Connection Rules of Thumb

[jheidt2543]

In checking some shop drawings I noticed that the outside detailer selected beam web stiffener thicknesses to match the beam's flange thickness. When checking the thickness required by calculation, the thickness I come up 5/8" vs his 7/8" as an example. I'm wondering if the detailer is just using a quick "rule of thumb"? If so, why match the flange thickness and not the web thickness, which is what we are trying to stiffen?

Also, when pricing your time to run connection calculations for a small to mid-sized steel project, do most of you work out the man-hours you think it will take or do use a "rule-of-thumb" like hrs/sf or hrs/ton of steel? Or, do you weight the set of plans <G>?

 

[nutte]

Well, the flange is thicker, and material is cheap, so why not use the thicker material?

In your example, if the flange is 7/8", the web is probably 1/2", less than what you calculated as being required. Looks like a good rule of thumb in this case.

 

[rowingengineer]

I would expect that the fabricator is using the same size stiffener as often as possible to reduce the number of different thickness plates required to be cut.

A clear conscience is usually the sign of a bad memory

 

[paddingtongreen]

rowingengineer beat me to it.

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

 

[GalileoG]

I have heard about this rule of thumb from fabricators many times in the past.

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[connectegr]

For small structures, using the beam flange thickness for stiffener plates can be a conservative "rule of thumb". OR A VERY BIG MISTAKE

Are the beams moment connected to the column flange? Rarely is the full moment capacity of the beams required, but with no additional information it may be the only choice. Neglecting any column strength the stiffener would also need be the same grade material to transfer the full axial load, and CJP welds would be required. What size welds are used (remember the weakest link will fail)? Simply providing stiffeners, does not address column shear? Are the moments due to wind (lateral loads) or gravity (cantilever loads)? Are the moments one-sided or on both sides of the column flange? Are moment connections intersecting the column web (the stiffeners should match the beam grade)(and be of additional thickness to allow for mill tolerances and erection tolerance)?

You stated that the web is the concern for stiffening. Does the beam have an axial load, or is the concern column shear as described above. Axial load would require prying check of the column flange and connection material. Column shear would require a web doubler plate for reinforcement.

The detailer may not have the background to even discuss the engineering behind a connection detail. And certainly should not be depending blindly on rules of thumb.

"Rules of Thumb" if adequately conservative to address a wide range of conditions, can cost a project and the fabricator big money. And in many cases do not address the loading and design considerations required for the connections.

How to estimate connection design costs? For small projects it can be as simple as counting the connections and types. And having the experience to know what design tools are available and the time required to run the connections. For large projects, it is impossible without defining a very specific scope of design. Cost per ton and cost per square foot are better determined at the end of a project. Projects vary dramatically, cost per ton for commercial construction does not compare with industrial. Power house construction does not compare with pipe line or water treatment facilities. I have worked on projects at $75-100 per ton, and architectural Frank Gehry designs that ran $1500-1800 per ton. I have worked on twin structures that varied 50% in engineering costs per ton, simply based on a recommended change in fabricator preferences. But, in this case the additional engineering time saved significantly in fabrication and erection, saved the owner money. We provide 99.9% of our services on an hourly rate basis.

http://www.FerrellEngineering.com

 

[connectegr]

The comments concerning standardizing material thicknesses are correct. Fabricators save money by purchasing large quantities of certain thicknesses. But, 5/8" and 7/8" is not typically a fabricator's "standard" thickness.

In my experience...
1/4", 3/8", 1/2", 3/4", and 1" (with +1/4" increments going up)

Material grade is another consideration. For our larger fabricators, they are purchasing Grade 50 only for 1/2" and up. If designing connections this can be an additional area for material savings by weight.

http://www.FerrellEngineering.com

 

[demayeng]

the OP's first sentence implies that they weren't using large quantities of a standard size stiffener..

I would say that if the engineer hasn't specified a size to use, then using the flange thickness is a reasonable conservative rule of thumb. If it's that critical it would be specified.

 

[csd72]

There is another possible reason for using this, if the stiffener is meant to be the same size as the flange then it is easy to spot if the incorrect size is used.

I disagree that this is a major cost factor as I would expect the labour cost of welds e.t.c to be far greater.

 

[jheidt2543]

Thanks for all the comments guys. This just seemed to be too much for a coincidence and I thought there might be a "rule of thumb" at work that I've never heard of.

 

[BAretired]

When a column is sitting atop a beam with webs aligned, I usually make the web stiffeners in the beam the same thickness as the column flanges.

BA

 

[rscassar]

Who's responsibility is it to sign off on the connection detail. If a member has a flange thickness of 7/8" and requires a web stiffener than my guess is that it is going to be transfer a significant force (enough to be considered dangerous in the event of failure) and therefore the plate thickness should have been specified by the certifying engineer.

By the sounds of it the steel detailer has selected a "chunky" plate. I haven't been out of college for long but a minimum stiffener thickness equivalent to the web thickness of the plate girder would seem intuitive to me.