Pre-Eng building over existing building

[rittz]

Our client wishes to design a new PEB spanning over a series of smaller buildings which make up his total operation (automotive dealership). The new building will be about 90’ x 110’ and will cover the existing plus part of the existing vacant lot. The idea is to build the building complete with a new office area and then move into the new office area. Thus he continues to operate during construction without disturbing the office and other operations.

We have not got to the building design stage yet. From past jobs the thrust at the base of the rigid frames may be in the area of 30 – 40 kips (unfactored) depending on the frame spacing. We normally use hair pins or thrust rods base to base to resist the base thrust. With the existing buildings in place neither of those methods can be employed. Obviously there are other ways to accomplish this. Have some of the readers here been involved in something like this? See attached sketch.

 

[msquared48]

Go to 22.5 foot bays for the mainframes and make the one at grid A an end wall frame. The one just to the right of grid A, make that a post and beam area, with a ridge beam spanning from grid A to the first interior mainframe at 45 feet. This should avoid the lateral thrust at the existing building area.

Mike McCann
MMC Engineering

 

[rittz]

The entire area from about 80 feet to the left of line A-C to 40 feet to the right of A-C is occupied existing space which is roofed and partitioned and will remain. It would difficult to build end wall frames or post and beam in that space. It almost seems at least 3 main frames are needed i.e. AC and 2 more @ 22.5 ft. If the floor plan ( not yet developed) allows it may be possible to do post and beam for the 2 frames to the left of BD.

We may be "jumping the gun" until the Arch comes up with the floor plan for the north inmost 50 ft.

 

[dhengr]

Rittz:
The whole thing sounds like a pretty wild scheme. Lifting heavy structural shapes and building construction in general over occupied space is not without risk. That sounds like one hell of a volume and structure to span the whole space. When some serious architectural planning, some new low-rise additions and some face-lifting on the existing buildings might be more practical and economical. Get complete plans of the existing buildings and foundations, including all mechanicals and any mech. tunnels, etc. So you really know what you’re dealing with. Then, for thrust rods, you could do one of two things: ( 1 ) You could use directional boring, 18" under the existing slab, from frame leg base pier to base pier. Pull a plastic casing through the bore from base location to base location; pull a tie rod through the casing and grout it after some post tensioning. ( 2 ) Depending upon the soil conditions you could drill from each base pier, into and under, the existing buildings, much like they drill, grout and tension tie-backs for tied back retaining walls/whalers in deep excavation work. The boring and casing might be easiest done first and then the piers cast around the casing.

 

[rittz]

You are so right Dhenar. It's wild. This is only what I gleaned from the first and only short meeting I have had with the client and Architect. There obviously has been no serious planning from them so far. I have already told the powers that be that "it's wild" and they have to look at a few realistic alternatives before we get very serious. Thanks for the response. I quite realize that my post lacked much to chew on.

 

[SteelPE]

I have been involved in many discussions in regards to covering existing buildings with new buildings. I don't think I have ever seen one of them get off the ground. There are usually large costs involved. Every time I bring up the discussion about "swinging large members above occupied space" and nobody has ever been worried about it (for what ever reason). Means and methods of construction are above my pay grade anyway.

In regards to your thrust problem. I would think the easiest solution would be to remove the thrust by installing columns at the ridge in areas where you can. Add a jack beam at the ridge in the area where installing columns may not be possible or possibly "fish" a column through the existing building onto a new foundation. Adding a support in the middle of the frames should almost eliminate the thrust generated by gravity loads.

 

[a2mfk]

You can also use micropiles such as a helical anchor that is installed at an angle to resist the horizontal thrust. These can be installed with small equipment in tight spots.

This does remind me of one of those projects where you waste a lot of time at meetings entertaining some big-dreaming client, until the owner gets into the meat of the costs and gets scared into buying some cheap land somewhere else and starting new.

And this is purely anecdotal and somewhat off topic, but it seems like here in Florida that any halfway successful car dealership has moved to a new location in the last 5 years or so. There have been a remarkable amount of dealership projects even in a down economy. I pulled into the local Ford dealership to have some work done lately and it was a ghost town, they had moved about a half mile down the same road. Besides a bigger piece of property, they could build the entire new facility with minimal interruption to the original business.

 

[rittz]

a2mfk (Structural)

Thanks for the response. What is going on with dealerships in Florida is just the same as here in Canada. Including your comments about wasting time on a "dream".

J

 

[tebodm]

I just read a brief article in Commercial Building Products where this was done at a casino in OK. They covered an existing fabric structure with the PEMB so that casino operations could remain open during construction. I'm not sure how they address the occupants while the steel was being swung into place. Granted this was in the south so thrust was minimal due to no snow loads.

http://www.nxtbook.com/nxtbooks/cbp/201309/#/22

 

[H57]

I have seen a concrete drag strut used rather than hairpins or direct ties. Basically a reinforced concrete grade beam cast perpendicular to the pad footings extending into the building. The lateral restraint is the friction along the sides of the grade beam. When I have seen this used it has been in metal buildings that don't have concrete slabs (equestrian or similar buildings).

In one project, our office received a call from the contractor after the slab was in asking what the pile of rebar was that they never used. Instead of installing direct ties between the columns per the plans, they bent some small bars and made their own hairpins. As the slab was installed the EOR reviewed and had them install the drag struts on the outside of the building. They became compression struts still using friction to resist the thrust.

You may be able to design a similar system, but it will depend on soil type, exterior grading and access.

 

[RFreund]

I have seen the helical peir method used but H57's method sounds like a similar concept.

EIT

http://www.HowToEngineer.com

 

[hokie66]

This is not really such a big deal. You are only covering an area a bit larger than a baseball infield. The thrust can be taken in a few ways, but I like a2mfk's approach of using inclined tension piles. Even for new construction, hairpins and full length rods present problems for later floor and service modifications.