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Gerber Beam
6

Gerber Beam

Gerber Beam

(OP)
I am a little confused about the different load combinations to check for a gerber system.  Is the load combination LL+DL whole length, DL whole length +1/2LL on cantilever and DL whole length +1/2LL middle?

Why 1/2LL?  Obviously LL+DL whole length would govern.

My other question is how to choose the length of the cantilevered portion?

RE: Gerber Beam

What's a Gerber Beam?

RE: Gerber Beam

(OP)
Are you an engineer?  How can you not know gerber beam?
 

RE: Gerber Beam

I'm an engineer and I don't know either.  

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

RE: Gerber Beam

The way I understand is gerber beam is a drop-in beam in a multi-span beam system. The drop-in beam is used to bridge the gap between 2 cantilevered beams.

RE: Gerber Beam

(OP)
NBCC says something about 50% Live load to be applied.  Do you know which section?

Thanks.

RE: Gerber Beam

DL+0.5*LL sounds like a service load case. The reason the live load is placed over the span and on the cantilever serperatly is because this will give greater deflections.  Placing a load on the cantilever will deflect more without the live load on the span to act as a counterbalance and vise versa for load on the span.

RE: Gerber Beam

(OP)
Which section in NBCC states this?  Do you recall?

RE: Gerber Beam

"How can you not know gerber beam?"

Presumably the same way that I don't know.

It's been said before, but this forum is on the WWW, not the USWW.

RE: Gerber Beam

I'm in the U.S. and I didn't know what a Gerber beam is.

RE: Gerber Beam

In 50+ years in the UK and USA, I have never heard of a Gerber Beam. Supports a baby food factory perhaps?

Going by slickdeals, I have designed many cantilever/span systems.

I always designed for the worst checkerboard loading for my own comfort; which, meant three cases for the cantilever beam, LL on the drop in span only, LL on the drop in and the cantilevers that support it (bottom flange compression case), and full LL over the length.

There were no special code requirements when I designed those, but I would not have done less, even if permitted by code.

Confirmed, it is cantilever/span.

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Gerber Beam

Anne...please note that terminology in engineering is not necessarily universal.  You have fallen into the trap that most of us Yanks are guilty of...thinking that if it applies to us (USA), it applies to everyone!! Not necessarily so!

While the term might be common in Canada, it is not so in the US and perhaps other areas, as noted by the responses. I have seen several different terms over the years to describe the beam you are dealing with...infill beam, connector beam, slot beam...

Unless your code dictates otherwise, this beam section should be subjected to the same loads as the other sections...DL+LL

The length of the cantilevered section is chosen at the point of moment inflection and where shear is moderate, but not maximum (commonly less than or up to 1/4 of span)

RE: Gerber Beam

Yes. I'm an engineer.  It was just a question.  
Thanks slickdeals, now I know what you are talking about.   

RE: Gerber Beam

Ron, I was taught to optimize. Look for a beam that would work as a continuous beam. Usually, the shape is a little stronger than required, so you increase the length of the cantilevers till it's strength is fully used, then you design the infill beam with a smaller shape. After all, the inflection points are where you put the hinges.

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Gerber Beam

Michael..I agree, but often the deflections get a little high when you stretch the cantilever.  To keep the brain damage down, the inflections work (as with a hinge).

RE: Gerber Beam

(OP)
rowingengineer's motto:  "Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it."

He is calling all engineers pigs. muhahaha, good job Rowing, keep on rowing.
 

RE: Gerber Beam

(OP)
rowingengineer's motto:  "Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it."He is calling all engineers pigs. muhahaha, good job Rowing, keep on rowing.  Actually why pig, why not monkey or gorilla or hyena?  Anyways keep up the great attitude.

RE: Gerber Beam

Add me to the list of people who did not know what a gerber beam was.

 

RE: Gerber Beam

I didnt know what a gerber beam was. All they taught us in College was sky hooks and structural paint.  

RE: Gerber Beam

3
As I recall, the Gerber beam was named after the guy who first proposed it.  I think he was Prof. Gerber, but can't remember for sure.  Could try Googling it.

Article 4.1.6.3 of the NBC is attached.  Usually Gerber beams are used for flat roofs of large, box type buildings.  

A few years ago, the NBC required 100% of snow load to be considered on any area and 0% on the remainder.  This was deemed to be unrealistic for snow load on a flat roof, so about twenty years ago, it was changed to 50% on the remainder.

If 100% of the snow load is placed on cantilevers and drop-in spans and 0% on the continuous spans, there would be negative moment across the entire span.  This was deemed to be an improbable condition on a large, flat roof so the code was changed to what you see attached.

BA

RE: Gerber Beam

As some have discussed already, the 'gerber' type system
is a just a term used for framing systems, usually roofs of industrial buildings - in my experience - and is about optimizing steel (ie. using cantilevers and 'drop-in' beams). Typically used with OWSJ. It is common in Canada. The CISC (AISC) likely has publications and guidelines with regard to the gerber system. I believe that the CISC has a set of seminars coming up for industrial buildings.

