Common Errors in Seismic Design & How to Avoid Them. T ...

1 2014 Annual Conference Proceedings _____. The Most Common Errors in Seismic Design And How to Properly Avoid Them Thomas F. Heausler, , Structural Engineer Kansas City, MO. Abstract The paper identifies the most Common Errors that structural engineers make when performing Seismic 1. Seismic Design Category A. Design and calculations. The paper demonstrates the [ ] [ ] [ ]. proper application of Codes and Standards so as to Avoid [ General Structural Integrity]. Errors and misapplications. The paper initially focuses When in Seismic Design Category A, you should not use any of on low Seismic risk Design categories (and the the provisions of Chapter 12. Instead, use the General Structural surprisingly numerous Seismic provisions which apply Integrity provisions of Section Note that the General therein) and then advances to the requirements unique to Structural Integrity provisions have some loads which may often high Seismic risk Design categories only.

2 Be erroneously unaccounted for. The forces required include 1%. dead load, 5% of dead plus live load for beam connections, and Introduction 20% of wall weight for wall connections. Non-Structural Components in Seismic Design Category A are exempt from This paper is based upon IBC 2012, ASCE 7-10, AISC Seismic Design requirements, as stated in Section 360-10, ASIC 341-10, ACI 318-11 (including Chapter 21 and Appendix D) and other Standards. A few of the 2. Importance Factor topics/ Common Errors that are addressed include: [ ] [Table ] [Table ] [IBC Table ]. Amplified Seismic force Omega o, Cd deflection The Importance factor is based upon Risk Category and the amplification, Modal analysis triggers, Rho redundancy associated Life Safety, Hazard and Essential nature of the factor, E for Allowable Stress Design , Ev Vertical structure.

3 Both the ASCE 7 and IBC tables should be reviewed. Seismic effect, Vertical distribution of base shear, A typical building can sometimes evolve into an Ie equal to Orthogonal effects on corner columns, Diaphragm or when occupancy or use expands. Examples include forces, Accidental torsion, Foundation ties, Unique relatively small churches (occupancy greater than 300), or a calculations and detailing for Special Steel Systems, building where hazardous materials are stored. It should be noted Concrete detailing and splice lengths when subject to high Seismic , Wood frame hold-downs, Masonry wall that for building Design Ie = , , or but for non-structural anchors, and Wind governed buildings. components Ip = , or only [ ] such that Ip may not equal Ie, and in Seismic Design and Errors some instances Ip may be less than Ie.

4 This paper is written is checklist format. It is intended 3. Continuous Load Path that an engineer could read the list so as to review and [ ] ASCE 7 has very specific provisions for many elements verify adequate knowledge of Seismic Design and such collectors, connections, diaphragms, walls, etc. However, in Common Errors , as well as on a per project basis when addition to the specifics, the engineer is required by Section checking a project by the engineer or by others. The to provide a continuous load path for all inertial forces from their basis of this paper is ASCE 7-10, and IBC 2012. origin to the foundation. Such load path shall conform to the Referenced sections are highlighted in brackets thus: relative stiffness and strength of the elements which exist in the [ASCE 7 Section Number].

5 Structure. 1. and ductility potential of the primary feature of the lateral 4. R Factor system. For example, in a steel braced frame system, in [Table and , 2] order for the diagonal brace to yield and dissipate energy The Response Modification Coefficient, R, is in a controlled and reliable manner, all other portions of part of a concept where an elastic Design may the load path ( connections, bolts, welds, gusset be performed but with due consideration of the plates, anchor bolts, columns and collectors) need to be overstrength and ductility inherent in the lateral stronger than the maximum anticipated strength, or force, force resisting system. In order to assure in the brace. Therefore, Omega zero amplification and reliability in the overstrenght and ductility, load combinations are specifically triggered in the many requirements are triggered with each R sections mentioned above and in Material Standards such factor.

6 The R Tables list the detailing as AISC and ACI. requirements triggered. The strings attached . to AISC R>3 structures and ACI Chapter 21 8. Redundancy - Rho structures can be extremely significant. Rho is a factor that penalizes structures that do not have redundancy. [ ] Rho = or Rho = for: 5. Omega o SDC B, C, for Drift calculations, [Table ] Fp (non-structural Components) forces, When diaphragms are flexible, you are allowed Collectors, Omega Zero Load Combinations, and to reduce the Omega zero factor by This Diaphragms. provision is listed in small print as footnote g, and is often overlooked by inexperienced users 9. Vertical Seismic Load Effect - Ev of ASCE 7. [ ] requires that a vertical load effect equal to Sds be applied to dead load. It is applied as a Dead Load 6.

7 Modal Response Spectrum Analysis Triggers Factor adjustment and may act downward or upward. It [ ] [Table Horizontal Irregularities] is at strength Design level so it may be multiplied by [Table Vertical Irregularities] for Allowable Stress Design (ASD). No Ie, Ip, nor Rho is The above tables describe various irregularities applied to Ev. and thus trigger specific provisions. One such provision is elimination of the option of the 10. Load Combinations and Allowable Stress Design . Equivalent Lateral force Procedure [ ] and E. the need to perform a Modal Dynamic For Allowable Stress Design (ASD) Load Combinations Response Spectrum Analysis. The tables [ ], Section [ ] shall be used in lieu of require close review to interpret the triggers. [ ] and [ ]. Earthquake forces are at strength The ASCE Design Guide Seismic Loads: Guide level so for the ASD combinations, use E (for LRFD.)

8 To the Seismic Provisions of ASCE 7-10 by use E). The E applies to Fp non-structural Finley Charney has a listing of the triggers in a component forces also. more user friendly format. 11. Orthogonal Effects 7. Omega o Triggers Earthquake forces shall be calculated for each of the two [ ] Load Combinations with Omega zero primary orthogonal directions. In order to consider the [ ] Cantilever Columns effects of earthquake forces at some angle other than [ ] Collectors Light Frame, Wood those two directions, orthogonal effects must be excepted considered. Section [ ] requires that irregular [ ] Columns, Beams Supporting buildings in SDC C, and corner columns in SDC D,E,F. Discontinuous Walls be considered with 100% of forces in one directions and [ ] Pile Anchorage 30% in other.

9 [AISC where R>3, ACI Chapter 21, Appendix D, etc.] It should be noted that IEEE 693 (Electrical Equipment). Omega zero is an amplification to the forces in applies Orthogonal Effects to all conditions, including certain elements in the Seismic load path. It is corner anchor bolts. required so as to prevent a weak link form occurring prior to the full energy dissipation 12. Effective Seismic Weight 2. Section [ ] defines the Effective Seismic The purpose of a Modal Response Spectrum Analysis is Weight, W. Except for as mentioned below, not to refine the magnitude of the Base Shear. Instead its live load is not included in the inertial force, purpose is to more accurately perform the following: however the Seismic force is later combined 1. Distributes Base Shear over height with dead and live loads in the load 2.

10 Horizontal Torsional Effects combinations. Section [ ] stipulates that 3. Higher Mode Effects W shall include the following masses: 25% of When a structural has significant vertical or horizontal Storage live load, Partitions 10 psf [ ], irregularities, the equation (triangular force Industrial Operating Weight (and unbalanced distribution) becomes inaccurate. conditions), 20% of snow > 30psf, and Roof Gardens. 17. Modal Analysis [ ]. 13. Period T Scaling of the results of Modal Response Spectrum Section [ ] has complex methods and Analysis is required and allowed. However, especially limits on calculating the Period T. It should be for buildings, the results of the Response Spectrum noted that it is acceptable to use T = Ta, for should be very similar to the Base Shear.