How Do I?

Note:- Any information in this page relating to interpretation of building codes and standards is offered as a guide subject to verification by the user. Readers may freely ask further questions or offer comments via e-mail.

How do I determine the load capacity of a single or a group of post-installed/
    drilled-in anchor bolts?
How do I determine the load capacity of a single or a group of cast-in-place
    anchor bolts in masonry?
How do I determine the load capacity of a group of cast-in-place anchor bolts
    in concrete?
How do I determine the tension in a diaphragm chord?
Can you give an example of how to calculate the uplift force on a roof truss
    anchor?

H0101: How do I determine the load capacity of a single or a group of post-installed/ drilled-in anchor bolts?
Up to the present, reliance has been placed on product approvals obtained by the bolt manufacturers on the strength of evaluations by independent testing agencies. However, ACI 318 in its 2002 revision presents in its Appendix D a rational and unified methodology for the determination of the load capacity of cast-in-place or post-installed anchor bolts in cracked or uncracked concrete which is expected to be adopted by the International Building Code in its 2003 revision. This approach may not be immediately applied to post-installed anchors because a k-factor needs to be determined by prequalification tests obtained by the manufacturer from an independent testing and evaluation agency. Until then, the only available procedure is the manipulation of the test data presented in the current product approvals. Tedious and error prone interpolations may be avoided by the use of free software downloaded from the manufacturer’s website such as the Hilti Anchor Program v2.4 at www.hilti.com .

H0102: How do I determine the load capacity of a single or a group of cast-in-place anchor bolts in masonry?
A working stress design procedure is specified in the Uniform Building Code, 1997 under Sec. 2107.1.5 Embedded anchor bolts and Sec. 2106.2.14 Placement of embedded anchor bolts. A similar procedure is stipulated in Sec. 2.1.2 – Anchor bolts solidly grouted in masonry of ACI 530-99 Building Code Requirements for Masonry Structures and in Sec. 2.1.2 of the Commentary which is adopted by the 2000 International Building Code. The latter provisions are basically the same as in the earlier ACI 530-95 which is adopted by the 1997 Standard Building Code. The conical concrete failure surface of an anchor is expressed in terms of its projected area on the surface from which the anchor protrudes. For anchors spaced less than twice the embedment length apart, the overlapping areas are deducted and for anchors having an edge distance less than the embedment length, the edge distance is used as the diameter of the projected area. The edge distance is defined as the least distance from the edge of the masonry to the surface of the anchor bolt. For an interior anchor adjacent to an anchor in proximity to a free edge, the calculation of the overlapping area of the circles of two different diameters does present some computation challenges which make a computerized solution desirable. The task becomes immensely more complex for an anchor group having different edge distances on two or more sides with overlapping areas in two directions falling partially upon each other to create redundant overlap deductions. Because of the brittleness of the material, the sparsity of consistent published data on masonry anchor groups and the complexity of the calculations, we do not recommend group placement where the anchors are spaced at less than twice the embedment length in each direction. Therefore, the Modstruct computer programs, AncBoltMas..., which comply with the codes and standards referenced above and which are available for purchase on this website, are confined to the simplest group configuration of a single row of anchors. However, these programs are capable of addressing anchors that are clear of or adjacent to free edges at distances less than an embedment length on one to four sides. Nevertheless, we recommend that wherever possible the anchors be spaced at least twice the embedment length apart to avoid interaction. The capacity of each of such anchors is, of course, that of a single anchor which can also be determined in the AncBoltMas... programs.

H0103: How do I determine the load capacity of a group of cast-in-place anchor bolts in concrete?
A procedure for the determination of the capacity of headed bolts and stud anchors in concrete by the Strength Design method has been prescribed in
Sec. A.9.6.2 of ASCE 7-95
Sec. 1914 of the Standard Building Code, 1997 with minor variations
Sec. 1923.2 & 1923.3 of the Uniform Building Code, 1997 with minor variations
and in Sec. 6.5.2 of the PCI Design Handbook, 5th Edition (1999) with some modifications.
The concrete failure surface of a bolt group is simplified as the surface of a truncated pyramid rather than a series of intersecting conical surfaces. Manual calculation examples may be found in the PCI Design Handbook. Adopted by the 2000 International Building Code under Sec. 1913 is the state-of-the-art Concrete Capacity Design or CCD Method which is prescribed in Section A.9.9.6 of ASCE 7-98.  Appendix D of ACI 318-02 now includes the most up-to-date determination of the load capacity of  cast-in-place anchors by the CCD Method.

H2312: How do I determine the tension in a diaphragm chord?
The uniformly distributed lateral load, w, on the edge of the diaphragm is obtained from manual calculations or from a computer program such as WBldg-98 (see FAQ #2312a). The chord tension, T, will be the moment in the diaphragm divided by the depth of the diaphragm. For a simply supported diaphragm (shear walls at both ends), if B is the building dimension normal to the lateral force and L is the building dimension parallel to the lateral force, T = wB^2/(8L).

H2322: 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-02, enter Basic wind speed = 140 mph; Hurricane prone region = 1 (yes); Classification category = 2; Exposure category = C (normally = B per ASCE 7-02 but = C per local amendment); Topographic factor = 1.00; Wind directionality factor = 0.85; Parapet Enclosure Classification = blank or 0 (since there is no parapet); 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 FAQ #2322d). 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 = -35.5 psf; Zone 2 = -50.3 psf; Zone 2 overhang = -79.7 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 = -79.7 psf x 2.00' x 2.00' = -318.8 lb; the uplift on the remaining portion of Zone 2 at the eave = -50.3 psf x 2.00' x 1.00' = -100.6 lb; the uplift on the interior Zone 1 = -35.5 psf x 2.00' x 11.00' = -781.0 lb and the uplift on Zone 2 at the ridge = -50.3 psf x 2.00' x 3.00' = -301.8 lb. Hence the gross uplift on the truss anchor will be -1502.2 lb. To arrive at the net uplift on the truss anchor, using the load combination 0.6D+W of Sec. 1605.3 of the Florida Building Code 2004 (Sec. 1605.3.1 of the International Building Code 2003), deduct 60% of the minimum dead load from the gross uplift = +0.6 x 15 psf x 2.00' x 17.00' -1502.2 lb = -1196.2 lb which is the force to be resisted by the anchor.