Live Load Reduction / Two Way Slabs 1

[mijowe]

I have a long span PT concrete office building that was being designed with a shallow beam and one way slab system. 45’ span beams and 28’ conventionally reinforced one way slabs. We are using an office load of 100 psf and due the large tributary area we used live load reduction to the beams on this 100 psf live load.

We have been asked to provide bay studies for a two way flat slab (with drop panels) to be compared and priced. Obviously to compare apples to apples I would need to use live load reduction for this two way slab as well. I have never considered reducing live loads to two way slabs, the bays typically are not large enough, however I suppose that it is possible. Does the code specifically restrict this? Does anybody regularly design two way slabs by reducing the live load?
Thanks for any input.

 

[youngstructural]

I don't know the US codes well enough to help you (and am presuming you are US codes from your units), however I would wonder about your vibration criteria from your description. Have you reviewed the proposed structural framing for walking excitation?

Also, on topic, I have applied live load reduction factors (LLRF)to two way systems because LLRF are based on the satistical chances that the floor is fully loaded. You must still apply them rationally, and it is entirely possible that not all of your members will benefit.

If no-one more familiar with US codes posts, I'll do a little sketch of what I mean. If you're curious, I'm familiar with design to NBCC as well as AS/NZS standards.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[JAE]

I think I'd be tempted to use a single bay size for the tributary area for two-way slabs.

 

[asixth]

Typically I stay away from using live load reduction when designing the slab because it may restrict the future use of this floor.

That isn't to say that it can't be used for two-way slabs. In fact, two way systems have more redudancy "to share the peaks of imposed load between the various elements" than a one-way system so if anything, you should probably be less inclined to use live load reduction for the one-way spanning system, particularly with such a large live load (I think 100 psf converts to around 4.5kPa).

Using a conventially reinforced flat slab system, you may be required to reduce your column grid spacing to 22' (6.5m) to be economical.

 

[JLNJ]

To me the code seems a bit arbitrary when it allows no reduction for a 101 psf and greater LL but allows reduction for 100 psf and less.

In the end you use your judgement.

 

[mijowe]

Thanks for the input

I have a copy of ASCE 7 in front of me now. It does define live load reduction for two ways slabs. The Kll value for two ways slabs is 1, and the value for interior beams is 2. So by code I can reduce the live laod, but it will be be reduced as much for the two way slab as it will for the slab and beam option.

 

[rapt]

mijowe,

Make sure you let the formwork designer know that the slab can only carry a reduced load. You cannot use live load reduction for formwork design so they will have to work with the lesser capacity for the design of the backptopping.

If you work it out, you will probably find it is better to design for the full load than have more expensive formwork.

 

[Taro]

rapt, that's very interesting. Is that just your opinion or have you performed actual cost comparisons of structure vs. formwork with regard to live load reduction?

I used to work for a large structural firm that designed a lot of PT office buildings. Their standard was to always use live load reduction and additional backprops as required. So I would be interested to see a cost study on this.

 

[rapt]

Taro,

I have yet to meet a builder who, when given the option of adding 10% more reinforcement to a slab or backpropping 6 floors instead of 3, has gone for the extra backpropping. Both for the cost of the backpropping and the interference with and delay of followup trades.

We also find these days that most clients with office buildings want the flexibility to place light weight compactus anywhere on the floor. This is not possible if live load reduction is allowed for in the design. For ease of letting of the building space, the more flexibility the better and that is where the money is. 10% more slab reinforcement is nothing compared to flexiblity of rental space.

We would only ever use live load reductions for columns and transfer memebers, never for floors.

 

[youngstructural]

Thinking of LLRF for columns, I have heard that a number of codes prohibit this, but do not know any specifically. Anyone aware of a code which does not permit live load reduction factors on columns? In my opinion it would not make sense, as while some floors may experience full live loads at some point through their lives, rarely if ever would a column see all of the supported floors loaded at full live load.

It would seem to me that the reverse would make more sense, and coincidentally align with rapt's preference for "rentability".

Any thoughts?

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[DaveAtkins]

I am just finishing the design of a large two way flat slab, and I did not use any IBC live load reduction. But I did use the ACI live load reduction. The ACI allows the use of 3/4 LL when patterning live loads (alternate spans, adjacent spans). You still use full LL when loading all spans.

