Old Notes

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Notes on Shear Wall Footings
Notes on Masonry Cantilever Retaining Walls
Notes on Masonry Screen or Fence walls
Notes on Wind Pressures on Components and Cladding of Enclosed and Partially
    Enclosed Buildings up to 60 ft.
Notes on Miscellaneous Subjects

Notes on Shear Wall Footing

06021a: What is the difference between FtgShrWall-00 and FtgShrWall-01?
For overturning and sliding, FtgShrWall-00 checks the traditional load combinations of D+L, D+L+W and D+W while FtgShrWall-01 checks the basic load combinations of the International Building Code 2000 and the Florida Building Code 2001 which are compatible with the state-of-the-art ASCE 7-98 load combinations.

06021b: What wind directionality factor should be used with FtgShrWall-01?
A wind directionality factor of 1.00 should be used because shear capacity and steel reinforcing in the footing are determined by the load combinations of ACI 318-99 Sec. 9.2 which is not compatible with ASCE 7-98. If the Simplified Procedure for wind loads is used, the table values must be divided by 0.85 .

06021c: What is the effect of FBC 2001 Sec. 1620.2 (High Velocity Hurricane Zone) on FBC 2001 Sec. 1609.4.1?
The 1.5 factor of safety against overturning in Sec. 1620.2 addresses the same issue as Load Combination 4, 0.6D+W, of Sec. 1609.4.1; so, one of these sections of the code is redundant. Accordingly, Sec. 1620.2 has been neglected in FtgShrWall-01.

06021d: What is the effect of FBC 2001 Sec. 1621.1 (High Velocity Hurricane Zone) on FBC 2001 Sec. 1609.4.1?
The allowable stress increase in Sec. 1621.1 addresses the same issues as Load Combination 3 of Sec. 1609.4.1 modified by the load reduction permitted in Sec. 1609.4.3, D+0.75(L+W); so, one of these sections of the code is redundant. Accordingly, the provisions of Sec. 1621.1 have been neglected in FtgShrWall-01.

Notes on Masonry Cantilever Retaining Walls

13821a: What is the difference between MWallRet-00 and MWallRet-01?
For overturning and sliding, MWallRet-00 checks the traditional load combination of D+H while MWallRet-01 checks the basic load combinations of the International Building Code 2000 which are compatible with the state-of-the-art ASCE 7-98 load combinations.

13821b: What is the difference between MWallRet-01 and MWallRet-01a?
For overturning and sliding, MWallRet-01a checks the basic load combinations of ASCE 7-98 while MWallRet-01 checks the basic load combinations of the International Building Code 2000 which are compatible with the state-of-the-art ASCE 7-98 load combinations. However, MWallRet-01 additionally includes a factor of safety of 1.5 against overturning and sliding per IBC 2000 Sec. 1610.2 which, incidentally, is not required in the Florida Building Code 2001. Florida design professionals should therefore use MWallRet-01a.

13821c: What wind directionality factor should be used with MWAllRet-01 and MWallRet-01a?
A wind directionality factor of 1.00 should be used because shear capacity and steel reinforcing in the footing are determined by the load combinations of ACI 318-99 Sec. 9.2 which is not compatible with ASCE 7-98. If the Simplified Procedure for wind loads is used, the table values must be divided by 0.85 .

13821d: What is the effect of IBC 2000 Sec. 1610.2 on IBC 2000 Sec. 1605.3.1?
The 1.5 factor of safety against overturning and sliding in Sec. 1610.2 addresses the same issues as Formula 16-11, 0.6D+W, of Sec. 1605.3.1; so, one of these sections of the code is redundant. Accordingly, the provisions of Sec. 1610.2 have been neglected in MWallRet-01.

Notes on Masonry Screen or Fence Walls

13861a: What is the difference between MWallScr-00 and MWallScr-01?
For overturning and sliding, MWallScr-00 checks the traditional load combinations of D and D+W while MWallScr-01 checks the basic load combinations of the International Building Code 2000 and the Florida Building Code 2001 which are compatible with the state-of-the-art ASCE 7-98 load combinations.

13861b: What wind directionality factor should be used with MWallScr-01?
A wind directionality factor of 1.00 should be used because shear capacity and steel reinforcing in the footing are determined by the load combinations of ACI 318-99 Sec. 9.2 which is not compatible with ASCE 7-98. If the Simplified Procedure for wind loads is used, the table values must be divided by 0.85 .

13861c: What is the effect of FBC 2001 Sec. 1620.2 (High Velocity Hurricane Zone) on FBC 2001 Sec. 1609.4.1?
The 1.5 factor of safety against overturning in Sec. 1620.2 addresses the same issue as Load Combination 4, 0.6D+W, of Sec. 1609.4.1; so, one of these sections of the code is redundant. Accordingly, Sec. 1620.2 has been neglected in MWallScr-01.

