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  • Flat-Slab Building Analysis




    I. Purpose of Investigation:

    The Test Lab will require that heavy testing machines and test specimens (electric motors) be located in a concentrated area of floor 1, Building 62. This investigation is to provide equipment layout and motor staging recommendations to the owner, in order to avoid overloading the floor structurally. Due to the age of the structure, and the limited ability to know for certain various properties, quantities, condition and location of all of the load resisting elements, this report makes no warranty or guarantee of future structural performance. It is intended to provide as much guidance as possible for the owner to minimize the possibility of structural problems developing under the proposed usage of the floor area under examination.

    II. Description of Building:

    Building 62 is a multi-storey factory structure, built with the “Flat-slab” reinforced concrete framing technique. This is a design that avoids the need for girders along the main column lines, by utilizing enlarged column tops (capitals), and a thickened slab at and around the columns (drop panels). This framing system was first patented in the U.S. in 1902. The owner’s construction drawings for Building 62 were prepared in 1916.

    III. Field Observations and Notes:

    The structure, in the area being considered, did not appear to have excessive concrete or steel reinforcing deterioration. Visible concrete spalling and steel oxidation seeping was minimal. The following should be monitored periodically as part of routine building maintenance:

    1) There is a crack propagating northward from column 34. This crack is in a region of structural compression and therefore does not significantly reduce the floor bending capacity. It may, however, be a shear crack, and a reduced allowable floor load may be appropriate. Shear cracks in concrete propagate at an approximate 45 degree angle within the slab. This crack, being slightly to the west of the line between columns 16 and 34, is in the location we would expect, if the 300 hp test stand was overstressing the floor slab in shear. One of the main problems with the flat-slab design is the transfer of shear from the slab, to the columns. This is the reason for the widening of the column tops and the thickened slab around the columns. Note that the crack ceases at the thickened slab. The original drawings do not indicate any steel reinforcing for shear within the floor slab.

    The top side of this crack may not be visible due to the 4” floor topping and equipment already in position on the floor. It is important to periodically verify that that this crack does not change in appearance, open up wider, or seep significant rust.

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    2) There is a rust stain at the north side of column 35. It is important to periodically verify that this stain does not change significantly in appearance, or seep significant additional rust.

    3) The rust staining that does exist throughout the ceiling should be monitored to verify that it does not increase significantly over time.

    The original north exterior wall has been removed. We have some concern over this, since buildings built with the flat-slab method typically rely upon shear walls for lateral stability. The north shearwall below the1st floor is still intact, and therefore provides
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    stability for the equipment loads under consideration. The impact of this shearwall removal would need to be addressed as part of an overall structural evaluation of the entire building, which is outside the scope of this investigation.

    It is important to verify that the slab between column 35 and the stairwell wall is bearing upon the stairwell wall. In order to achieve proper bearing, the steel reinforcing needs to extend over the wall. This is not possible to verify visually, and we have not been able to verify this in the original drawings.

    IV. Structural Analysis:

    Three procedures are being used in assessing the floor’s ability to resist the equipment loads:

    1) Comparison of the actual equipment load in pounds per square foot, to the floor live load capacity.

    The greatest floor pressure is 235 psf, created by the 300 hp test stand. This is less than the 400 psf original design load, and the 290 psf proposed limit.

    2) Comparison of moments and shears created by the equipment alone, to those which are created by a uniform 290 psf floor loading. (400 psf = stated original design live load. 400 x (0.90/0.125) ~ 290 psf, reflecting change in ACI moment coefficients.)

    There is one location where this comparison shows an overstress. Midspan bending of the floor area supporting the 300 hp test stand exceeds the limit by 21%. The equations used are for point loads, and overestimate the actual bending somewhat. We consider this to be a good example of a loading that is at or slightly above safe loading of this floor. Combined with the question of the column 34 crack, it may be a good idea to re-position the 300 hp test stand.

    We were not able to find a routine set of beam equations for the case of a fix ended beam with a partial uniform load anywhere along the span. The equipment loads have been modeled as point loads in the analysis, so that routine simple beam equations could be utilized. This allows for a quick check of the effects of moving the equipment to various locations on the floor, as well as changing the load magnitude. However, somewhat conservative results are obtained this way.

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    In order to refine the procedure, it is necessary to perform a numerical analysis of each possible load configuration. We are looking at available software which may be able to provide this more refined analysis.

    3) Comparison of moments and shears created by the equipment, to the reinforced concrete member capacities, as calculated by modern procedures.

    The floor slab flexural reinforcing in the center of the 25’ x 25’ floor regions (5/8” square bars at 10-1/2” o.c., per the original drawings) is insufficient under modern concrete design standards. This has nothing to do with strength. The purpose of minimum reinforcement is to ensure that failure is ductile.

    The strength checks show one location of overload – slab midspan adjacent to the 300 hp test stand is 34% over the limit. The equations used are overestimating the forces somewhat, but this is another indication that the 300 hp test stand might best be re- positioned.

    V. Recommendations:

    There is reason to reduce the allowable floor live from the original 400 psf. Design procedures in use at the time of the Building 62 design allowed for reduced design moments, which have since been increased by the governing specification (ACI-318).

    The increase translates to an allowable floor live load of approximately 290 psf.

    Though less than our limit, discreet loads can cause greater internal stresses than continuous uniform loads, at certain locations within the slab. These effects can be minimized by loading the floor in as uniform way as possible. This means, spread the equipment and test motors being staged throughout the region evenly. Avoid loading up one 25’ x 25’ area bounded by columns, with adjacent areas unloaded.

    In particular, the 300 hp test stand may need to be re-positioned.

    In order to be more certain of these findings, a comprehensive numerical analysis is needed. Specialized computer software is required for this. Since the flat-slab design method is not often used in modern times, there is limited information and resources available. Modern reinforced concrete structures of this type typically utilize girders in between the columns. These girders provide, essentially, a depository for shear forces, which alleviate the tendency for the columns to punch through the floor. The need to enlarge the column tops, and build drop down panels are, in themselves, an indication of an inefficient design, and one that has been replaced over the years with designs that better utilize structural materials to transfer load.


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