Cable Truss

[RebuildSudan]

Hi All

I work for an organization building schools in Southern Sudan. Limited site access and local supply availability severely limit the types of building materials available to us. An architect working with us has suggested a novel solution for a second floor with a 20 foot span. It resembles external post-tensioned concrete but could be best described as multiple cable slings under the floor with compression struts between the cable and small steel plates bolted through the slab. The slab itself is lightweight laminated ferro-cement. The suspended floor area is 6x8 meters.

Back-of-the envelope calcs suggest that it could work, but no one has seen anything like it, so we're doubtful. The PE on our team only said he didn't have enough experience with either cable tensioning or ferrocement to comment.

Where (or should) we begin to assess the feasibility of this design?

Blake

 

[JAE]

Where to begin looking for this system, especially in Sudan, I'm not sure.

Cable tensioned systems like this have been done - these can take the form of bowstring trusses with flat top chords and curved bottom chords. Or they could be straight tendons along the bottom of the slab, stood away from the slab by struts and only curved up at the ends - or the slab end could have its bottom curve/slope down.

Some bridge departments in the US have used straight tendons along the bottom of existing steel wide flange girders to upgrade bridge capacities.

Key things to think about are:
1. Availability of tendons, anchorages, etc...the material availability.
2. Are there firms that can properly install, stress and test such a "custom" design?
3. Is there a fire protection issue in that your primary reinforcement has no concrete cover?
4. Would the building owner/user, in the future, not understand that the cable is important and cut it to get more space for something new?

Just some thoughts.

 

[hokie66]

Gut feeling...complicated solution for simple problem. Sudan is not quite the location where I would think of doing this.

 

[RebuildSudan]

Wow - Thanks for the quick responses!

I'm glad to hear that it's at least been done before.

hokie66, I think you're half-right on - it is a very complex solution to a simple problem. However the notion of simple solutions don't seem to exist in Southern Sudan. Most building materials are trucked 1,000 miles from Kampala Uganda - a trip that takes two weeks - anything that can't fit into the back of a pickup truck might not make it at all if it rains....

In a nutshell, we're willing to look to a very unorthodox solution as we have some rather unique challenges to overcome. Anything made of wood is destroyed by termites. It would be nearly impossible to source and transport an I-Beam. Steel trusses are possible if fabbed on site, not without risk. Reenforced concrete is the local solution, but after what happened in Haiti, everyone is very leery of going that route. (or course, that wasn't exactly reenforced...)

Most likely, we will end up abandoning the planned second floor with a shrug and say, "it sounded like a good idea at the time".

In answer to JAE:

1. No - Everything would need to be shipped in. But it could fit in the back of a pickup better than an I-Beam. Everything is shipped in anyway.
2. No - Any design needs an abundance of margin to accommodate midskilled workers - picture trail crews building a small suspension bridge.
3. No - We'd have to weigh the fire risk - though minimal from inside the building, possible with brush fires.
4. Unknown - though having the supports in plain view is a pretty good precaution. We would not consider internal cabling for this reason.

One question - I completely understand the complexity of this type of engineering and construction as it would apply to reenforcing bridges as mentioned. Does that same level of complexity exist with a 20 foot span and a 500 square foot slab supporting 50#/sqft live load? Is it more complicated than designing a conventional truss to do the same thing?

Thanks again for your insights.

Blake

 

[hokie66]

You said the slab itself is "light weight laminated ferrocement". That in itself is complicated. Are these precast panels?

If reinforced concrete is the local solution, use it. It is easy to span 20 ft with a flat plate. Perhaps you just need someone there who knows concrete construction to control the quality.

 

[RebuildSudan]

hokie66 - you wanna volunteer in Sudan?

RCC is the preferred local solution - but it's very poor quality. I wouldn't want to spend much time in a building built with local know-how. Like I said, a second floor is looking less and less likely...

Could you explain more about your experience with ferrocement? I understand it can be difficult to model with engineering tools, but in my experience it's pretty forgiving in the field. Failures are rarely sudden or total and there is a growing body of knowledge that suggests it has many positive properties under dynamic loads.

Blake

 

[hokie66]

Blake,

Thanks, but not thanks. I'm too old for that sort of caper.

Ferrocement can mean different things to different people. My only experience with it was only experimental, and involved building water storage tanks with shotcreted walls, reinforced with small gage wire fabric or steel fibres.