MSL93
Structural
- Aug 10, 2017
- 3
This question has to do with the proper selection of wind load chapter that is to be applied to a silo type structure.
The structure in question is a 50 ft diameter x 100 ft tall cylindrical structure that is open on both the bottom and top (i.e. no floor and no roof). The structure is supported on a number of legs which are simply I-beams with a base plate that is bolted to a concrete foundation. Stiffening rings are located at various elevation to help combat ovalling of the cross-section and excessive circumferential compressive stress. Below is generally how this would look.
Based on my reading and to use the terminology of ASCE 7-16, I would simply classify this structure as a "circular bin, silo, or tank".
ASCE 7-16 has two separate criteria for applying wind load on a circular bin, silo, or tank.
1. Wind load that will be transferred to and through the MWFRS (main wind force resisting system) in chapter 29.4.2.
2. Wind load that will be applied to the components or cladding in chapter 30.12.
My understanding of chapter 30 are that these are for items that are not part of the load resisting frame of a building or structure. For example, for the "skeleton" of a building, the wind load applied would be per chapter 29. For the "skin" (i.e. walls) or things like windows on said building, the wind load would be applied per chapter 30.
Further, 30.2.3 of chapter 30 states that "components and cladding elements with tributary areas greater than 700 ft² shall be permitted to be designed using the provisions for main wind force resisting systems". In other words, if the element in question in larger than 700 ft² in surface area, then using the more highly localized wind load from chapter 30 is not required.
With all of this background information, my professional opinion is that this type of structure only needs to use the wind load provisions from chapter 29. The reasons are as follows.
- The cylindrical shell is part of the main wind force resisting system. There is no "skeleton" that this shell is attaching to. The shell IS the structural element through which wind load will transfer down to the support columns and then into the foundation. Therefore, the walls of this cylindrical structure would not be classified as "cladding".
- The cylindrical structures that I deal with will almost always have a projected area (i.e. tributary area) greater than 700 ft². In the case of this example, the projected area for wind on the cylindrical shell would be 50 ft * 100 ft = 5000 ft². Therefore, the note in 30.2.3 of ASCE 7-16 would indicate using chapter 30 is not required.
The big counterpoint that I have come up with is that the wind load criteria within chapter 29 for "circular bins, silos, or tanks" in seemingly only for structures with closed bottoms (floor plate or ground) and tops (roofs or top closures). Since the structures I am dealing with have an open top AND bottom then there would be a contribution from an internal pressure coefficient - which chapter 30 gives guidance on in 30.12.3 (although it only states for open TOP - not bottom - structures). If I was to use chapter 29, I would use a force coefficient (Cf) of 0.63 and be done with it. If I was to use chapter 30, the magnitude of wind load around the circumference of the cylinder would vary greatly. See below for how this would look.
This variable wind load around the circumference of the shell would make using classical structural analysis (to find the maximum axial, bending, and circumferential stresses at any elevation in the system) either much more difficult or totally unfeasible. The analysis required then would be to do a finite element analysis on the structure to explicitly account for this variable wind load around the circumference of the cylinder.
My goal is obviously to not just do what is convenient from an analysis perspective, but to do what is going to be the most correct method for modeling how wind load is acting on the structure. What do you all think about this general topic and my opinion?
The structure in question is a 50 ft diameter x 100 ft tall cylindrical structure that is open on both the bottom and top (i.e. no floor and no roof). The structure is supported on a number of legs which are simply I-beams with a base plate that is bolted to a concrete foundation. Stiffening rings are located at various elevation to help combat ovalling of the cross-section and excessive circumferential compressive stress. Below is generally how this would look.
Based on my reading and to use the terminology of ASCE 7-16, I would simply classify this structure as a "circular bin, silo, or tank".
ASCE 7-16 has two separate criteria for applying wind load on a circular bin, silo, or tank.
1. Wind load that will be transferred to and through the MWFRS (main wind force resisting system) in chapter 29.4.2.
2. Wind load that will be applied to the components or cladding in chapter 30.12.
My understanding of chapter 30 are that these are for items that are not part of the load resisting frame of a building or structure. For example, for the "skeleton" of a building, the wind load applied would be per chapter 29. For the "skin" (i.e. walls) or things like windows on said building, the wind load would be applied per chapter 30.
Further, 30.2.3 of chapter 30 states that "components and cladding elements with tributary areas greater than 700 ft² shall be permitted to be designed using the provisions for main wind force resisting systems". In other words, if the element in question in larger than 700 ft² in surface area, then using the more highly localized wind load from chapter 30 is not required.
With all of this background information, my professional opinion is that this type of structure only needs to use the wind load provisions from chapter 29. The reasons are as follows.
- The cylindrical shell is part of the main wind force resisting system. There is no "skeleton" that this shell is attaching to. The shell IS the structural element through which wind load will transfer down to the support columns and then into the foundation. Therefore, the walls of this cylindrical structure would not be classified as "cladding".
- The cylindrical structures that I deal with will almost always have a projected area (i.e. tributary area) greater than 700 ft². In the case of this example, the projected area for wind on the cylindrical shell would be 50 ft * 100 ft = 5000 ft². Therefore, the note in 30.2.3 of ASCE 7-16 would indicate using chapter 30 is not required.
The big counterpoint that I have come up with is that the wind load criteria within chapter 29 for "circular bins, silos, or tanks" in seemingly only for structures with closed bottoms (floor plate or ground) and tops (roofs or top closures). Since the structures I am dealing with have an open top AND bottom then there would be a contribution from an internal pressure coefficient - which chapter 30 gives guidance on in 30.12.3 (although it only states for open TOP - not bottom - structures). If I was to use chapter 29, I would use a force coefficient (Cf) of 0.63 and be done with it. If I was to use chapter 30, the magnitude of wind load around the circumference of the cylinder would vary greatly. See below for how this would look.
This variable wind load around the circumference of the shell would make using classical structural analysis (to find the maximum axial, bending, and circumferential stresses at any elevation in the system) either much more difficult or totally unfeasible. The analysis required then would be to do a finite element analysis on the structure to explicitly account for this variable wind load around the circumference of the cylinder.
My goal is obviously to not just do what is convenient from an analysis perspective, but to do what is going to be the most correct method for modeling how wind load is acting on the structure. What do you all think about this general topic and my opinion?
