Alternate Approach to the Pole Building Design
- Another way to analyze the pole buildings is to consider the diaphragm action of the metal deck sheathing on the roof and the diaphragm action of the metal deck sheathing on the wall, shear walls. The shear values in National Frame Buildings Association Inc. , NFBA. We can use the results of the extended panel since we require stitch screws.
- In most cases, the roof consists of gauge 29 steel metal deck. The metal deck with 1 ½ inch screws spaced at 6 inches on center can act as a diaphragms to resist a hear of up to 125 plf. So, you could design the pole/column as a propped cantilever if the reaction form all poles/ columns, from one side of wall only, at the top do not produce shear more that the above value. A probed cantilever has a simple support at the roof level.
- If the pole building is big and the top reaction form the pole design and the wind force on the roof, see item f below, give a shear more than the above value, the pole/column must be design as a true cantilever without a support at the roof level.
- However, in rare cases, the contractor/owner may ask for OSB sheathing under the metal deck. In this case, you can design the pole/column as a probed cantilever. The roof diaphragm shear resistance shall be that of the wood sheathing BLOCKED diaphragm since the presence of the metal deck above the OSB will transfer the shear.
- To calculate the shear on the roof diaphragm, you need to:
- Get the column top reaction from analyzing the pole/column as a propped cantilever, Rt.
- Calculate the wind force on the roof. This force equal the vertical projection of the roof multiplied by the wind pressure and multiplied by the spacing between the poles/columns, Rr.
- Add to two above forces together Rtotal= Rt + Rr.
- Divide the R total by the width, narrow, dimension of the building, diaphragm to get the shear on the diaphragm.
- If the resulting shear obtained the step above less is or equal to the allowable shear given in the National Frame Buildings Association Inc. , NFBA, 125 plf, then we are OK . We can design the poles/columns as a propped cantilever. Please continue with the sections below. Remember, if you rely on the diaphragm to support the pole/column, the diaphragm will deliver the top reaction from all the poles/ columns to end walls, half the total reactions to each wall. So, you have to check the end walls as a shear wall. Note that some of the end walls are full of opening and you have to consider the net meat or the solid portion of the wall excluding the opening. If theses walls fail in shear, you have to use other measures to resist the lateral loads. Please see section 29-1-600 “if the walls failed as a shear wall” below for more details.
Pole/Column could be designed as:
- If the shear on the roof diaphragm is within the allowable values as outlined above, you can design the column as a propped cantilever. It is a cantilever with a simple support at the eave of the roof. You will get smaller section compared to if you designed it as a true cantilever. Also, most contractors would like to use 6×6 PT Him Fir whenever possible. Remember, when you design the pole as a propped cantilever, you are saying that the roof diaphragm will support the pole/column at the roof eave in addition to the wind forces on the roof. You can use Wood Seizer or RISA software to design the pole/column.
- If The diaphragm can NOT support the reaction from the poles/columns. In this case, the pole/columns will be designed as a true cantilever. The pole/columns size will be bigger and you do not have to worry about the roof diaphragms or the shear walls. You can use Wood Seizer or RiISA software to design the pole/ column. Please refer to the beginning of this chapter for the design procedures.
Shear on walls in pole buildings:
- Shear on side, long wall is usually small and usually is less than 125 plf allowed test results obtained form by the National Frame Buildings Association Inc. , NFBA.
- Shear on the end walls:
- When you rely on the diaphragm to support the pole/column, the diaphragm will deliver the reaction from the roof diaphragm to the end walls, half the total reactions to each wall. So, you have to check the end walls as a shear wall.
- Note that some of the end walls are full of opening and you have to consider the net meat or the solid portion of the wall excluding the opening. If theses walls fail in shear, you have to use other measures to resist the lateral loads.
If the walls failed as a shear wall in pole building:
If the shear stress on the walls exceeds the allowable shear stress, there are several methods that we have used successfully to resist the lateral loads. Here they are:
- Increase the number of poles/column in the wall and turn the poles/column so the strong direction is in the plane of the wall.
- Use OSB sheeting on one or both sides of the wall.
- If you can not add multiple columns in the wall as in item 1 above, use multiple poles/ column near the end of the side walls near the building corners.
- Use Wood moment resisting frame.
- Add multilevel Wood moment frame.
- Use Knee braces.
- Try open front structure method of design allowed by both. UBC Section 2315 or IBC section 2305.
- Try cross bracing with rods or cables.
- Try cross bracing with 6×6 or so diagonal wood poles.
- Try a shear wall above the opening to force the poles/columns to act as cantilever at the top. Remember, the poles/ columns are already fixed at the bottom due to the embedment in the concrete footing.
- Try to sheet or cross bracing above and when possible below the openings. This will force the poles columns to act as fixed at the top and bottom without the need for embedment into the foundation. Refer to Appendix A figure 11 to 11.xx.
- Try steel pole/ column true cantilever. The steel pole could be fixed at the bottom by embedment into the foundation.
- Try steel pole/ column true cantilever. The steel pole could be fixed at the bottom. The fixation at the bottom could be achieved by a grade beam.
Open Hay Barns
Same as enclosed pole building above, except: Since hay barns do not have walls, wind forces become small and seismic forces in the roof most likely will control.
Same as enclosed pole building above, except:
- Hay barns do not have walls, so poles/columns cannot be analyzed as propped cantilever at any time.
- Since hay barn do not have walls, there is not uniform wind load on the pole/column. Only concentrated load at the top of the top of the pole/column. This concentrated load must be divided by 2 because it is shared by two poles/columns.
Contact us for more information on pole structures. We will gladly discuss project details and design options with you.