RE: Gerber Beam

Anne-
I find it quite amusing that you use a vaguely known engineering term for a beam system that you don't know how to design, while acting extremely condescending to those whom have never heard of it. A little humility goes a long way, even for geniuses.  

RE: Gerber Beam

Been around engineers and structural engineeing for 50 years and never heard the term either.

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Gerber Beam

Thanks BA.

You know, I learned years ago that the more you know, the less you know.  

From what I hear here, I guess the term is applied in bridge construction and steel/glulam link-span systems?

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Gerber Beam

BA/Mike...yeah I've got 34 years and never heard it.  I guess Anne has figured out you can't get away with much in here!

RE: Gerber Beam

BA/Mike/Ron... I had 51 years in when I retired. I was just going to say something about being a bunch of old dinosaurs, but I don't think the youngsters thought of me that way, if they did, I didn't get a hint of it, in fact, I don't think they though of any of the old guys as dinos, more as a source of insights from experience.

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Gerber Beam

paddington..

you are correct..  many of us on this forum appreciate the experience of the elder statesmen on this forum..   

RE: Gerber Beam

I second that, the elder statesmen definitely have a better sense of structural behavior because of working in the pre-computer era. I learn so many cool things from one of my bosses (he is about 60 now). Experience + ability to visualize problems is a great asset.

RE: Gerber Beam

"I guess the term is applied in bridge construction and steel/glulam link-span systems?"

Not in the bridge construction I've been involved with.  We call 'em "drop-in sections".

Then again I can't remember the names for any of the truss systems, so I probably don't count as an engineer anyway--but at least I know which terms I've heard before, and "Gerber" ain't one of them.

Hg

Eng-Tips policies:  FAQ731-376: Eng-Tips.com Forum Policies

RE: Gerber Beam

Anne,

As others have pointed out, you do indeed need to consider partial loading on the gerber system per your local Code.

Typically, I've found that cantilevers on the order of 15-20% of the interior span work well.

Unless you're designing an element that repeats in large numbers, I've found that the optimization of the member sizes is not that important.  In my market, contractors simply like the beam-over-column detail for steel.  Of course, they like it a little less when I make them brace the bottom flange at the columns which is very important.

Older versions of the wood design manuals (Canada & US) provide information on the Gerber systems, including recommendations regarding the cantilever dimension.

RE: Gerber Beam

I agree, KK that partial loading must be considered. For large flat roofs, a partial loading of 50% may be reasonable but for parking, it will not do.  The floor should be designed for 100% or 0% live load on any area so as to produce the most critical case.  

On April 23, 1988 an 80' x 80' section of the Save On Foods store  in the Station Square Development in Burnaby, BC collapsed on opening day.  The opening was attended by a number of senior citizens.  There was one photographer who managed to take a few amazing photographs considering that the roof was in the process of collapsing above him.

A comprehensive report was written about the reasons for the failure.  The main reason attributed to the collapse was the lack of lateral bracing of the bottom flange of the cantilevered beam where it passed over the column.  Other factors were the underestimate of dead load and the overestimate of yield strength of the beam.  The report puts the entire blame on the engineers responsible for design and inspection.

The photographs suggest to me that several joists were not welded to the top of the beam.  This, however is not mentioned in the report and has not subsequently been acknowledged by anyone who assisted with the preparation of the report.
 
Page 1 of the attachment shows the collapsed area after the event.  Page 2 shows the failure in progress.   

BA

RE: Gerber Beam

So what kept the area of roof in the top right corner of page 1 from collapsing?  It appears to be similar in loading.  

 

RE: Gerber Beam

CTW,

The beam which failed was a W24x76.  It had been identified, along with one other beam as requiring remedial measures.  A mill certificate indicated a yield strength of 55,700 psi.  The design strength was 44,000.  The engineers concluded that remedial work was not required after all as the beam was 26% stronger than their design had assumed.

The report stated that the mill certificate was based on coupon tests taken from the web of the beam.  The strength of the web is always greater than that of the flange, so that decision was criticized.

Elsewhere in the building, beams were of different sizes and different spans.  

BA

RE: Gerber Beam

Pad/MM/BA-
I have only been around for a decade, and spent another 8 years in the building/contracting trade (where I feel I picked up a tremendous amount of valuable experience).

It seems to me that there are two kinds of "older guys" in the profession.
There are guys that I can't believe have worked 35 years and can't answer very simple questions in areas in which they are supposed experts.
Those, like you guys seem to be, that seem to be full of insight.
The former, I believe, mastered nothing more than how stay in a job for 35 years.
I don't think those types would bother logging onto any forums.  

RE: Gerber Beam

Some people have thirty five years experience, others have the same five years repeated seven times.

Michael.
Timing has a lot to do with the outcome of a rain dance.

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