Thin Concrete wall structures 2

[KCRatnayake]

Dear All,
I have to do some design checks, stability check etc. of a concrete wall structure. There are two types.

1) 4-storeyed building. Walls are 2.8m apart and parallel. 100mm thick grade 25 concrete with 1 layer BRC8 r/f mesh in the middle.

2) 14-storeyed building. Wall are 3.0m apart and parallel. 140mm thick grade 25 concrete with 2 layeres of BRC7 r/f mesh.

All slabs are 100mm thick 1-way slabs.

Can somebody please help me to find some literature regarding this concrete wall structures?

Regards,
Keerthi

 

[youngstructural]

What you're talking about are tilt-up buildings; I have designed a number and find the description you've given shocking. I am suprised to hear of such narrow and tall structures, but I suppose the 100mm thickness would be possible with only a 2.8m span; I'm just surprised anyone would find such a structure useful or worthwhile building.

All that aside, you really want a design guide for tilt-up buildings. The UK Concrete Centre has published "Tilt-Up Concrete Buildings - Design and Construction Guide", the National Precast Concrete Association Australian has published "Precast Concrete Handbook" (I know and can recommend this publication) and the US Tilt-up Concrete Association has the 5th Edition of Tilt-Up Design & Construction Manual out now.

http://www.npcaa.com.au/html/Publications/PubDetails3.html

I would recommend the Australian publication, if only because I know and have worked with it. Also, many codes (the joint AS/NZS concrete code comes to mind) have sections on tilt-up, and often address stability concerns.

Just one quick comment, if I might: Watch your overturning with those buildings!

Good luck,

YS

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

 

[KCRatnayake]

Thanks YS,
But, these concrete walls are not pre-cast. They are cast-in-situ. Any comments?

Regards,
Keerthi

 

[csd72]

I dont think you will find any specific guidance on this type of structure.

Analyse whole thing as a frame and check the walls and slab to your local concrete code (there will be separate sections for both).

 

[concretemasonry]

Are these buildings in Brazil or India?

Are they apartment buildings?

Are there any interior walls perpendiculat to the parallel walls you described?

The first sounds like a "ground plus 3" in India. these are a combination British material standards and some American techniques.

The second sounds like a common concrete block ot concrete apartment building in Brazil, but they usually use two-way slabs. Many of these use the basics of C318 or ACI 530.

Both types really rely on structural continuity in all directions.

Dick

 

[hokie66]

In addition to your design checks for strength and stability, I would query the constructability of the 100 mm walls. How do they propose to compact the concrete?

 

[NerdSE]

Out of plane:
SEAOC produced a publication offering guidance on slender wall design around 1980. This is the basis for the out of plane analysis used in the 1997 UBC and ACI 318. Although, the UBC and ACI made some rather coarse simplifications. This is a good proven method for design of slender walls out of plane. Also look at the PCI design handbook. Pre-casters like to make walls out of double T’s and other slender precast systems. PCI has devoted a significant amount of effort to creating a very good concrete design guide.

In plane:
What type of loading do you anticipate? Is this in an area of high seismicity? The reason I ask is because the intent of seismic design is to allow the cantilever shearwall to become non-linear in a flexural manor. This causes inelastic deformation at the ends of the walls which causes a loss in stiffness. (remember that your buckling equation is significantly impacted by stiffness) This then can create an unstable situation where the wall may buckle out of plane as it cycles. Pauly and Priestly: Seismic design of reinforced concrete and masonry buildings has some minimum slenderness criteria for concrete and masonry shearwalls used in areas of high seismicity.

 

[KCRatnayake]

Thanks for replies.

The developer is a Malaysian guy and the original design has been done in Malaysia and to be built in Sri Lanka. All are low cost apartment buildings.

There are number of perpendicular walls. 100 mm thick walls have only one BRC mesh in the middle.

Generally, Sri Lanka is free from earth quakes. But, we do detailings for minor earth quakes. We basically follow Australian seismic code (Intra-plate). But, this is fore-shore area. I anticipate some cyclone effects. Corrosion can also be a problem as sections are slender.What changes can we make on the design to impove the resistance to Tsunami etc.? Any comments?

Regards,
Keerthi

 

[concretemasonry]

Tsunami?

Deep spresd footings with reinforced (8" wide) block piers perpendicular to the coast line - Minimum water resistance and maximum strength. Put block "blow-out" walls bewteen the piers to permit interior finishing - Seen this in a number of rebuilds after the 28'(+ or -) Katrina storm surge.

On all these slender walled apartment structures, continuity, cellar wall arrangements and two way slabs do wonders for real structural integrity.

4" walls bother me from a practical standoint. From what I have seen 6" partial reinforced block walls are a far more efficient use of materials up to 15 floors or so. Your developer probably has some local traditions to follow.

Just make sure the "pond curing" of the slabs is carried out rigorously to get all the strength you need from the 4" slabs.

 

[youngstructural]

I think that hokie66 has a very important point here: We CANNOT simply get all analytical and hope for the best... This is not going to be easy to build. How do you pour this amount of concrete without seggregation? What about bar shift in the forms? What about curing? Where do you choose to have your cold joints if you pour in multiple lifts? At the floor levels is a negative bending, and your points of positive bending mid-storey and intra storey are going to be theoretical positions. If you're pouring an insitu concrete frame, that's entirely different, but this IS essentially a tilt-up structure, but just not tilted up.

I think casting walls like this will have to be done with a great deal of caution, and I would not attempt it myself. And what's more there is no reason to try: Employ tried and tested tilt-up construction techniques. They are simpler, easier to work with and do not even require a crane if done right. Although I have not been involved in one as yet, I have always thought that it would be possible to build a tilt-up multi-storey building with the winches and manual labour typical of developping nations.

Please be careful; As an Engineer you hold a great deal of responsibility to the welfare of the public and the labour involved. I have known several Sri Lankans and enjoy the friendship of a fellow Canadian Engineer who's family emigrated from Sri Lanka years ago. My father also worked on projects in Sri Lanka for CIDA (Canadian International Development Agency) doing work on improving infrastructure, etc. Everything I know about the country tells of hard working industrious people who have had the benefit of a Britannic tradition, but who suffer the yoke of corruption and a separatist cause reminicent of the Irish Troubles. And I would further worry a great deal about something unique or unconventional I was trying to acheive if corruption (however unlikely) was a possible factor in the construction. I would probably worry about tilt-up as well, but less so. Either way, you could try and find out what type of construction is typical and that the labour forces are familiar with.

I would encourage you to involve a local Sri Lankan Engineer (Structural, of course). They should be able to tell you what can be acheived and how to go about preparing and presenting your final design. They're most probably going to be top-notch as well, since again the British influence will most probably have left behing a quality engineering tradition with well educated and dedicated professionals available to anyone who takes the time to look.

I apologise in advance if I've politicised our little technical utopia; And I know that my personal politics should not be colouring my technical advice: But I'm only human ladies and gents, and I just had to say it.

I await your comments and criticism, but my gut feeling is something's not 100% with regard to the situation described.

YS

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

 

[KCRatnayake]

YS,
I really appreciate you comments. I do not want to talk politics here as you have tremendously explained the situation. Anyway, corruption is most difficult thing to fight with.

Segregation is one of problems to deal with. I thought of using a concrete mix that has more than 150 mm slump but, without increasing the water cement ratio. We'll have to use some admixtures.

Thought of using shutter vibrators. Controlling the free fall of concrete will be a challenge. Using tremie pipes to pour concrete is not also possible as wall thickness is just 100mm.

Anyway, I ask the contractor to provide me with a method statement for concreting.


Keerthi