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Building Code Complexity
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- Subject: Building Code Complexity
- From: "Sherman, William" <ShermanWC(--nospam--at)cdm.com>
- Date: Thu, 6 Nov 2003 15:56:13 -0500
This email is submitted for general comments on building code complexity. (Comments on specific code requirements are also welcome.) I have been developing design requirements for a single story pre-engineered metal building for a water treatment plant in New England. The structure is intended to be a concentrically braced steel frame for lateral loads, although rigid moment frames will be used to resist vertical loads. The lateral bracing is intended to avoid seismic requirements for moment frames resisting earthquake forces. Although we have requested preliminary foundation reactions from local pre-engineered metal building suppliers, they will not provide this information for "free". Although we would be willing to pay for a preliminary design, legal counsel has said that if we pay a supplier for a design, that supplier should be excluded from bidding as they would have a competitive advantage over other bidders (for a public works project). Thus we plan to develop our own preliminary design to get foundation reactions. Developing the required design load cases has been a lesson in building code complexity. The governing building code is the 2000 IBC. The building is an L-shaped building and will have non-symmetrical bracing and irregular bay sizes, due to equipment layout. I have currently come up with a "minimum" of 83 load cases to be evaluated! (I guess I can understand why the building manufacturers no longer want to design for free.) FYI, following is a summary of the code requirements. Seismic Design: Basic Seismic Design Parameters per IBC 2000: - Seismic Use Group III, Importance Factor = 1.50 (per IBC Table 1604.5, required for fire suppression) - Site Class D (based on Soil Profile, per Geotech Report and IBC Table 1615.1.1) - Sds = 0.401; Sd1 = 0.163 (based on zip code) - Seismic Design Category D (per IBC Tables 1616.3(1)&(2)) Per IBC Section 2212.1.2, the steel structure must conform with AISC Seismic Provisions, Part I (LRFD) or Part III (ASD). It is proposed to use AISC Seismic Provisions Part I, Section 14.5 for Low Buildings (recommended by a building manufacturer). Members and connections are to be designed for Load Combinations 4-1 and 4-2 (with overstrength factors), as given in Section 4.1 of Part I. It should also be noted that Part III for Allowable Stress Design uses the same load combinations, so these may be used for LRFD or ASD. In these equations, the required load factor for seismic (omega-zero) is 2.0 (i.e., 2.0E is used). Orthogonal effects need not be considered with these load combinations. However, standard load combinations using 1.0E must also be analyzed, including orthogonal effects. Thus, there are 2 standard load combinations and 2 special load combinations for seismic effects. Note: Where Allowable Stress Design is used in accordance with Part III of the AISC Seismic Provisions, allowable stresses must be factored up by a factor of 1.7 and then multiplied by a resistance factor which varies by type of stress and component (e.g., flexure vs shear, tension vs compression, welds vs bolts, etc). ASD load combinations include the same load factors as for LRFD. Thus in effect, the same analysis is performed for LRFD and ASD but different formulas are used for effective strengths. One effect of this is that standard allowable stress design checks built into software for ASD design, cannot be used directly since the varying resistance factors have not been incorporated. Thus member design must be done manually from the analysis results for forces and stresses, when ASD is used. However, if the software includes LRFD code requirements, that may be used for member design. (I guess AISC will succeed in forcing LRFD upon us.) Due to the L-shaped building, plan irregularities must be accounted for in accordance with IBC Table 1616.5.1. Type 1a or 1b irregularity must be evaluated due to differential story drift at the long end of the building vs the short end of the building. Type 2 irregularity must be evaluated due to the re-entrant corner. (Type 5 irregularity may also apply due to bracing not being symmetrical about the major axes.) IBC Table 1616.6.3 permits the equivalent lateral force method to be used "with dynamic characteristics included in the analytical model". Equation 16-35 will be used to determine the seismic response coefficient (i.e., the peak seismic response; Equation 16-36 does not govern based on the estimated period per 1617.4.2.1). Redundancy must be evaluated in accordance with IBC Section 1617.2.2. Torsion must be evaluated in accordance with IBC Section 1617.4.4 (including torsion due to mass and stiffness distribution, accidental torsion, and dynamic amplification of torsion). Where orthogonal effects must be evaluated, design must conform to IBC Section 1620.2.2. Seismic effects should be evaluated in two directions for each orthogonal axis, to get the maximum tension and compression reactions at braces and at foundations. Furthermore, positive and negative accidental torsion should be evaluated for each of these cases. This results in 4 basic load cases * 2 axes * 2 directions * 2 torsion eccentricities = 32 load cases. Additional combined load cases must also be evaluated on a case-by-case basis to account for orthogonal effects (such as where orthogonal braces attach to a single column base). Story drift must be evaluated in accordance with IBC Table 1617.3 (max allowable drift = 0.010*h) and Section 1617.4.6; but for drift the redundancy coefficient should be equal to 1.0. If the redundancy coefficient calculated from IBC Section 1617.2.2 is greater than 1.0, additional load cases should be evaluated. It is proposed to select the most critical cases for building deformation from the original strength design load cases by judgment and evaluate drift as needed. Wind Design Parameters per IBC 2000: Wind Load: - Design per ASCE 7-98, Chapter 6, Method 2 - Analytical Procedure - Basic Wind Speed = 100 mph, Importance Factor = = 1.15 - Exposure Category B (wooded area) - Topographic Factor, Kzt = 1.0 Note that this building is not a "simple diaphragm" building or "regular shaped building" due to lack of symmetry. There are 3 basic load combinations in the 2000 IBC including wind. However in accordance with ASCE 7-98, cases with positive and negative internal pressures must be considered and, as shown in Figure 6-4 of ASCE 7-98, two cases must be evaluated for each of the four building faces (similar to "accidental torsion" for seismic design). This results in 3 basic load cases * 2 internal pressures * 4 faces * 2 cases = 48 load cases. Additional Basic Load Cases: There are 3 basic load combinations to account for dead load, floor live load, and roof snow load, without lateral wind or seismic loads. Although there are no "floor live loads" acting on our building frame, one bay will be affected by monorail loads. Also, collateral loads should be included to account for lighting and HVAC hanging from the ceiling. Snow Design Parameters per IBC 2000: - Ground Snow Load = 50 psf, Importance Factor = 1.20 - Terrain Category B, Partially Exposed, Thermal Factor = 1.1 at process area and 1.0 at admin area - Roof slope varies 4:1 or 8:1 (14-deg to 7-deg) For gable roofs, balanced and unbalanced snow loads must be evaluated and wind from all directions must be accounted for to determine unbalanced loads. Thus 4 wind directions must be evaluated for unbalanced snow conditions plus one full balanced snow load case, for 5 snow load cases. However, the 4 unbalanced snow load conditions should be able to be included in wind load combinations already accounted for. It appears that this single story steel building design will require 32 seismic load cases, 48 wind load cases, and 3 basic load combinations, as a minimum. (For reference, for a rectangular building with symmetrical bracing, the load cases could be reduced to 23: the seismic and wind load cases could each be reduced by a factor of 4, including a factor of 2 by avoiding torsional eccentricities in two directions for each case and a factor of 2 by avoiding loads in each direction for north-south or east-west load cases. Results for the most critical stresses on one side of the building would be used for the opposite side of the building.) William C. Sherman, PE CDM, Denver, CO Phone: 303-298-1311 Fax: 303-293-8236 email: shermanwc(--nospam--at)cdm.com ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. 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