Design and Maintenance of Green House 3(2+1)

Lesson 7 Design Load Calculations: Part II

7.1.2 Definitions

• Windward -toward the wind; toward the point from which the wind blows

• Leeward -the side or point to which the wind blows

• Simple Diaphragm Building –While traditional greenhouse coverings are not considered diaphragm materials, a horizontal truss system at the roof level will transfer lateral loads to vertical lateral force-resisting systems and can be considered a diaphragm.

7.1.3 Design Procedure

Design wind loads for greenhouses shall consider:

• The basic wind speed, V

• The velocity pressure q_z, where z is the height, which is calculated taking into consideration the exposure category, the surrounding terrain, the wind directionality, and the occupancy of the structure.

• The design wind pressure p, which is calculated taking into consideration the direction of the wind, the exposure category, the height of the building or element, and the openness of the structure. 

7.1.4 Calculation of Wind Loads

7.1.4.1 General: The design wind loads, pressures and forces are determined by the appropriate equations given in ASCE 7, Section 6.5.12 or 6.5.13; or in the case of the simplified procedure, found in Figures 6-3 and 6-4 of ASCE 7.  Gust effect factors and pressure coefficients are found in figures and tables in ASCE 7.

7.1.4.2  Basic Wind Speed: The basic wind speed, V, in miles per hour, for the determination of the wind loads shall be found in a figure in the referenced code or standard being used.

7.1.4.3  Importance Factor: Greenhouses shall be assigned a wind load importance factor, I_w, in accordance with the following Table:

Table 7.1 - Classification of Greenhouses for Wind Load Importance Factors

Category ASCE 7    IBC	Nature of Occupancy and Location of Greenhouse	Wind Factor Iw
II               I	All commercial greenhouses that are not in ASCE 7 Category I (IBC Category IV)	1.00
I              IV	Production greenhouses in non-hurricane prone regions and in hurricane prone regions with V = 80 -100 mph and Alaska	0.87
I              IV	Production greenhouses in hurricane prone regions with V >100 mph	0.77

 

7.1.4.4 Wind Speed-up Over Hills and Escarpments, K_zt

Wind speed-up over isolated hills and escarpments that constitute abrupt changes in the general topography shall be considered for buildings and other structures sited on the upper half of hills and ridges or near the edges of escarpments.  The effect of wind speed -up shall not be required to be considered when hill height to distance upwind of crest of hill ration H/L_h < 0.2, or when height of hill H < 15’ for Exposure D, or H< 30’ for Expo sure C, or H < 60’ for all other exposures. Factor K_zt shall not be less than 1.0.  Refer to Sec. 6.5.7 of ASCE 7 for further information.

7.1.4.5 Wind Directionality Factor: A wind directionality factor, K_d, shall be used in the analytical method of determining the wind velocity pressure, q_z, per Sec. 6.5.10 and 6.5.4.4 of ASCE 7.

Care should be taken in applying the wind directionality factor, which is a number less than 1.0.  By ASCE 7 definition, the factor is to be used with ASCE load combinations, and is contrary to use of the IBC load combinations.  

7.1.4.6 Exposure Categories: For each wind direction considered, an exposure category that adequately reflects the characteristics of ground surface irregularities shall be determined for the site at which the greenhouse is to be constructed. For a site located in the transition zone between categories, the category resulting in the largest wind forces shall apply. Account shall be taken of variations in ground surface roughness that arise from natural topography and vegetation as well as from constructed features. For any given wind direction, the exposure in which a specific greenhouse is sited shall be assessed as being one of the exposure categories A, B, C, or D.

7.1.4.7 Enclosure Classifications:  All buildings are classified as enclosed, partially enclosed, or open.  Whether the IBC or ASCE 7 is used to determine wind loads, the enclosure classifications are essentially identical.  In wind -borne debris regions, special consideration is given to glazing with respect to the determination of openness.  ASCE 7 continues beyond the basic definitions to provide for clarification of buildings that fall under multiple classifications, by stating if a greenhouse by definition complies with both the “open” and “partially enclosed” definitions, it shall be classified as an “open” building.  A greenhouse that does not comply with either the “open” or “partially enclosed” definitions shall be classified as an “enclosed” building.

7.1.4.8 Velocity pressure q_z : When using the analytical method in calculating the wind loads, the velocity pressure at height z is calculated by factoring the given basic wind speed with the velocity pressure exposure coefficient K_z, the wind speed -up factor K_zt, the wind directionality factor, K_d, and the importance factor I.  Refer to Sec. 6.5.10 of ASCE 7.

7.1.4.9   Internal & External Pressure Coefficients and Gust Effect Factors, G_cpi: Internal and external pressure coefficients, and gust effect factors are needed when using the analytical method of determining wind pressures.  The factors are found in Sec. 6.5.11 of ASCE -7, based on physical characteristics of the structure and the site. 