DaveAtkins

 

[hokie66]

Dave,

Patterned live load and live load reduction are different concepts. Patterned live load is to establish an envelope for design, while live load reduction recognizes the improbability of full live load being applied to the whole area at any one time.

mijowe,

I would think you would have to reduce the 45' column spacing to use a flat slab. Thus you would reduce the area, and the advantage of live load reduction. I wonder about your 100 psf for an office building. You may gain more benefit by using a lower value initially than trying to rationalise what reduction should be taken. I would rather use 60 psf live load (in Australia we use 3kPa), with a generous allowance, say 20 psf for superimposed dead load, and forget about the live load reduction for the floors. By all means use the permissible reduction for the columns.

 

[Taro]

rapt, I understand the rational for using a stronger floor slab to avoid additional shoring costs. What I am asking is if you have done or seen a comprehensive parametric economic analysis to quantify this or are you just shooting from the hip? Is "10% more reinforcing vs. 3 additional levels of backprop" a universal rule of thumb? Does the answer vary on multi-story buildings where the backprops can be reused frequently? Are there other factors to consider?

The decision of whether to design for loads in excess of the minimum code requirements should never be made by the engineer in a vacuum. I would explain the pros and cons to the owner and let them make the call with additional input from their contractor with regards to the shoring issue. Perhaps the owner does not wish to pay for a future tenant's potential desire for high-density file storage. Maybe they do. But I would not presume to make that decision for them.

 

[rapt]

Dave, Hokie66 is correct, that is not live load reduction.

Hokie66, 45' column spacing is ok for a flat slab as long as you are willing to make it deep enough. I have done 16m which is about 50'but the slab was about 425mm thich (about 17"). But at these spans you are better off if you design as partial prestressed and that is not possible using the ACI design method for flat slabs because you are not allowing for the moment concentrations at columns etc. As long as column strip concentrations are allowed for, partial prestress would make it a much better design.

Taro,
No I do not have numbers on it and it varies for every design. The deeper the slab, the more back-propping is going to be needed and the higher the live load reduction the more back-propping. So for longer spans which allow more LL reduction the backpropping is progressively worse. And as time goes on everyone wants longer and longer spans. If the slab is 8" thick (weighing about 100psf) and the design live load 80/2 = 40psf then backpropping might be 3 or 4 floors. If it is 12" thick (about 150psf) then it might be 5 or 6 floors etc. Most cases I have been involved in are longer spans and thicker slabs so the backpropping would be farcical if designer for LL reduction.

Reuse is not the problem, they are always reused, it is the cost of supply for a longer period and the delay to followup trades that adds the costs. That is why no builder in my experience has opted for the LL reduction.

Yes, it is the clients choice, but most clients in my experience are building for someone else and they want the flexibility for easy rental at higher rates. For example all government departments in my experience insist on the extra loading capacity and they pay well and are numerous and their number and size always expanding.

 

[hokie66]

rapt,
I might consider a flat slab for a 45' square grid, but for a 45' x 28' grid, I would instinctively always go with the one way system with band beams in the long span direction.

 

[rapt]

Hokie66,

Agreed, it makes the 28' span direction very uneconomical but he wants to compare a flat slab with drop panels with band beam and slab.

I just hope when the design is done with drop panels, the ultimate strength of the tendons away from the drop panels is going to be done based on the depth of the slab, not the drop panels, unlike many such designs done to ACI code logic.

 

[hokie66]

I am confident mijowe wouldn't do something that silly.

 

[rapt]

hokie66,

It is standard practice in US and in the the software developed there and is unfortunately being copied by designers in other parts of the world who use that software. I have seen it done in designs in Singapore, Malaysia, Thailand, India and Middle East as well as by US designers. Gives nice cheap designs!!! Too bad about strength and ethics!

 

[hokie66]

So are you saying for middle strip tendons at the column line they use the slab depth as the depth of the drop panel?

 

[rapt]

hokie66,

Yep, the effective depth used is the depth to the bottom of the drop panel, even though a tendon or reinforcing bar might be 3-4m from the drop panel and completely unaffected by its presence.

I have seen the same done with distribution tendons in the slab parallel to band beams and with the full panel width of the slab used as the effective flange of the band beam as well!

No need to worry about how things actually work. Really makes the design cheaper. Too bad the client is paying for something with significantly more strength and less deflection and cracking than he is getting!