Trussed Bars?? 1

[DMWWEngr]

I was looking through an existing buildings plans to familiarize myself with the design. It is an 8" thick flat slab with drop panels at the columns.

When looking at the reinforcing for the slab it calls out the number of bars, the size of the bars, and designates top or bottom. Below this is a number in parentheses. For example:

14-#4B (bottom)
(7)

I undertand the top line but I don't understand the number in parentheses. The notes indicate that the number in parentheses is the number of "trussed bars". What is a trussed bar??

TIA!!

 

[Qshake]

Where have we gone now?! Forgetting (or perhaps not even knowing) what trussed bars are! Just kidding!

Trussed bars are bars that were pre-bent to aid in placing reinforcing for negative and positive moment areas of typical slab applications. This was done with one bar rather than clutter up the slab with single bars each for positive and negative moment areas. It also cut out the need for anchorages in those areas too.

Of course, those bars were used in conjunction with other bars too where necessary but the idea was to balance the moments thereby using a single size bar.

 

[DMWWEngr]

SO....

You're saying it's the bars that "step-up" from the bottom to the top (or vice versa). I saw these bars shown in the details and this was my "guess" but I wanted to check with you guys.

 

[Taro]

Yes, the bars are bent up or down so that a single reinforcing bar can be used for both positive and negative moment resistance, depending upon location in the span. This practice is not used much anymore in the United States because the labor costs required to bend and place the trussed bars outweighs the material savings by far.

In older slabs, besides trussed bars, there are often bars arranged radially around column, placed at diagonals to the direction of the span, and all sorts of other peculiar arrangements. Early concrete designers and contractors each had their own pet methods of design, many of which didn't quite satisfy statics. It took several decades and much debate before it was widely accepted that 2-way slabs needed to resist 100% of the load in each direction.

 

[Ingenuity]

Taro,

You bring an interesting topic of early flat slabs that did not satisfy statics...

C.A.P Turner and his "mushroom slab" concept for flat slabs back in 1905 really started much debate and argument in the engineerng community...

It was Nichols in 1914 did little more than apply the equations of statics to a flat plate panel and arrive at the Total Static Moment, then Weestegard got involved etc etc.

It was not until ACI 1971 that more rational and statically correct design rules were adopted by the code - quite a time span!

Several early test slabs yielded values of the total static moment of less than 70%, and sometime as low or lower than 50%, ie "lets design for only 50% of statics".

It was not until ACI 1971 that more rational and statically correct design rules were adopted.

Although this is off the topis of the original thread, the subject is historically interesting....

 

[DMWWEngr]

Thanks everyone for there help...and small history lesson

 

[trilinga]

An additional use of these 'Trussed' bars (also called 'Bent-up' bars)is that these bars resist the diagonal tension due to the shear near the supports. Eventhough this arrangement is not widely used nowadays, this was followed in earlier days both in slabs and beams. In beams, we used to calculate the number of bottom bars to be taken straight through the support based on the bond stress requirement and bend the remaining bars up to take care of the diagonal tension and also as a part of the negative reinforcement.

The habit of bending up the alternate bottom bars at the support is still followed in India.

 

[austim]

Hi, Qshake.

We never stop learning, do we? In more than 44 years of engineering practice (in the UK and in Australia) I had never seen 'bent-up' bars called 'trussed' bars.

As a matter of interest, the current Aus concrete code prohibits the use of bent-up bars for shear reinforcement.

 

[LPPE]

Here's my two cents - I've never heard them called Trussed bars either.

 

[trilinga]

Frankly, I admit that I did not know until now that these bars are called 'Trussed bars'. I guess that the reason for the name could be that the bent up bar resembles the truss action in which the bent portion acts like a diagonal bracing member of a truss resisting the shear along with the vertical members.

 

[Qshake]

No fellas we all learn something new each day! And thank goodness for that...there are some that have just given up learning altogether.

Ingenuity, thanks for the tidbits about statics and slabs really interesting how designs have evolved.

 

[Ingenuity]

I just happened to open one of my bridge enginering texts on a detailing page that shows a transverse section of a concrete bridge deck to NY State DOT details and it shows 50% of the bars bent up. The text was published in 1995!

I have not seen this used in current forms on construction...and i too have not heard the term "trussed bars"....

 

[JAE]

While I was in Texas, the term used there was "crank bars" as the bars resembled a crank handle for the front of an old car.

The concept of using them for shear is certainly true. The problem is, they only resist the shear at the area where they are turned up. On either side of the diagonal there is generally no shear reinforcing.

I've reviewed old plans of buildings where these bars were used and have never relied on the shear capacity provided by these bars as their actual location along the length of the span is always questionable. This ends up reducing the calculated capacity of the floor members and gives owners the creeps when you tell them that their floor is very safe flexurally, but in shear there is limited live load capacity. And the shear failure is abrupt which adds to your concern.

 

[evelrod]

Hey Qshake, Now you have gone and done it---you made me remember a bunch of stuff I thought I had forgotten, or something like that!!! Yuck, I hate rebar. I hate being cold, wet and tired! Yuck, I hate rebar!!!


Rod