2-way foundation mat - rebar distribution question

[RHTPE]

I'm looking for eaveryone's thoughts on proper rebar detailing for a foundation mat slab that was designed as a two-way slab.

There are additional bars called out for the intersection of the column strips. Since this is a foundation mat, these additional bars are added at the bottom. Column strip width is defined as L/4 each side of column for a total width of L/2 (assuming uniform bays, which this building desn't really have).

Should the additional bars be distributed over the width of the column strip? Given 36' bays, L/4 = 9', column strip width = 18', there are (18) additional bars called for, therefore spacing = 12" c/c.

OR

Since the additional bar lengths are determined by 0.30Ln (or 0.22Ln) past face of support, where Ln is the clear span, should the additional bars be distributed over a width of 0.30Ln + support + 0.30Ln? Assuming a 24" wide support this becomes 10.2' + 2' + 10.2' = 22.4', giving a bar spacing of 15" c/c.

Since this is a foundation mat and not an elevated slab (or essentially an elevated slab turned upside down), would it be safe to assume that the "support" width is equal to the column base plate dimension?

I have designed formwork & shoring for, and helped to construct, a number of projects with two-way flat slabs but have never been involved in the design of them. I am reviewing rebar shop drawings which apparently assume that the rebar placer understands how to distribute the additional bars. Consequently no information regarding their spacing or end locations has been given on the placing drawing. I am not comfortable with this knowing the education level of those who will be placing the bars.

Can anyone offer some insight please?


Ralph
Structures Consulting
Northeast USA

 

[hokie66]

They should be distributed within the column strip, therefore @ 12" centres. 22.4' is the length of the bars, and does not define the spacing. The ends of the bars typically project beyond the column strip.

 

[csd72]

Check that they have not been relied on for shear resistance

 

[RHTPE]

csd72 - I need to embellish a placing drawing for an under-educated crew who will be installing the bars and wanted to confirm my understanding of the distribution of the added bottom bars.

As I said, this is a foundation mat. There are thickened areas under all columns and along the perimeter under the foundation walls, all of which have additional reinforcing whose spacing & position is easily understood. The 12" mat has typical reinforcing consisting of #6 at 8" c/c each way top & bottom. Due to the bay geometry, this typical reinforcing could be as close as 4" c/c because of splayed (or "fanned") arrangements in some bays. It's the added bottom steel that is simply labeled as "9 #6x24'-0(BL) BOT.ADDL", etc. on the placing drawing. No spacing, no indication of bar end locations if not centered about the column lines.

Ralph
Structures Consulting
Northeast USA

 

[hokie66]

Ralph,
Reinforcing is often shown in that manner, with typical bars and added bars where needed. In the absence of further information about how they are to be placed, they should be distributed across the strip. In the areas where congestion would occur, you should refer your query to the design engineer. One possibility would be to use larger bars in these areas to reduce congestion.

 

[RHTPE]

hokie66 - I understand that the bars are often shown that way. I've seen many, many rebar drawings through my career, but never had occasion to think about the guy trying to read and understand the placing drawing. Until my current project, that is. Were it not for the unique bay geometry, I would not be pursuing a clarification.

Something about the way the rebar detailer handled a portion of the mat doesn't feel right. Although the EoR approved the shop drawing without a single notation or correction of significance relating to rebar (only a correction to a wall corner geometry), my gut tells me it wasn't really looked at. I just want to be sure it's correct BEFORE the bars hit the site.

My client is a small sub, who is working for an international CM. The only way to the EoR is through them - the process is very political and bureaucratic.

Ralph
Structures Consulting
Northeast USA

 

[hokie66]

You have my sympathy. That type project organisational nonsense inhibits lines of communication which are essential to good outcomes.

 

[BAretired]

Are you adding 18 bars or 9 bars to each bottom mat? What is BL...both layers?

BA

 

[RHTPE]

BA -

The labels are from the placing drawing done by the rebar detailer. I'm assuming that "BL" means Bottom Layer - there is no complete legend on the drawing. Typical interior column locations get (18) additional bars each way bottom.

Ralph
Structures Consulting
Northeast USA

 

[BAretired]

Ralph,
You might want to take a look at the engineer's drawing too. Normally, we talk about Top and Bottom mats. Each mat has a top and bottom layer so that, in total there are usually four layers of steel.

I would have expected additional bars in the top and bottom layers of the bottom mat.

BA

 

[RHTPE]

BA You need to remember that this is a foundation mat, not an elevated slab. Everything is essentially flipped upside down - concentrated loads (columns) on a uniform support (soil). The typical mat reinforcing is #6@8" EW T&B. The bars of concern are the additional 18 #6's EW under the columns.

I am not checking or questioning the EoR's design (it is not my place to do so) - I am only verifying the distribution of the additional bottom bars to insure that my client's field personnel install them correctly the first time. I have prepared a sketch for the EoR to review so that he/she gets an opportunity to comment and so that there is something in the record for the inspector's enlightenment should he question what gets installed. Sort of a preemptive strike if you will.

Ralph
Structures Consulting
Northeast USA

 

[BAretired]

Ralph,
I understand that the slab is inverted from what you would expect on an elevated slab. I understand what you mean when you say additional 18 - #6 EW under the columns.

What I don't understand is the note "9 #6x24'-0(BL) BOT.ADDL".

BA

 

[RHTPE]

BA: It was an example of the labels that the rebar detailer put on 9 of the 18 additional bars. Quite honestly, I don't know what his intent was by that, and I have asked him to explain. His labeling was inconsistent throughout the plan.

Ralph
Structures Consulting
Northeast USA

 

[beton1]

It may mean 9 additional bars, to the bottom mat, bottom layer. Not clear and in fact a little vague if you ask me.

 

[RHTPE]

BA: The labeling of "(BL)" indicates black (uncoated) reinforcing bars. It is an automatic label applied by their detailing software to categorize the bars for the cutting & bending list for the shop.

"B" or "Bot" indicates bottom. I have critiqued the rebar detailer on the inadequacy of his drawing legend and the inconsistencies of his labeling.


Ralph
Structures Consulting
Northeast USA