Gerber Beam 6

[Anne651]

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?

 

[LowLax]

What's a Gerber Beam?

 

[Anne651]

Are you an engineer? How can you not know gerber beam?

 

[rowingengineer]

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

 

[slickdeals]

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.

 

[Anne651]

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

Thanks.

 

[asixth]

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.

 

[Anne651]

Which section in NBCC states this? Do you recall?

 

[apsix]

"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.

 

[Lion06]

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

 

[paddingtongreen]

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.

 

[Ron]

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)

 

[LowLax]

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

 

[paddingtongreen]

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.

 

[Ron]

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).

 

[Anne651]

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.

 

[Anne651]

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.

 

[frv]

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

 

[FFENGINEER]

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

 

[BAretired]

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