Recommended Indiana Limestone Wall Heights
In the past, building designs utilized bearing wall construction where large stones were installed one course at a time from grade to considerable elevations. In contrast, modern construction designs utilize relatively thin stone veneer and a variety of back-up materials from CMU to stud systems. A question often posed is how high can limestone panels be safely stacked without relieving angles.1
Compressive strength of the stone is one factor in answering this question. The minimum compressive strength of Indiana Limestone is 4000 psi. The minimum safety factor recommended by the Indiana Limestone Institute for bearing stress is 8 to 1. Allowable compressive stress is therefore 500 psi. If perfectly uniform full bearing were achieved at the bottom bed—where load is the highest—Indiana Limestone could theoretically be stacked about 500 feet high with ideal conditions. But several other factors must be considered, each of which limits this theoretical height. A partial list follows:
- Local building codes must always be consulted and they may mandate maximum stack heights.
- The back-up structure must have been designed and constructed to comprehend, accommodate and permit the intended stacking height.
- Even if full mortar beds are specified, truly uniform bearing is seldom achieved.
- Setting buttons used with mortar beds should be relatively compressible so the buttons will deform thereby transferring load to the mortar bed. Also, setting beds are rarely perfectly flat or parallel, and
the selection of bearing pads must accommodate the likelihood of high spots.
- Anchors in bedding joints should be designed and installed to avoid point loading.
- Ledges, shelf angles and bearing pads must be sized to provide adequate bearing.
- Designs must accommodate differential volume changes where materials other than limestone support or abut stone.
- Relief joints in the back-up structure should be matched with those in the cladding.
- Out-of-plumb conditions or vertical offsets can result in concentration of stress along panel edges.
- The higher the stack, the more relative movement there will likely be between the cladding and the back-up. Accommodation of this greater relative movement requires more complex anchors.
- Deflection, or “drift”, of the back-up structure at full design load may limit stacking heights, especially when considering corners and returns. Special joint patterns and larger joint sizes may be required.
- Allowable stack heights may be affected by lateral seismic design loads.
- As stone is stacked higher, the margin of tolerance for even minor errors in either design or installation drops significantly.
ILI’s general rule is to recommend the weight of the stone be carried at each floor level. Relieving angles must be adequately supported by the back-up structure, and joints must be sized to accommodate actual deflection and sealant capabilities and performance.
If there are no intervening floors and assuming that bearing is adequate, that the anchoring systems have been designed and installed properly, and that the back-up structure has been appropriately designed to permit these heights, ILI offers the following maximum wall heights between gravity supports as conservative guidelines:
for 4 inch limestone — 25 to 30 feet
for 3 inch limestone — 20 to 25 feet
for 2 inch limestone — 15 to 20 feet
ILI recommends that limestone panels not be less than 2 inches thick. In all cases, regardless of how high they are to be stacked, stone panels must be sized and properly anchored to the back-up to handle wind loads, seismic loads, and other required design factors. This will sometimes require design by an experienced cladding designer. The back-up must also be properly designed to receive these loads, to permit the stacking height and for attachment of the stone anchors.
There will be instances, dictated by the situation, where stone may be safely stacked higher or should be stacked lower than indicated in these general rules. In all cases, the stone and its support and anchorage system should be properly evaluated to assure a proper and safe design.
For most installations, compressive stress or shear stress at the beds will control. But for tall stacks of thin stone, column buckling may control. Very little research or empirical data is available for this condition and ILI recommends avoiding tall stacks of thin stone.
Additional information about the use and installation of Indiana Limestone may be found in other ILI publications.
1A relieving angle is defined here as an angle designed and installed to carry the weight of the cladding material above. It has a soft relief joint below of sufficient width to assure there is no load transfer to the cladding material below. Typically a relief joint is caulked.