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?

 

[beton1]

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.

 

[ToadJones]

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.

 

[msquared48]

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

 

[BAretired]

Mike,

Didn't realize you were such an old codger. These links will provide all you want to know about the Gerber System but were afraid to ask:

http://www.google.com/search?q=CISC+Gerber+system&hl=en_US&sourceid=gd&rlz=1Q1IRFA_enCA356CA356&aq=t

BA

 

[msquared48]

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

 

[Ron]

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!

 

[paddingtongreen]

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.

 

[frv]

paddington..

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

 

[slickdeals]

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.

 

[HgTX]

"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

 

[KootK]

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.

 

[BAretired]

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

 

[BAretired]

The attached pages actually form one plate in the report. The second page fits to the right of the first.

BA

 

[CTW]

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

 

[BAretired]

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

 

[ToadJones]

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.

 

[paddingtongreen]

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.