advice on spanning 30-40 feet with concrete floor

[SMAJWK]

I represent a missionary effort in central Mexico. A three story 25,000 sq building has been proposed. I am looking for technical assistance in developing concrete floors which can span up to 40 feet. Can steel tube trusses and metal pans be used more economically than precast concrete? At this point the bearing walls are also open for design to accommodate the issue of the floor spans, issues relating to sheer, and other movement within the building.

Thanks SMAJWK

 

[Ron]

As much as I admire your efforts, trying to offer such advice in a forum such as this is dangerous. A three-story building of this size MUST be carefully designed by a competent structural engineer, contemplating the loads that can occur in the use and locale of the building, including material dead loads, usage live loads, wind loads, seismic loads, etc.

We know nothing of your building or its actual location. We do not know the local available materials or the construction competence in the area. All of these must be considered in the design.

 

[concretemasonry]

Ditto on the lack of information. Without knowing what is available, we do not have much to offer as far as options (probably opinions).

I have been fooler many time by associated a country (Mexico) as a assumed criteria and only found out later that they knew more about materials, their use and the applications since they have been around longer than the USA.

"missionary effort" rings of cheap, expediant and getting local involeved if being supervised, but being tough to be confronted with in areas that have easily solved the situation.

What kind of structure is it? Three story churches of 25,000 sf/floor are not very common anywhere. Are the upper levels of use that have walls that are used for class rooms/housing, that could be used to reduce the load on the first floor that seems to control the entire structure?

There are always local resources and engineers, that surprisinly are ahead of traditional designs due connunications and the internet. - They use U.S. "standards/codes", but just use them "better" to adapt to the local availability of materials, workmanship and technology.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[SMAJWK]

Ron,

I understand your concerns, your points are well taken. However, in the third world buildings are constructed with a different mind set. For the most part, there are few codes or inspectors. That does not relieve one of building responsibly. If left to their own devices, a local contractor would lay block bearing walls with concrete intermittent post and beam reinforcements, then erect a plywood "ceiling" supported by anything from 2x4s to tree limbs. Once the form is in place, the concrete is mixed (usually by hand) and carried up to be poured onto the form creating the floor of the next story. They prefer to span in increments of 4' (plywood size) and will pour up to 8" thick (usually reinforced with steel, although there is no set code to determine how much steel) Spans longer than 18-20' are simply not done (thankfully).
I have seen tube trusses used in Honduras. The building was more State-side than third world in its construction. Mexico has been greatly influenced by the US, making more progressive building techniques and materials available - in the larger cities. I was hopeful of gaining an understanding of how to safely span about 30 feet through the expertise of people more knowledgeable than myself.

 

[SMAJWK]

Dick
The building is basically divided into two areas. The city building lot measures 95' x 95' and has a 10' fall from one side to the other. The foundation would be stepped. The resulting two levels of the ground slab create two areas of the building. One area measures 55 x 95 "side A", the other 40 x 95, "side B". Side B would house a school on the first two stories. The third story is for sleeping dorms used by visiting teams who help out from time to time. Side A first floor has a garage and apartment. Side A second floor is a multipurpose room (55 x 95) There is no third floor in Side A as the multipurpose room is two storied. The floor for the multipurpose room is amply supported by walls beneath, and is not in question. The basic question of spans relates to Side B. The second and third floors and the flat roof over the third story need to be spanned. I trust that this information helps.

 

[Ron]

SMAJWK....30 feet is not a large span for a tubular steel truss. Of course it must be designed for that. The floor system you use on top of the trusses will somewhat dictate the spacing of the trusses. Reinforced concrete could be used, steel deck with concrete topping could be used, or wood or a variety of materials, depending on availability.

The entire floor system could be done in wood, as well, using wood floor trusses or "engineered" wood components.

 

[dik]

What are the loads? Do you have a Hollowcore caster locally, one that can do 12" HC slabs? Do you have cranes?... Steel trusses are likely the least expensive, but rely on greater manpower skills.

Dik

 

[SMAJWK]

Thanks to all who are helping me think through the issues associated with this building. I will be making a trip down to central Mexico, to the site, in three weeks. I will be meeting with several local contractors, researching materials, and related verdors (precast suppliers, crains, ect.) I'll know a great deal more upon my return.

SMAJWK

 

[concretemasonry]

SMAJWK -

Good luck on you trip and please provide a follow-up.

I have been surprised (good and bad) on many trips using pre-conceived ideas. Some were real eye-openers on how far behind we are in the U.S. and others made me realize that some advice I got ("If it does not make sense, it must be true") and everything inbetween.

Make sure you understand the site conditions and organizing AND the capabilities of the suppliers.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[hokie66]

SMAJWK,
Why not use the local methods, but with a proper design? Cast in place concrete floor slabs are used all the time to span 30 ft. May just need to incorporate some wide, flat band beams.

 

[SMAJWK]

Please explain what a band beam is.

SMAJWK

 

[msquared48]

Get a structural engineer.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[hokie66]

SMAJWK,
A band beam is just a concrete beam which is wide relative to its depth. Say 96" wide x 16" deep.

 

[darthsoilsguy2]

When I worked in Central America, I would get the contractor(s) to tell me how they would build it, then I checked the design and detailed it. Not as efficient, but unless closely supervised... they may build it the way they want to anyway. Apart from materials and methods the next biggest distinction is that in missionary work, there are no do-overs only retrofits. I would try to find a similar building erected by the same contractor too.

Another major point is that, the Owner needs to agree with the design too. There are plenty examples in the world of half-baked charity work. Everybody knows of some project in another town that didn't work out. The local Contractor needs to be the champion of the project and will promote it to skeptical members of the community.

During my time in Central Amer, I saw an existing building built by charities where the roofs sagged due to the contractor splicing the main support beams because the local mill didn't have big enough trees.... I saw water mains that did not deliver water because the concrete water tank construction location was moved to a new site uphill of somebody who knew somebody (okay this one broke my "no do-over rule").... I stopped a school I designed from being built on 6 feet of uncompacted fill on one side due to the community's inability to assign a good site for the project, and to this day I don't know if it was built.... I caused a political problem by blacklisting the local CMU fabricator from supplying block because I could rip them apart like they were crackers... One side of a classroom on a school i designed had to be infilled with block instead of windows because the contractor misinterpreted an elevation drawing. (He stuck all the 16" o.c. wood rafters inside the concrete support beam right above the bottom rebar because the projection showed them lower... at the eave. The beam looked like an up-side down comb and nearly fell apart before they even got to putting the roof tiles on). point being there are a lot of pitfalls and they attack randomly and familiarity by the contractor and community will be your best defense.

You'll probably get a different grade of contractors working on a 3-story with erection, but i'd definitly make every effort to go with a design they were accustomed to build.

 

[SMAJWK]

I wish to thank those who responded to my original question about span issues in concrete buildings, and offered tips,advice, or council. I have recently returned from Mexico, having met with several builders and architects. The upshot of all the meetings was a workable plan to coordinate efforts with an engineer for the stress and loads on the building, an architect who will combine my design/function plans with those of the engineer and provide a set of building prints. These will conform to the city building codes, recently overhauled and increased in 2006. He will also walk the plans through the permit process. There were two builders very capable of the construction, although a decision as to which one will occur closer to the start of construction.
The above will enable me to confidently present this community center to those receptive to helping in its funding.
Thank you once again for your interest and help.

 

[Ron]

SMAJWK....good luck and best wishes. Please keep us updated on progress.

 

[EliteConcrete]

I was very pleased to read this thread and please updated on progress.

Elite Concrete Restoration, Inc.

http://www.eliteconcreterestoration.com/

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