the heat shield and springs on the new retaining bolts to keep it from rattling. Eliminate the rigid connection and the stress goes away.

I also feel your pain on the stainless thing. My car had stainless exhaust but the maker used carbon steel hardware. Every couple of years I'd get under the car and crumble the remains of the hardware off with my fingers and install replacement clamps. I finally went to stainless clamps, but by then the rest of the car had developed terminal rust in places that could not be reached. If it's not on thin-wall carbon steel tubing I would expect stainless to make a joint that survives a long time.

 

[WARose]

The one that has been driving me nuts for years now is the notion that good structural designers aren't needed. (I.e. lets just run with the red to black drafters.) You can't put a price tag on those guys.....they save you money at the end of the day.

 

[1503-44]

How about not realising that increasing the clips cross section increases the thermal restraint force, even though thermal stress remains the same. Strengthen the strap only to shear the bolt.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Tomfh]

That contractors are idiots and that the engineer always knows best.

 

[phamENG]

Come on, Tom - everyone knows it's true!

 

[dvd]

On the topic of galvanic corrosion mentioned above, attached is a nice reference - which refers to small CAR or small cathode-to-anode ratio as a condition to avoid galvanic corrosion.

 

[Just Some Nerd]

Really does feel like 80% of the time it is true....that being said, nothing worse than fixing the mistakes of a bad engineer. Sometimes gotta look at the silver lining that they're no longer at the company when that happens


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Why yes, I do in fact have no idea what I'm talking about

 

[ElliottJames]

"Tack welding of rebar shall always be prohibited." -- Never mind that the rebar details in the footing are driven by temp and shrinkage, not ultimate or service loads, and whatever small strength that is lost at the rebar connections are insignificant and immaterial.