Asce 7-16 Section 29.4.4

[Latarsha Dorrance]

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I would like to explore a little bit more Chapter 29.4.4 of ASCE 7-16 "Rooftop Solar Panels Parallel to the Roof Surface on Buildings of All Heights and Roof Slopes". In particular the introduction of the two new factors; γE = array edge factor and γa = Solar panel pressure equalization factor. Why this? This is a major step compared to 7-10 since this section is dedicated specifically to Solar PV Modules.

As you can see, these 2 factors are directly proportional to the final design wind pressure, these 2 factors combined can yield a multiplier from 0.4 to 1.2, that is why is so important to know how to take advantage of these system and design the array in a way that your combined factor is as low as possible.

These tests have shown that when the pressures in the underside of the PV modules approach those on the upper surface, the net pressure acting across the layer can be substantially reduced due to pressure equalization that occurs across the surfaces of the array. This effect occurs thanks to 2 unique characteristics, the cavity space and a small gap between modules. As the tributary area of the array increases the net array wind loads decrease, thus there is a significant benefit to connecting the modules together to reduce net wind load.

This equalization doesn't occur equally across the full array, observations have shown that modules close to the edge of the array experienced higher loads that interior modules. To represent the effects of equalization a multiplication design factor γa is introduced, it is called "solar panel (or array) pressure equalization factor". ASCE 7-16 uses the terms "panel" and "array" interchangeable which might bring confusion if the concept of "equalization" is not clear enough. The value for γa is plotted in FIGURE 29.4-8 as a function of the Effective Wind area of the structural element considered.

Here the code lack a bit of clarification if the distance d1 could be measured to the ridge edge or hip edge for roof with a tilt greater than 7 degrees. An argument could be made that the definition of "roof edge" should be consistent with the same definition use from fig. 30.3-2A-I to calculate the External pressure coefficients, this is, from ridge and hip edges.

As seen, we are interested in keeping γa*γE as low as possible in order to reduce load on the array and reduce costs in things like number of attachments, type of module used, number of rails, attachment spans and attachment type. Here is a few ideas to keep this pressure low;

It is important to be able to not only understand the new ASCE 7-16 29.4.4 chapter but also to be able to take advantage of it. I hope this article have shed some light on this section and can help you improve your design quality and save you money.

Disclaimer: I'm not an structural engineer and this is only my personal opinion based on my interpretation of the code. I'm not responsible for any decision taken based on this article, ASCE 7-16 is complex and filled with nuances, the intent of this article is not to cover every inch of the code, but to give a broad view about this topic. Thanks.

ASCE 7 has a method for calculating wind loads on rooftop Structures and equipment for buildings, and this article will describe that method. Fortunately, the method is relatively straight forward, not too complicated, and is covered in ASCE 7-16 Section 29.4.1

In section 29.4.1 of ASCE 7, it coveres all rooftop structures and equipment EXCEPT solar panels and structures identified in section 29.4 (chimneys, tanks, open signs, single-lane open frames, and trussed towers). We have covered the method for calculating the wind load on solar panels in a separate article. The most common rooftop equipment that would fit this criteria would be Heating Ventilation and Air Conditioning (HVAC) units.

The lateral force acting on the equipment is calculated using equation 29.4-2. An corresponding excerpt from ASCE 7-16 Sec 29.4.1 is shown with Equations 29.4-2. This is a very simple equation made up of the terms GCr, qh and Af.

The vertical uplift force on the equipment is calculated using equation 29.4-3, which is shown in the corresponding excerpt. This equation is very similiar to Equation 29.4-2, which is used for lateral force. The value of GCr is determined based upon the value of Ar, and if 0.1*B*L For the lateral force calculation we now need to determine the GCr, and to do that we need to determine where Af is in relation to 0.1*Bh and Bh. B is the width of the building normal to the wind direction, and so B will be different for each direction we consider.

For the vertical force calculation, we need to determine GCr, and this is dependent upon Ar. These calculations are also dependent upon B that we used earlier, but it also uses L which is the building dimension in the direction of the wind.