Notes on Wind Pressures on Components and Cladding of Enclosed and Partially Enclosed Buildings up to 60 ft.

23221a: What is the correlation between wind pressures on components and cladding obtained by the analytical method of ASCE 7-98 and the simplified wind pressures of the IBC 2000 Table 1609.6.2.1 or the FBC 2001 Table 1606.2?
The simplified wind pressures of the model codes are basically expanded versions of the ASCE 7-98 Table 6-3A, the Simplified Procedure table applying to enclosed buildings only. The wind pressures in all of these tables are derived by the analytical method on the basis of prototype values assigned to a number of parameters listed in Sec. C6.4 of the Commentary. Provision is made for adjustments for height and exposure. Other than that, the applicability of the Simplified Procedure is limited to the conditions dictated by the prototype parameters. To re-create the tables using your favorite components and cladding program such as Modstruct's WCladLo-98 or by manual calculations, enter a basic wind speed 85-170 mph, a wind directionality factor Kd=0.85, a building classification=2 (importance factor I=1.00), an exposure category=B (Kh=0.70), a topographic factor Kzt=1.00, an enclosure classification=enclosed (internal pressure coefficients GCpi=+/-0.18), a mean roof height h=30.00' (for each roof slope, you may need to manipulate the eave height and the least horizontal dimension of the building to obtain the 30' mean roof height), a roof slope of 0 degree (representing the interval between 0 and 10), or a slope of 20 degrees (between >10 and 30), or a slope of 40 degrees (between >30 and 45). The tabular wall pressures are obtained from the run using a roof slope between 10 and 30 degrees. Your results should match the corresponding values in the Simplified Procedure tables. But wait a minute! There's a hidden pitfall here: the use of the 0.85 wind directionality factor implies that these wind pressures will be used in the basic load combinations of IBC 2000 Sec. 1605.2 & 1605.3 or FBC 2001 Sec. 1609.3 & 1609.4 . If this is not so, you will need to multiply each of these pressures by the reciprocal of 0.85. To find out why, see the next question below.

23221b: Is there any restriction on using the 0.85 wind directionality factor for components and cladding from ASCE 7-98 Table 6-6?
Because this factor is calibrated with the load combinations of ASCE 7-98 Sec. 2.3 & 2.4, Section 6.5.4.4 as well as the footnote of Table 6-6 stipulate that it shall be applied only in conjunction with those load combinations. The basic load combinations of IBC 2000 Sec. 1605.2 & 1605.3 and FBC 2001 Sec. 1609.3 & 1609.4 are compatible with the ASCE 7 combinations. If, for example, you intend to use a wind uplift pressure to determine the resistance of a footing using the 0.6D+W load combination, the 0.85 factor is appropriate. On the other hand, if you intend to use the wind pressures to design a concrete element using the ACI 318-99 1.05D+1.28L+1.28W and 0.9D+1.3W load combinations or a steel element using the ASD load combinations D+W and D+L+W with allowable stress increases, then you must use a 1.00 wind directionality factor (see also IBC 2000 Sec. 1605.2.1, Exception 1). By the way, the ASCE 7 load combinations have been updated and moved from Appendix C of ACI 318-99 to Chapter 9 of ACI 318-02 in which case the 0.85 wind directionality factor is applicable.

23221c: Why would there be differences in the wind pressure values between one computer program and the next?
Barring programming errors, there may be minor differences because, for instance, one program uses the stepped values of ASCE 7-98 Table 6-5 to determine velocity pressure exposure coefficients while another program such as Modstruct's WCladLo-98 uses the more precise Equation C6-3 of the Commentary. The method described in Old Notes #23221a to re-create the Simplified Procedure tables should be a good check on the accuracy of your favorite components and cladding program.

23221d: What is the difference between the effective wind area and the tributary area of a component?
The effective wind area, which is used only to determine the average wind pressure on a component, is the span length L multiplied by an effective width B which need not be less than one third of the span and is therefore equal to the greater of LxB or L squared divided by 3. The total wind load acting on the component will be the average wind pressure multiplied by the tributary area which is simply LxB. However, for cladding fasteners (nails, screws etc.), the effective wind area shall be equal to the tributary area.

23221e: How can I use the WCladLo-98 module to schedule wind pressures on windows and doors?
After entering the data for the building as a whole, determine the effective wind areas corresponding to the various window and door sizes and enter them in the A.e column of the program. Then select the negative and positive pressures, 5N and 5P, for all openings within the corner zones having the zone width a.z and 4N and 4P for all openings within the interior zones which lie between the corner zones.

23221f: Where can I obtain the wind pressures on balconies, canopies and carports attached to a building?
Because wind pressures on balconies, canopies and carports attached to the main structure are not addressed in ASCE 7, it should be at the discretion of the designer to use the roof overhang pressures (Zones 1No, 2No and 3No in WCladLo-98) even if the slopes are different from the main roof slope. Note that the roof overhang pressures include contributions from both the upper and lower surfaces.

