Kahn Truss Bars? 1

[JAE]

I've been working on a project involving structural stabilization of some old buildings (circa 1911-1921). These are cast-in-place reinforced concrete systems (both joists and beams). Part of my efforts involve determinatino of existing floor capacities - which is easy enough to do as there have been numerous "past" reports, some field investigation involving chipping into the concrete to see what reinforcing is actually there, and original design drawings.

On the original drawings, there are beam schedules that call out a weird bar - the schedule looks like this:

2 - 3 1/2 K

and then the bar is indicated as a bent truss-bar

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A previous engineer's report indicated that these were "Kahn Truss bars" and I have found a little info on them. Apparently they are a square bar with webs rolled up on them from two diagonally opposite edges...kind of like a diamond shape with horizontal wings extended from each side. Stirrups were formed by shearing these webs/wings on each side for part of their length along the bar and then turning them up. Four sizes were mentioned in the info source that I found -

1 1/2" x 1/2"
2 3/16" x 3/4"
3" x 1"
3 3/4" x 1 1/4"
Yield strength = 46 ksi.

None of these match my schedule - 3 1/2" K

Do any of you have any more info on this system...I'm simply trying to determine the area of a 3 1/2" K.....I think its about 0.5 sq. in. as you can only count on the diamond part as the wings are turned up as stirrups.

 

[haynewp]

My office has some extremely old books, I will look and see if I can find something.

 

[austim]

JAE,

A Google search for "Kahn concrete reinforcement bars" will turn up a number of encouraging leads.

http://www.glias.org.uk/news/182news.html

includes a reference to 'Concrete engineers pocketbook, first edition 1911. It says that the Kahn system was widely used in the UK by Truscon.

good hunting.

 

[Mattman]

From the Concrete Engineer's Handbook, Hool and Johnson, 1918:

Kahn Trussed Bars- New Section Bars

Size in Inches, a x b: 2 x 3-1/2
Weight in #/ft: 10.2
Area: 3.00
Length of Diagonal in Inches
Standard: 36
Special: 24, 30, 48

The bar is rolled with flanges, to which diagonal bars are attached. In the above table, "a", the 2" measurement, appears to be the length of a side for the truncated-diamond shaped rod. "b" is the flange width.

The area appears to be for the rod with flanges, and does not include the area of the diagonal (bent) bars attached to the flanges, if I am reading the sketches correctly.

 

[JAE]

Thank you all for the replies...

Mattman - where did you get that book? Is it still available somewhere like Ebay or something?

 

[Mattman]

You're welcome.

I got my beat-up copy from an old timer at a firm that I formerly worked for who retired and gave me all his books, catalogs, etc.

I did check

http://www.bookfinder.com

(author: Hool, Title: Concrete Handbook) and found a number of copies for under $20.

By the way, the book has 3 illustrations of the Kahn system, including a cut-away perspective drawing that shows the system quite well.

 

[JAE]

Is there any way you could fax those illustrations or sheets to me? Assuming its legal to do so.

fax 402-493-7951

 

[Qshake]

God Bless Old timers!!

 

[JAE]

We're all headed that way.

 

[jheidt2543]

My reference: Handbook of Building Construction, by Hool & Johnson, McGraw-Hill, 1929, vol 2, p. 1017

Kahn Trussed Bar System, named for its inventor, is a rolled steel reinforcing bar with flanges which are bent up to resist the shear in a concrete beam. The normal nomenclature is a x b which stands for bar square bar width in inches x width of bar “flanges”. The table in the book shows the following:

Size a x b, inches Weight, plf Area, in^2 Length of
diagonals,in

1/2 x 1-1/2 1.4 0.41 12

3/4 x 2-3/16 2.7 0.79 12, 24

1-1/2 x 2-1/4 4.8 1.41 12, 24, 36

1-3/4 x 2-3/4 6.8 2.00 36

2 x 3-1/2 10.2 3.00 36


Hope this is some help! Uoops, I see some others have found it too, well consider this a confirmation! Bye the way, George A. Hool was a professor of structual engineering at the University of Wisconsin and wrote 12 books on structures, along with others, that are worth having if you can find them.

 

[whyun]

God bless the old timers who leave behind treasures.

Also bless those young ones who maintain the treasure in safe and accessible places.

It is unfortunate to see many irreplaceable treasures lost forever, never to be found again. Thank God for the internet and providing us a place where we can share our treasures with one another.

 

[JAE]

Hey - thanks for the replies (again) - this is great - and is what makes Eng-Tips so great....

Very helpful....