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#1 Top Question Asked

What are the updates with the ASCE 7-10 programs?

ASCE 7-10 Wind Speed Map Updates

·         The wind speed map for all locations has been revised & Importance Factors have been removed.

·         There are 3 new wind speed maps.  Wind speed maps are provided for each Risk Category as opposed to a single map with importance factors (300 year period, 700 year return period, 1700 year return period). The difference is that all of the previous ASCE 7 Standards had referenced only one wind velocity map for all Risk Categories.

·         New wind speed maps have replaced the existing maps that are directly applicable for determining design wind pressures using the strength design approach.  Different maps are provided for different Risk Categories instead of a single map with importance factors to be applied for each Risk Category.

·         Wind speed values are now represented in the ASCE 7-10 as "Ultimate" wind speeds.

·         Strength design level wind speeds replace the ASD level wind speeds.

·         Comparative hurricane wind speeds are lower than those given in the ASCE 7-05.

·         The wind speeds in the maps are much higher than those in previous editions, the Load Factor on "W" in Section 2.3.2 is now 1.0 instead of 1.6 as established in the ASCE 7-05.

·         Importance Factors have been used in previous editions of the ASCE 7 to adjust the velocity pressure to different annual probabilities of being exceeded. However, the use of Importance Factors was an approximate means for adjusting the return period because the slope of the wind speed vs. return period curves differ. The distance inland where the hurricanes can influence wind speeds increases with the return period. This situation was not adequately addressed by using Importance Factors from a table.

Updates for allowable Wind Speeds That Supersede Wind Speeds Provided In The ASCE 7-10

·         The basic wind speed shall be increased where records or experience indicate that the wind speeds are higher than those reflected in Fig. 26.5-1 of the ASCE 7-10.

·         Mountainous terrain, gorges, and special wind regions shown in Fig. 26.5-1 of the ASCE 7-10 shall be examined for unusual wind conditions.  The authority having jurisdiction shall, if necessary, adjust the values given in Fig. 26.5-1 to account for higher local wind speeds.  Such adjustment shall be based on meteorological information and an estimate of the basic wind speed obtained in accordance with the provisions of ASCE 7-10 Section 26.5.3.

·         For areas outside hurricane-prone regions, regional climate data shall only be used in lieu of the basic wind speeds provided in the ASCE 7-10 Fig. 26.5- when (1) approved extreme-value statistical-alaysis procedures have been employed in reducing the data; and (2) the length of record, sampling error, averaging time, anemometer height, data quality, and terrain exposure of the anemometer have been taken into account.  Reduction in basic wind speed below that of Fig. 26.5-1 shall be permitted.

·         In hurricane-prone regions, wind speeds derived from simulation techniques shall only be used in lieu of the basic wind speeds given in the ASCE 7-10 Fig. 26.5-1 when approved simulation and extreme value statistical analysis procedures are used. The use of regional wind speed data obtained from anemometers is not permitted to define the hurricane wind-speed risk along the Gulf and Atlantic coasts, the Caribbean, or Hawaii.

·         In areas outside hurricane-prone regions, when the basic wind speed is estimated from regional climatic data, the basic wind speed shall not be less than the wind speed associated with the specified mean recurrence interval, and the estimate shall be adjusted for equivalence to a 3-second gust of wind speed at 33 ft (10 m) above the ground in Exposure C.

ASCE 7-10 Importance Factor Update

·          In the equation for Velocity pressure (qz) the importance factor has been removed, and the coefficient 0.00256 (0.613 in SI) shall be used except where sufficient climatic data are available to justify the selection of a different value of this coefficient/factor for a design application.

ASCE 7-10 Category “D” Update

·         Surface Roughness Category "D" now applies to all water surfaces including water surfaces in hurricane prone regions.

ASCE 7-10 Updates for the State of Hawaii

·         Entire State of Hawaii has its own special wind region.