23221g: How can I find the uplift force on a roof truss anchor?
WCladLo-98 delivers only the wind pressures (psf or lb. per sq. ft.) on various effective wind areas of components and cladding. The determination of wind forces (lb) goes beyond the scope of this program. However, for those who still need a little nudge on this question, the answer to the next question may assist your understanding of the issues to be addressed.

23221h: Can you give an example of how to calculate the uplift force on a roof truss anchor?
Given a typical interior roof truss spaced at 2'-0" on centers with a minimum dead load of 15 psf, a span of 29'-4" center-to-center of 8" exterior walls, a gabled roof slope of 6:12, an eave height of 9'-0" above ground and an eave overhang of 2'-0" in a 30'-0" x 60'-0" residential building located on a relatively flat site in a suburban area in Broward County, Florida: using manual calculations or a computer program such as Modstruct.com's WCladLo-98, enter Basic wind speed = 140 mph; Hurricane prone region = 1 (yes); Classification category = 2; Exposure category = C (normally = B per ASCE 7-98 but = C per local amendment); Topographic factor = 1.00; Wind directionality factor = 1.00 (because anchor capacity will include the 33% allowable stress increase for wind); Building Enclosure Classification = 3 (enclosed); Angle of plane of roof from horizontal = 26.57 degrees; Height of eave above ground = 9.00 ft; Height of parapet above the eave = 0.00 ft; and Least horizontal dimension of the building = 30.00 ft. The result of the calculations thus far should read: Calculated value of the mean roof height = 12.75 ft; Zone width = 3.00 ft. To get the wind pressure (psf or lb. per sq. ft.) on any component, you will need to enter the effective wind area, not the tributary area of the component (see Old Notes #23221d). The length of the effective wind area of a truss anchor will be one-half of the truss span out-to-out of walls plus the eave overhang. Therefore, enter Effective wind area = 17' x 17' /3 = 96 sf. Read: Zone 1 negative pressure = -41.8 psf; Zone 2 =   -67.8 psf; Zone 2 overhang = -93.7 psf. To get the uplift force (lb) on the truss anchor, you will need to multiply each value of the negative pressure by the portion of the tributary area upon which it is acting. The overhang overlaps Zone 2 at the eave. Therefore, the uplift on the overhang = -93.7 psf x 2.00' x 2.00' = -374.8 lb; the uplift on the remaining portion of Zone 2 at the eave = -67.8 psf x 2.00' x 1.00' = -135.6 lb; the uplift on the interior Zone 1 = -41.8 psf x 2.00' x 11.00' = -919.6 lb and the uplift on Zone 2 at the ridge = -67.8 psf x 2.00' x 3.00' = -406.8 lb. Hence the gross uplift on the truss anchor will be -1836.8 lb. To arrive at the net uplift on the truss anchor, using the load combination 0.6D+W of Sec. 1609.4 of the Florida Building Code 2001 (Sec. 1605.3.1 of the International Building Code 2000), deduct 60% of the minimum dead load from the gross uplift = +0.6 x 15 psf x 2.00' x 17.00' -1836.8 lb = -1530.8 lb which is the force to be resisted by the anchor.

Notes on Miscellaneous Subjects

H23221a: Where did the 1.5 factor of safety against uplift and overturning due to wind go in the IBC 2000 and FBC 2001?
These factors of safety have been replaced by the basic working stress load combinations of Sec. 1605.3.1 of the International Building Code 2000 and Sec. 1609.4.1 of the Florida Building Code 2001 including the combination 0.6D+W. Note that if 0.6 times the dead load effects is required to resist the wind load effects, then 0.6D=W which makes the factor of safety effectively 1.67 against uplift and overturning. For a discussion in greater depth, see the last paragraph of ASCE 7-98 Commentary C2.4.1, IBC 2000 Sec. 1609.1.3 and the commentary on p. 24, Ghosh & Chittenden: 2000 IBC Handbook, Structural Provisions. Incidentally, the wind directionality factor of 0.85 which is applicable in this case (see FAQ #2322b) reduces the factor of safety to 1.42 relative to traditional procedures. As the French say, the more you change the more you come back to the same. But there's more! If, rather than overturning and sliding, the maximum toe pressure on the soil is the governing condition and if you have been using an allowable stress increase in the soil resistance, because stress increases are not permissible in the load combinations cited above, the footing will be somewhat larger. And the water gets muddier: a 25% reduction in the combined effect of two or more transient loads is permissible which substantially cancels the loss of the allowable stress increase but not in the case of a single transient load acting in conjunction with the dead load such as D+W. Things do not always remain the same after all.

H23221b: Is a live load considered a transient load?
In an evaluation of the permissible load reduction for the combined effect of two or more transient loads, a live load is considered a transient load. See Sec. 2.4.3 of ASCE7-98.