7.1.4.10 Design Loads and Wind Pressures:  No matter which method is used in determining wind loads on a structure, the goal is to determine the worst case loading on the main wind force resisting system and on the components and cladding. 

Using the IBC simplified method of Sec. 1609.6.2, design wind pressures are given in Tables 1609.6.2.1 and are multiplied by the appropriate factors for height, exposure, and importance.

When using the simplified method of Sec. 6.4.2 in ASCE 7, design wind pressures are found in Tables 6-2 and 6-3, and are adjusted by importance, exposure or area reduction factors.

When using the analytical method of Sec. 6.5.12 in ASCE 7, design wind pressures are calculated by factoring the wind velocity pressure with internal and external pressure coefficients and gust effect factors.  Sec. 6.5.13 in ASCE 7 gives the equation that is used in determining the design wind force for open buildings.

7.1.5 Wind and Seismic Detailing

The IBC requires that lateral force-resisting systems shall meet seismic detailing requirements and limitations prescribed in the code, even when wind code prescribed load effects are greater than seismic load effects, per Sec. 1609.1.5.  Seismic requirements in the IBC (Sections 1616.4 & 1620.1) state that all parts of the structure shall be interconnected. These connections are designed to resist the seismic force, F_p, induced by the parts being connected.  Any smaller portion of the structure shall be tied to the remainder of the structure with a connection that shall be capable of transmitting the greater of 0.133 times the design, 5% damped, spectral response acceleration for short periods (S_DS ) times the weight of the smaller portion, or 5% of the weight of the smaller portion to a larger portion of the structure. Each beam, girder, or truss member shall be provided with a positive connection to its support for resisting horizontal forces acting on the member.  This support connection shall have sufficient strength to resist 5% of the dead and live load vertical reaction applied horizontally.  Similar seismic detailing requirements are found in ASCE 7, Section 9.5.2.6. 

7.2 SEISMIC LOADS 

7.2.1 Seismic Design -Background

Seismic design no longer uses the concept of seismic zones. Instead it uses maps, soil type and occupancy. The seismic maps in the building code and ASCE 7 are based on recent work by the US Geological Service. Some areas of the country have had their seismicity reduced. A number of areas are now in seismic zones that never were considered as areas having seismic potential. Seismic design requires determination of the Seismic Design Category (SDC). The SDC is a classification assigned to a structure based on its occupancy (Seismic Use Group) and the level of expected soil modified seismic ground motion. The SDC is determined by: 

• the anticipated earthquake ground accelerations at the site,

• the type of soil at the specific site and

• the Seismic Use Group (SUG)

Because earthquake design seldom governs for greenhouses, designers may find that the use of default values may reduce the amount of calculations. All greenhouse structures would be Seismic Use Group I. The default soil type, Site Class D, simplifies the determination of the SDC. Designers will have to determine the site ground shaking (S_s and S₁) by use of the applicable seismic map. These seismic maps are contained in the building code and ASCE 7.

Using S_s and S₁ and the Site Class (soil type), coefficients S_DS and S_D1 are computed. Then based on these computed values and the Seismic Use Group, the Seismic Design Category can be determined from the tables in the code or ASCE 7. The SDC directs users to specific code requirements. SDC A has minimum requirements, whereas an SDC E structure would have numerous analysis and detailing requirements.

Exceptions in the seismic design requirements (IBC 1614.1, Exception 3) include exemptions for agricultural storage buildings intended only for incidental human occupancy, areas with low S_s and S₁ values and for computed S_DS and S_D1 with low values. Most production greenhouses should qualify for the agricultural exemption. However individual state and local regulations may still require design of all agricultural structures.

Once the seismic design category is determined, an R-value ( a measure of the ductility of the structure) is determined from the building code (IBC Table 1617.6 or ASCE 7 Table 9.5.2.2). Greenhouse structures appear to qualify as ordinary steel concentrically braced frames, which have an R-value equal to 5. If a greenhouse is mounted on the roof of another structure, the R value for the greenhouse is independent of that underlying structure. The connection reactions for the greenhouse shall be applied to the underlying structure’s roof, just as roof-mounted equipment would be, and the supporting structure’s roof shall be designed for those loads, considering all applicable load combinations. 

Designers will have to determine whether such earthquake design loads, and the installed equipment, are critical compared to wind loads. For a greenhouse, this will depend on the location and mass of the structure and its equipment compared to the exposed areas that the building presents.

7.3 Other Loads

7.3.1  Flood and hydrostatic

7.3.1.1  Soil and hydrostatic pressure and flood loads - Local regulations will identify flood design zones. Whether such criteria are critical for a greenhouse will depend on FEMA and local requirements.

7.3.2  Other Loads

Other design factors the engineer should consider in individual structures include:

• Thermal expansion and the need for joints

• Rainwater

References:

1. www.ngma.com : Design Considerations, Chapter 2-7.