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Girder Loading Question

Girder Loading Question

Girder Loading Question

(OP)
I have a question regarding girder loading.  At the risk of sounding stupid I'll ask it, because I'm starting to second-guess my own thinking.

If you are manually loading a girder, say if you make changes to a truss the girder is supporting, and you have a reaction for the supported truss, you want to put the load of that reaction on the girder, and you want to split it into LL and DL components within the software.

The software I'm using sometimes gives results for two L+D reactions.  On a few trusses, I've noticed the reaction from L2 + D2 is larger than the reaction from L1 + D1.  So here is my stupid question, which should be obvious to me, but I'm beginning to doubt my thinking (not good...)

You want to load the girder with the components from the larger of the two reactions (1 vs. 2) right?  I'm actually working on truss-to-truss connections right now, another person here does the individual designs, and I'm seeing that some of the girders are loaded with the lesser reactions, not the components (L & D) from the larger reaction, which is the reaction given on the truss seal of the supported truss.

The loads on the girder should match the reactions given on the seal of supported trusses, correct?

Before I ask him whether he manually loaded these and just put the wrong loads on, or whether our software is doing something funky, I want to make sure my reasoning is correct.  I'm noticing what I think are incorrect loadings on several trusses on this building and believe he may have goofed (he tries to work really fast and doesn't want to give me any of his work...another issue...) but I want to be careful because we had a long meeting yesterday and when I brought up the need for a checking process (this office doesn't have one...everyone does their thing and nothing is checked before it's sent out) they all looked at me and said, basically, that they don't make mistakes.

Yeah, that's a thread all on it's own, I know...   

RE: Girder Loading Question

Yes, the girder should carry the maximum reaction from the truss it supports.  The loads on the girder should reflect the reported reaction on the supported truss shop drawings.  

Most truss designers are not engineers.  I would point this loading issue out to the designer without being condescending. Just because they THINK they don't make mistakes doesn't mean they don't.

RE: Girder Loading Question

(OP)
I know...everyone makes mistakes, we're human.  I've always been grateful when someone points out something to me that I don't have right...especially before it's built and hanging over someone's head!

If you had been in the meeting yesterday, though, where I mentioned the "what exactly is the checking process here?" question, and had about 10 pairs of eyes glaring at me, you might be hesitant to ask this guy any questions, either.

When asked how my truss-to-truss connection creation was coming along (the company has been around for years but they have no standard details and they have no "library" of what they've done in the past, so they keep paying for the same thing over and over) I mentioned that by increasing the gauge of the connecting webs we'd get higher capacities from the screws, this designer answered me with, "increased gauge material costs more money."  Yes, but so does having to put 20 screws into each connection plate versus 10...  I am hitting a brick wall here.

I asked how they handled high-load connections before I started and the answer was, oh, we never had any...  ???

Thank you for confirming my reasoning.  You know how you sometimes look at a word and it is spelled incorrectly but you can't quite remember the correct spelling of it?  That's how I feel here.  I know in my head what is correct but I'm seeing someone else's work on paper and I know it isn't correct but I start doubting my own training when I keep seeing it over and over.

I've worked on about 10 truss-to-truss connections so far this morning and so far I've found mixed up loadings on about half of the girders.  In one case, two 3k loads are missing from a girder and now I just found another girder 2" too long.  This is going to be a long day.  



RE: Girder Loading Question

If your company hired you to do engineering and they never had anyone on staff to do it before you got there, of course there will be ruffled feathers.  It's a 2-way street: I'm sure there is something you can learn from the designers, and they can probably learn a lot from you - if they want.  

RE: Girder Loading Question

I am typically the EOR on jobs where truss guys have provided ther design to me.  I always check the girder trusses because that is usually where the mistakes are made.  Some programs have trouble with the auto loading, and other times the technician puts in the individual truss loads as plf, thereby doubling the loads.  Girders are the biggest source of errors I have seen.

RE: Girder Loading Question

I run a three man shop - not truss engineering - but I was once in that field.....we do scaffolding - not much different..except scaffolding comes down after a few months!!

I DEMAND that my junior engineers check my work and I obviously check theirs --- like you said - we ALL make mistakes.

Set up a series of checks and balances.   ALL good companies have quality controls.. and this is what you need.  It does not need to be fancy - but MUST be written down and enforced.  It will not affect production - at first it will slow things a bit - but will save you big time in the long run.  You might not realize it - but catching one simple mistake - might keep you out of a million dollar lawsuit!!!

Get everyone involved in the "process" and few feathers will be ruffled.  When you as the boss - ask for help -- people will respect you and understand the "pressures" to get it right!!

Good Luck!!

RE: Girder Loading Question

(OP)
I'm not the boss - If I were, it would be easier to start a checking process here.  

I am ok with "ruffling feathers" if the need arises.  I worked at a truss company about 7 years ago and because it was a small company, I did everything from estimate to layout to running a screw gun if the shop needed help.  Unfortunately, after the meeting yesterday, no one at this new company - including the boss who owns the company - sees a need for anyone to check each other's work.  At the other company, 3 of us did the same type of work and we'd check each other's work.  We had checklists for everything and I wish I'd saved copies of them.

At the meeting, one of the estimators here summed it up by saying "if everyone became intimately involved with each project by checking everything we'd never get any jobs done."  That's a direct quote - I wrote it down.  

The first project I worked on here involved drag truss loads that the designer neglected to add to the truss while designing.  The fix was awful and it has the potential for litigation for this company.  The drag truss loads were actually printed right on the truss layout in the structurals, the designer just didn't know what they were.

He has worked here 10 years - I'm going on 2 months.  Everything is blamed on a detailer who does contract work for us, whether he made the mistake or not.  The fact that no one checks his work, either, was lost on them all yesterday.  When I agreed with an EOR to redesign some trusses that didn't leave room for HVAC, the owner's nephew told me to "quit badmouthing the company".

Mike, are you hiring at all?  I'm good with power tools...

RE: Girder Loading Question

I can find a place for ya, if you want to come to Maryland.....sounds like you need to find a new job....yikes.

RE: Girder Loading Question

I agree w/CTCRAY --

A new job might be in order....  That comapny with their "pass the buck" attitude is a good candidate for a large lawsuit and bankruptcy.

Sorry - no openings here -- right now.

At least your head is in the right place  - but it might not fit in where you are at!!!

RE: Girder Loading Question

Depending on the software you are using, if the girder is loaded from layout, the program (in my case MiTek) will do the following.  If the common trusses framing into the girder truss are spaced say 2'-0" o.c., and the girders standard load spacing is 2'-0", MiTek will take the maximim gravity reaction of the common truss and subract out the overlapping tributary area load.  (2' X DL of the bottom chord).  I believe they could subtract the spacing X TL.  In general the amount of difference is 20#.  Automatic loading is an issue for some reason if you have open jack trusses.

Where you have to really watch it is when the layouts change and the girders are not reloaded....it happens quite often.  I find them regularly.

Ctcray, I am also in MD

woodengineer

RE: Girder Loading Question

woodengineer,
We use Robbins, and it is similar.  It doesn't subtract the top chordloading since it assumes the tributary area for the top chord is not affected.  That is, there is a valley truss adjacent to the girder.      

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