·         The reasoning behind this is due to the highly complex three-dimensional topography in the State of Hawaii. This conclusion was reached by numerous studies. The topography has speed-up effects that cannot be adequately portrayed by a single statewide value of wind speed nor at the macro-scale of a national map.  The State of Hawaii has addressed this issue with the development of wind maps for each local jurisdiction.

·         For these new "special region maps" for the State of Hawaii, you must reference the Hawaii State Building Code.

·         Although the probabilistic reference wind speeds are provided for Hawaii in the ASCE 7-10, the intent is that the actual design wind speeds are to be further modified as determined from the authority having jurisdiction. Wind speeds are identified simply to provide the reference wind speed for each Risk Category, and also ensure that the wind-borne debris region criteria in the ASCE 7-10 is appropriately triggered by the net value of net effect wind value.

Reference: General Requirements for Determining Wind Loads & ASCE 7-10

#2 Top Question Asked

What is the difference between the Standard Edition and the Building Permit Edition for both ASCE 7-02 & 7-05 programs?

The Standard Edition strictly calculates the wind load pressures according to the Building Information that is entered in.  The Building Permit Edition (BPE) has additional information that is required by your building permit department.  Diagrams and the required information have been added.  There are separate sheets that are automatically created based on the input.  For example, if you have a Gable Roof, then you would select the Gable Roof tabbed sheet and the information and diagrams are automatically created.  All you have to do is hit ‘print’ and you have all your wind load information to submit to your Building Permit Department.  The same is true for all other roof types, windows, and doors.  This makes the wind load permit process easy for you and your permit department.  Winloadcalc.com has created the BPE to make the rigorous wind load process a breeze.

#3 Top Question Asked

What is the difference between the ASCE 7-98 and the Latest ASCE 7-02 & 7-05?

First off, your building permit department will deny your information if it does not abide by the latest ASCE 7.  Versions of ASCE 7-98 and lower are obsolete.  They should not even be practiced.  There have been major changes.  Below are some major changes that would affect your design according to C&C.

- Method 1 has been modified.  Roof slope calculations have been modified.

- Exposure A is no longer used.

- Parapet calculations were added.

- Permission to interpolate the exposure category for the surface roughness length parameter, using an acceptable/reasonable procedure.

#4 Top Question Asked

What is the difference between the Zones for the Walls and Roof? (You can also see examples on our Instructions page (Link to Instructions page).

The “Zones” are the locations on the walls and roof of a structure that identify the locations that have more pressure applied to them than other locations.  The Zones that have see the most pressure are the corner Zones of the walls (Zone 5), and the perimeter Zones of the roof (Zones 2 & 3).  The interior Zone pressures do not have as much pressure applied to them as the corner and perimeter Zones.  The interior Zone of the wall is Zone 4, and for the roof is Zone 1.

The value of “a” (which is automatically calculated for you in our program) will be your starting point.  This value (a) is the horizontal dimension that provides you with Zone 5 for your walls and Zone 3 for (the corner of) your roof.  You measure from the corner, inwards.  For your walls, Zone 4 will be the remaining area between the Zones 5’s measured from each corner, and for your roofs, Zone 2 will be the remaining area between the Zone 3’s measured from each corner.  Also, for roofs, Zone 1 is the entire area left over from Zones 2 & 3.  Additionally for the roofs, you can divide up the areas for the Zones (1,2, & 3) to be the areas measured between your rafters; since this is the area between the tie-downs. 

Below is a diagram that should help you visualize how to break up the zones.

Note:

1. Zones 1, 2, & 3 are always applied to the roof.

     a. Zone 3: are the edge or corner sections; always equal to the value of “a”.

     b. Zone 2: are the perimeter sections; minus the Zone 3 (edge/corner) areas.

     c. Zone 1: are the interior sections. Basically the interior area left over after

         subtracting the areas of the Zone 3, and Zone 2.

