My ongoing ISBU (Intermodal Steel Building Unit) Design:
I have been working on a Container design that I intend to start a build someday. After reviewing countless websites on design, I have begun to become confused as to the minimum requirements on foundation needs.
The idea is to create a sensible design that incorporates maximum structural integrity while minimizing cost. ie; lowest cost per sqare foot for a fortress (if you will) that will be built in North Carolina.
The foundation is obviously critical. Pier design allows for flexability and less concrete but the spans I am trying to incorporate (maximizing living space) are large.
My plan is to also minimize the structural alterations to the containers to maintain their intrinsic robustness. I have read that the walls of the containers are non structural.
I also want to minimize ground swell & hired labor but to enable easy future modifications\ repairs for myself when I retire resulting in a low maintenance design.
I am trying to incorporate a combination of 8 and 12″ piers with a slab on grade 4″ foundation.
would support 8″ SIPs but would not have columns below them. The bearing weight would be minimal but allow for a decent coefficient of thermal insulation with minimal labor or future maintenance.
Finally, the concrete wall ( only 3 courses in height, mortarless SBC dry stacked, also for minimal labor (cost) but to maximize structural integrity.
My questions are:
Could I forgo the quantity of piers of I moved them under the concrete wall on the perimeter and use a combination of them on the inside corners of the ISBU’s (still using the 12″ piers to allow for space for the leveling jack screws) or must I use it the way that it is currently designed to eliminate foundation shifting and cracking?
Keep in mind that I intend to use 2.5″ polished concrete floors inside of all 3 ISBU’s (incorporating radiant heat tubing in each and in the center of the main slab).
Is there a very reliable way to eliminate the concrete slab under the ISBU’s (only but keeping the central pad) to reduce cost while supporting SIP panels?
Your opinions are appreciated.
ISO shipping cargo containers are tested in accordance with the requirements of International Standard ISO 1496/1 which stipulates static and dynamic design load factors to be complied with. In the case of a 20′ steel container, it is designed to have a maximum gross weight of 52,910 lbs (typically has a tare weight of around 5,000 lbs and a payload (P) potential of 47,910 lbs). The container when loaded to its maximum gross weight must be capable of withstanding imposed loads of 2g downwards, 0.6g lateral and 2g longitudinal plus be able to withstand eight similar containers loaded to maximum gross weight stacked on top of it in a ships hold or at a land terminal. It therefore has a very sever operational life and, notwithstanding its low tare weight it is very strongly built.
The side walls and end walls/doors have to withstand loadings of 0.6P and 0.4P respectively, these values equate to 28,746 lbs and 19,164 lbs based upon the payload given above. The side wall area in contact with the load is 146.56 sq. ft. giving a pressure of 196 lbs/sq. ft. Corresponding figures for the end wall/doors are 51.78 sq. ft. and 370 lbs/sq. ft. These figures are well in excess of the 20 lbs/sq. ft. wind load required for structures less than 50 ft. high. A wind of 100 MPH produces a pressure of only 30 lbs/sq. ft.
The roof load test is 660 lbs over an area of 2′ x 1′ applied to the weakest part of the roof. The load is usually applied at the center of the containers positioned with the 2′ dimension aligned longitudinally. Thus the roof is able to support an imposed load of a minimum of 330 lbs/sq. ft. The design is easily capable of supporting the basic snow loads of 30 lbs per sq. ft. evenly distributed.
It is difficult to quantify uplift and suction forces. Unlike a building, the roof of a container is an integral part of the structure; it is continuously welded around its entire periphery and is itself made from sheets of corrugated 14 ga. Cor-Ten steel also continuously welded together. This steel, also used for the side and end walls has a minimum yield strength of 50 ksi, and tensile of 70 ksi. The probability of the roof being removed by these forces is practically zero as the entire container structure would have to be destroyed for this to happen.
However, it is not unusual for the complete container to be lifted or blown over if it is not secured to the ground in storm or hurricane conditions. This would be prevented by adequate foundation design which is the responsibility of the customer. As you know when containers do blow over in container yards the resulting damage is almost always minimal, another testimonial to their strength.
The floor is design to pass a concentrated load test of 16,000 lbs over a foot print of 44 sq. inches. The floor has also been designed to pass a test at twice its rated payload capacity of 47,895 for a 20 container and 58,823 lbs for a 40′ container when evenly distributed.
The boxes are suitable for earthquake areas of seismic rating of up to the California standards.