In the commentary of ASCE 7-16 (Section C29.4.1) there is some discussion about Mechanical Equipment Screens. These are often used to conceal plumbing, electrical, or mechanical equipment from view. They are defined as rooftop structures not covered by a roof and are located away from the edge of the building roof such that they are not considered a parapet. These can be solid or porous panels, the latter of which allows some wind to flow through the panel. The commentary states that little research is available to provide guidance for determining wind loads on these screens as well as the equipment behind the screens. Consequently, ASCE 7-16 recommends that the screens (whether solid or porous) and all of the equipment behind the screens should be designed for the FULL wind load determined in accordance with Section 29.4.1. The only exception to this is when the situation has been appropriately analyzed using the Wind Tunnel Procedure in Chapter 31.

The first section of this article explained the basic science and forces that act upon buildings and roofs. You may now be wondering how to apply that information? How are forces, fastening patterns, and adhesive application rates determined for roof systems? This is a complicated topic, one on which thousands of pages of detailed information have been published by testing agencies and engineering firms. This article is intended as a light introduction to the topic.

As you would expect (with all other factors being equal), the roof of a short wide building, like a warehouse, will experience far less winds and stresses than those imposed on the roof of a tall skyscraper. (Think about the last time you traveled from home on a mildly windy day to a downtown area with tall buildings where the mild winds had morphed into gusts).

The ASCE 7-16 building specifications break a roof into three separate roof zones: corners, perimeters and center. The corners and perimeters experience much greater uplift pressures than the center of the roof and typically require increased fastening rates. Individual uplift pressures and fastening requirements are calculated separately for each of the roof zones.

The ASCE 7-16 specifications include the mathematic formulas necessary to calculate uplift pressures. A component of the formulas are pressure multipliers linked to risk factors. ASCE 7-16 includes a couple charts listing conditions and their associated multipliers.

Buildings deemed to have a lower risk to human life such as storage buildings will have smaller multipliers and thus be required to meet less stringent uplift pressures than buildings considered essential such as hospitals.

In conclusion, many calculations and factors must be considered in order to follow the proper building codes for a commercial roof. The roofs specifications greatly depend on the environment and shape of the building. These factors make it necessary to insure that you have a quality roofing contractor working for you. Read Part 3 here. Part will be coming soon. Subscribe to our emails to get them in your inbox.

Doctrines of earthquakes and also the latest approaches of earthquake resistant building design in standards need to be revised periodically. While the revisions and updates in the American standards occur over periods of three or five years including limited subjects, in Turkey the same revisions are done once over long periods including the whole subjects of the standards. As examples the standards of 1975, 1998, 2007 and finally 2018 could be given. Especially, in 2018 standard (TBEC-2018) many changes were made over concepts and criteria. The procedure of calculating the earthquake loads in 2018 standards is similar to the one in the American standards of (ASCE-7-16), however for the element design the changes are shown as developments over the one of 2007 earthquake standard (TEC-2007). The changes made by 2018 standard for calculations of earthquake loads and their effects on civil engineering are very important factors of new building design. The earthquake load affecting a building which is the first factor of earthquake resistant building design shows important differences according to the condition changes in the standard. Based on this motivation in this study reinforced concrete frame type buildings of different elevations were researched by using ETABS (structural software for building analysis and design) according to linear equivalent seismic load method. According to the analysis results of the chosen buildings, a comparison forthe base shear force, top displacement and relative story displacement between TEC-2007, TBEC-2018 and ASCE 7-16 standards was carried out. From the analysis results, it is found that for most of the soil classes while the maximum base shear forces in 3 and 5- story buildings are achievedat TEC-2007, the maximum base shear forces in 7 and 9- story buildings are achieved at TBEC-2018. Also, it is predicted that the higher increment in the design forces of buildings with higher elevations is obtained at TBEC-2018 for strong soils, and at TEC-2007 for weak soils. By considering cracked sections at TBEC-2018 the calculations displacement and period was affected as periods in TBEC-2018 were increased by almost 34% respected to TEC-2007. The same increment ratio was determined for ASCE 7-16 as 45%. Also, as a response for the increments in period, the spectral acceleration determined from the elastic spectrum diagram was decreased. At the end of the study, nonlinear performance analysis was also performed and performance points were determined according to the demand spectra of the seismic codes. ASCE's demand displacement values are in any case lower than Turkish codes. TBEC-2018 reveals less displacement demands in high-rise buildings than TEC-2007. The closest results for the three regulations occurred on the softest grounds.According to the results obtained from the static pushover analysis, a ductile behavior occurred in all of the structural systems and plastic hinge mechanism started from the beams firstly.

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