2. Zones 4 & 5 are always applied to the walls.

     a. Zone 5: are the corner sections; always equal to the value of “a”.

     b. Zone 4: are the interior sections; or the area remaining after subtracting the

         Zone 5 sections.

3. Remember: The Value of “a” is always applied to the roof and wall corners.

4. Remember: Any door or window dimension that falls within the area of “a”

    (edge/corner = Zone 5) section must have the Zone 5 applied to it.

5. Remember: Any opening dimension that falls with the area of “a” (edge/corner =

    Zone 3) on the roof (such as a sky-light) must have the Zone 3 applied to it.

 #5 Top Question Asked

Can you explain windward and leeward pressures?

 It is assumed that the force the wind could exert directly on the windward side of the building will act as a vacuum force on the lee side of the building and it is this vacuum that causes lift.  Therefore, it is taken that the uplift force is equal to the force applied to the windward side of the building and conventionally this is given a negative sign.

Frequently Asked Questions about the Wind Provisions of ASCE 7

1. Why have the Velocity Maps changed in various ASCE Standards?

You may be wondering why the maps have changed from the previous wind velocity maps. Well, since the development of the model used for the ASCE 7-98 wind speed map, significantly more hurricane data has become available which improves the modeling process.  The new hurricane hazard model indicates that the hurricane wind speeds given in ASCE 7-05, 7-02, and 7-98 are conservative.  This conservatism is evident even though the overall rate of intense storms produced by the new model is higher compared to the rate of intense storms produced by the model used to develop the wind speed maps in ASCE 7-98 through ASCE 7-05.

2. Is it possible to obtain larger scale maps of basic wind speeds (see Figures 26-5.1A-C) so that the locations of the wind speed contours can be determined with greater accuracy?

No. The wind speed contours in the hurricane-prone region of the United States are based on hurricane wind speeds from Monte Carlo simulations and on estimates of the rate at which hurricane wind speeds attenuate to 90 mph following landfall. Because the wind speed contours of these figures represent a consensus of the ASCE 7 Task Committee on Wind Loads, increasing the map scale would do nothing to improve their accuracy.

3. IBC Figure 1609 gives the 3-s wind speed at the project location. However, according to the Notes, Figure 1609 is for Exposure C. If the project location is Exposure B, what is the proper wind speed to use?

The basic wind speed in IBC Figure 1609 or ASCE 7 is defined as a 3-s gust wind speed at 33 ft above ground for Exposure Category C, which is the standard measurement. The velocity pressure exposure coefficient, K_z, adjusts the wind speed for exposure and height above ground. However, for simplicity the coefficient is applied in the pressure equation, thus adjusting pressure rather than wind speed. Use of K_z adjusts the pressures from Exposure C to Exposure B.

4. If the design wind loads are to be determined for a building that is located in a special wind region (shaded areas) in Figures Figures 26-5.1A-C, what basic wind speed should be used?

The purpose of the special wind regions in these figures is to alert the designer to the fact that there are regions in which wind speed anomalies are known to exist. Wind speeds in these regions may be substantially higher than the speeds indicated on the map, and the use of regional climatic data and consultations with a wind engineer or meteorologist are advised.


5. Do I consider a tilt-up wall system to be components and cladding (C&C) or MWFRS or both?

Both. Depending on the direction of the wind, a tilt-up wall system must resist either MWFRS forces or C&C forces. In the C&C scenario, the elements receive the wind pressure directly and transfer the forces to the MWFRS in the other direction. When a tilt-up wall acts as a shear wall, it is resisting forces of MWFRS. Because the wind is not expected to blow from both directions at the same time, the MWFRS forces and C&C forces are analyzed independently from each other in two different load cases. This is also true of masonry and reinforced-concrete walls.

6. When can I use the one-third stress increase specified in some material standards?

When using the loads or load combinations specified in ASCE 7, no increase in allowable stress is permitted except when the increase is justified by the rate of duration of load (such as duration factors used in wood design). Instead, load combination #6 from Section 2.4.1 of ASCE 7 was added for the case when wind load and another transient load are combined. This load combination applies a 0.75 factor to the transient loads ONLY (not to the dead load). The 0.75 factor applied to the transient loads accounts for the fact that it is extremely unlikely that two maximum events will happen at the same time.

