Multi-Lat™ Design Assumptions

Design Assumptions:

1. User’s of Multi-Lat™ are expected to have a solid education and professional experience in the field of structural engineering. As with any software, the user should posess a solid professional intuition as to the accuracy of the results obtained. Errors do occur in the design of most software and while we seek the help of beta testing, the results can be misinterpreted or falsely obtained if the user errs in the input of information, geometry or design assumptions. Multi-Lat™ is a tool that if misused can result in serious structural damage.
2. Note: The initial version of Multi-Lat™ assumes the building shall be restricted to a maximum of 3-stories as stated in note #3 below. Tables, loads, and other information is provided in the workbook to allow for future expansion of features. The inital release will be recommended for residential light-framing as noted in ASCE Section 12.3.1.1 below.
3. Multi-Lat™ assumes that the structural diaphragms can be idealized as flexible.
   • ASCE Section 12.3.1.1 Flexible Diaphragm Condition. “Diaphragms constructed of untopped steel decking or wood structural panels shall be permitted to be idealized as flexible in structures in which the vertical elements are steel braced frames, or concrete, masonry or steel shear walls.
   • Diaphragms of wood structural panels or untopped steel decks in one and two-family residential buildings of light-frame construction (wood or cold form steel) shall also be permitted to be idealized as flexible.”
4. Note 2 above is discussed in some depth in the article by S.K. Ghosh, PhD and Susan Dowty, SE in the August 2006 Code Simple Column Section 1602.1 Definitions (Diaphragm, flexible) and Section 1617.5.3 Horizontal Distribution . Essentially, IBC 2006 revised ASCE 7-05 Section 12.3.1.1 by identifying that the code should define a building by its construction rather than its occupancy.
   1. It will often be argued that residential buildings must be maintained as dynamic in nature – in other words, easily changed to fit the needs of the owner or occupant. A residence that is not easily changed will become, in this engineers opinion, an exposible structure. In other words, the cost to identify the lateral load restraining system of a building designed as rigid or non-rigid would require that original design criteria (analysis and drawings) be maintained and that each change be recorded and justified for additional future revisions. With Flexible analysis, the diaphragm transfer’s shear simply as it would a uniform load on a simply supported beam. Rigid analysis requires that the original deisgn assumptions be understood by future engineers and designers or that the structure be exposed sufficiently to allow the costly reverse design of the lateral restraint system. In a non-rigid analysis, a combination of rigid and flexible may be used that was roughly called an “envelope solution” at the time this argument originated with the 1997 Uniform Building Code.
   2. In short, some structures must be “flexible” enough to allow afforable modification with the assumption that buildings following flexible analysis for nearly 200 years of conventional framing and which are built within the specifications of the engineered design have performed well during high seismic and wind events. Failures occur when construction defect increases or when the designer/engineer does not adequatly detail the lateral restraint system starting at the roof and working down to an adequate foundation. These buildings have performed well since log cabins gave way to stick framed platform or balloon framing.
   3. Problems were identified in high seismic regions when raised floors on cripple walls or without appropriate anchorage to an adequate foundation existed. These were identified as unreinforced wood frame structures prior to 1969 that exhibited the greatest amount of post hurricane or earthquake damage. Retrofit standards are in place, but most important is that flexible analysis was not the reason for failure – inadeuqate detailing or construction defects were the primary cause.
   4. Finally, to tie this ideology into a neat package, one needs to look at the materials and method of construction to determine the design criteria comparing historic performance. The restriction should not be by occupancy since the performance of the structure is not dependent on how someone uses a building but on the way the building was constructed. Therefore, the provisons of ASCE 12.3.1.1 was modified by construction – no by occupancy.
5. The spreadsheet offers many choices that can not be used in the design of structures with flexible diaphragms. These choices noted on the “Seismic” worksheet as a choice for the “Main Structural System” and “Specific Restraint Type” that are not in compliance with flexible analysis are left in the spreadsheet to allow for future expansion that may allow for rigid analysis. Comments are added to warn the user when the type of building or the geometric layout does not comply with simplified/ flexible design analysis. Until the upgrade is made, the program assumes that the user will design the building to comly with fleible analysis, not exceed 3-stories or have a mixed lateral load resisting system. The User must be cautioned to study this area closely and the recognition for deisgn is left to the decretion of the engineer in responsible charge.
6. Allowable Stress Design (ASD) methods assume that the provisions of ASCE 7-05 Section 2.4.1 for basic load cases be adhered to. In general the stress reduction of E/1.4 from the prior 97 UBC is equivalent (inverse function) of the indicated 0.7E. “The same E from Section 12.4 is used in both Sections 2.3.2 and 2.4.1. Refer to the Chapter 11 Commentary for the Seismic Provisions.”
7. Multi-Lat™ allows the structure to be broken into geometric blocks and each block to contain up to four stories (not to exceed 65-feet in height from fixity) with up to ten lines (grid) of shear in each orthogonal direction and up to five shear resisting elements in each line of resistance.
8. Where the structure is reduced to geometric blocks, the user must manually input the results from the “cut-line” results to the workbook containing the adjacent block. The “cut-line” is assumed to be a physical line of shear resistance, however, this may be a “virtual” line of resistance as long as the design engineer can show that the diaphragm at each level is capable of transfering the shear to the nearest lateral load resisting shear element.
9. Shearwalls will not exceed 40-feet between lines of resistance in each orthogonal direction.
10. Lines of shear and shear resisting elements are not required to stack in each story. Where walls do not stack, the design engineer shall provide additional design and detailing to show how the element transfers shear and resists deflection, Tension and Compression on the diaphragm below.

TO BE CONTINUED – UNDER CONSTRUCTION……. 2/04/2008

~ by structuralist on January 23, 2008.