7. Why can the wind directionality factor (K_d) only be used with the load combinations specified in Sections 2.3 and 2.4 of ASCE 7?

In the strength design load combinations provided in previous editions of ASCE 7 (ASCE 7-95 and earlier), the 1.3 factor for wind included a "wind directionality factor" of 0.85. In ASCE 7-98, the loading combinations used 1.6 instead of 1.3 (approximately equals 1.6 x 0.85), and the directionality factor is included in the equation for velocity pressure. Separating the directionality factor from the load combinations allows the designer to use specific directionality factors for each structure and allows the factor to be revised more readily when new research becomes available.

8. What pressure coefficients should be used to reflect contributions for the underside (bottom) of the roof overhangs and balconies?

Sections 28.4.3 and 30.10 specify pressure coefficients to be used for roof overhangs to determine loads for MWFRS and C&C, respectively. No specific guidance is given for balconies, but use of the loading criteria for roof overhangs should be adequate.

9. Under what conditions is it necessary to consider speed-up due to topographic effects when calculating wind loads?

Section 26.8 of the Standard requires the calculation of the topographic factor, K_zt, for buildings and other structures sited on the upper half of isolated hills or escarpments located in Exposures B, C, or D where the upwind terrain is free of such topographic features for a distance of at least 100 h or 2 mi, whichever is smaller, as measured from the crest of the topographic feature. K_zt need not be calculated when the height, H, is less than 15 ft in Exposures D and C, or less than 60 ft in Exposure B. In addition, K_zt need not be calculated when H and L_h is less than 0.2. h and L_h are defined in Figure 26.8-1. The value of K_zt is never less than 1.0.

10. What constitutes an open building? If a process plant has a three-story frame with no walls but with a lot of equipment inside the framing, is this an open building?

An open building is a structure in which each wall is at least 80% open (see Section 6.2). Yes, this three-story frame would be classified as an open building, or as "other" structure. In calculating the wind force, F, appropriate values of C_f and A_f would have to be assigned to the frame and to the equipment inside.

11. Flat roof trusses are 30 ft long and are spaced on 4-ft centers. What effective wind area should be used to determine the design pressures for the trusses?

Roof trusses are classified as C&C since they receive wind load directly from the cladding (roof sheathing). In this case, the effective wind area is the span length multiplied by an effective width that need not be less than one-third the span length or (30)(30/3) = 300 ft². This is the area on which the selection of GC_p should be based. Note, however, that the resulting wind pressure acts on the tributary area of each truss, which is (30)(4) = 120 ft².

12. Roof trusses have a clear span of 70 ft and are spaced 8 ft on center. What effective wind area should be used to determine the design pressures for the trusses?

Following the approach of question #12, above, the effective wind area is (70)(70/3) = 1,633 ft². The tributary area of the truss is (70)(8) = 560 ft², which is less than the 700-ft² area required by Section 30.2.3 to qualify for design of the truss using the rules for MWFRS. The truss is to be designed using the rules for C&C, and the wind pressure corresponding to an effective wind area of 1,633 ft² is to be applied to the tributary area of 560 ft².

13. Metal decking consisting of panels 20 ft long and 2 ft wide is supported on purlins spaced 5 ft apart. Will the effective wind area be 40 ft² for the determination of pressure coefficients?

Although the length of a decking panel is 20 ft, the basic span is 5 ft. According to the definition of effective wind area, this area is the span length multiplied by an effective width that need not be less than one-third the span length. This gives a minimum effective wind area of (5)(5/3) = 8.3 ft². However, the actual width of a panel is 2 ft, making the effective wind area equal to the tributary area of a single panel, or (5)(2) = 10 ft². Therefore, GC_p would be determined on the basis of 10 ft² of effective wind area, and the corresponding wind load would be applied to a tributary area of 10 ft². Note that GC_p is constant for effective wind areas less than 10 ft².

14. A masonry wall is 12 ft in height and 80 ft long. It is supported at the top and at the bottom. What effective wind area should be used in determining the design pressure for the wall?

For a given application, the magnitude of the pressure coefficient, GC_p, increases with decreasing effective wind area. Therefore, a very conservative approach would be to consider an effective wind area with a span of 12 ft and a width of 1 ft, and design the wall element as C&C. However, the definition of effective wind area states that this area is the span length multiplied by an effective width that need not be less than one-third the span length. Accordingly, the effective wind area would be (12)(12/3) = 48 ft².

15. If the pressure or force coefficients for various roof shapes (e.g., a canopy) are not given in ASCE 7, how can the appropriate wind forces be determined for these shapes?

With the exception of pressure or force coefficients for certain shapes, parameters such as V, K_z, K_zt, and G are given in ASCE 7. It is possible to use pressure or force coefficients from the published literature provided these coefficients are used with care. Mean pressure or force coefficients from other sources can be used to determine wind loads for MWFRS. However, it should be recognized that these coefficients might have been obtained in wind tunnels that have smooth, uniform flows as opposed to more proper turbulent boundary-layer flows. Pressure coefficients for components and cladding obtained from the literature should be adjusted to the 3-s gust speed, which is the basic wind speed adopted by ASCE 7.

16. Section 26.2 of the Standard provides definitions of glazing, impact resistant; impact-resistant covering; and wind-borne debris regions. To be impact resistant, the Standard specifies that the glazing of the building envelope must be shown by an approved test method to withstand the impact of wind-borne missiles likely to be generated during design winds. Where does one find information on appropriate test methods?

Section 26.10.3.2 of the Standard refers to two ASTM standards. These standards give test method and performance criteria of glazing, doors, and shutters when impacted by wind-borne debris.

17. The Standard does not provide for across-wind excitation caused by vortex shedding. How can one determine when vortex shedding might become a problem?

Vortex shedding is almost always present with bluff-shaped cylindrical bodies. Vortex shedding can become a problem when the frequency of shedding is close to or equal to the frequency of the first or second transverse of the structure. The intensity of excitation increases with aspect ratio (height-to-width or length-to-breadth) and decreases with increasing structural damping. Structures with low damping and with aspect ratios of 8 or more may be prone to damaging vortex excitation. If across-wind or torsional excitation appears to be a possibility, expert advice should be obtained.

18. If high winds are accompanied by rain, will the presence of raindrops increase the mean density of the air to the point where the wind loads are affected?

No. Although raindrops will increase the mean density of the air, the increase is small and may be neglected. For example, if the average rate of rainfall is 5 in./h, the average density of raindrops will increase the mean air density by less than 1%.

19. What wind loads do I use during construction?

ASCE 7 does not address wind loads during construction. Construction loads are specifically addressed in the standard SEI/ASCE 37-02, Design Loads on Structures during Construction.

20. Can the pressure/force coefficients given in ASCE 7-10 be used with the provisions of ASCE 7-88, 7-93, 7-95, 7-98, -02, or 7-05?

Yes, in a limited way. ASCE 7-88 (and 7-93) used the fastest-mile wind speed as the basic wind speed. With the adoption of the 3-s gust speed starting with ASCE 7-95, the values of certain parameters used in the determination of wind forces have been changed accordingly. The provisions of ASCE 7-88 and 7-02 should not be interchanged. Coefficients in ASCE 7-95, 7-98, 7-02 and 7-05 are consistent; they are related to 3-s gust speed.

21. Is it possible to determine the wind loads for the design of interior walls?

The Standard does not address the wind loads to be used in the design of interior walls or partitions. A conservative approach would be to apply the internal pressure coefficients GC_pi = ±0.18 for enclosed buildings and GC_pi = ±0.55 for partially enclosed buildings. Post-disaster surveys have revealed the failure of interior walls when the building envelope has been breached.

Some of the FAQ questions above are reprinted from "Guide to the Use of the Wind Load Provisions of ASCE 7-02"
By Kishor C. Mehta and James M. Delahay

Q) Is there a typo in the new ASCE 7-05 Standards on Figure 6-21 for the ‘Round ([D * sqrt of qz] > 2.5). In the '"Very Rough" row and the "25 – h/D column"?

A: Yes. Instead of 0.2, it should be 1.2

Signs

Q) Why do I have to enter a frequency and damping ratio for both the sign and the supports?
A) Each part of the sign has its own frequency and damping ratio. If both are used, they are averaged into the calculation.

Q/A) If you have all the “Sign Information” filled in to the best of your knowledge, but your force output is zero……Verify your “Open Area of Sign” for the value of sigma is most likely not between 0.1 & 0.7.

Q/A) Value of “Kd” is only used in conjunction with load combinations specified in ASCE 7-02, Section 2, 2.3 & 2.4. So, if your calculation consists of a combination of loads, say for ice, flood loads, select “Yes” from the pull-down list for “Kd.”

Q) Can the Signs Program be used for Solid Free Standing Wall & Lattice Framework calculations?
A) Yes, but be sure to follow the ASCE 7-02 handbook on pages 70 &71.

Q) Is the equation for Rl = Rb correct?
A) No. The value of ‘B’ is missing. It should be: Rl = Rb = (4.6*n1*B)/Vz (bar on both).

Walls

 Q) Why does my “Effective Area” value equal something other than what I inputted?

A) The program calculates the necessary adjustments, according to the ASCE 7-02 handbook. This typically results in cathedral type windows; very tall and narrow.

Helpful Information for Windload Design Program 

Program works in Windows Excel only.

·                    Opening information for maximum and minimum pressures are given for all openings with any given Building Information.

·                    Components & Cladding Wind Loads are given per code requirements for all heights above and below 60 feet.

·                    You can adjust the program to fit your computer screen by adjusting the percentage on your ‘STANDARD’ toolbox.  Or go to the ‘VIEW’ pull down menu.  Go to ‘TOOLBARS.’  Select ‘STANDARD.’  Now you can adjust your screen size percentage.

·                    If you notice that your value(s) in the cell(s) for ‘Total Area’ do not coincide with your values in the width & height columns, it is because the values have been adjusted for the most appropriate value for that given area.  According to Florida Building Code, 2001 catalog in Section 1606.1.5 under Effective Wind Area; page 16.6.

·                    When needed, information is interpolated to give the most precise value.

EXCEL imported into AUTOCAD

Follow these steps to insert an Excel spreadsheet into AutoCAD.

When you are finished with your Excel spreadsheet:

a.   Go to FILE

b.   Select Print Area

c.   Select Set Print Area (Select what you want to show up in AutoCAD)

d.   Select Edit

e.   Select Copy

2.         Leave Excel spreadsheet open.  (If you save after you do the Copy command,       

Go back to Edit and select Copy.  In other words, Copy should be the last thing done.)

Go to AutoCAD 2000, 2002, 2004, 2005, 2006

a.   Select Edit

b.   Select Special Paste (after this, a window pops up on your screen)

c.   Select Paste Link

d.   Select OK

Right-click on the spreadsheet just inserted

a.       Select Properties (here you can scale your inserted Excel spreadsheet)

                                                               i.      Just a hint.  Typing in the value 0.2 works well when scaling down, under the text size area. (i.e.: Arial      18 or 10   =   0.2)

From this point on, any changes made in the excel spreadsheet will be updated automatically in AutoCAD.

However:  If you make changes outside of the selected Print Area, those changes will not appear in AutoCAD.  Meaning, if you added onto a schedule be sure that it is within the selected Print Area.  If not, you will have to go through the procedure again.

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