CEE 4606 - Capstone II Structural Engineering

CEE 4606 - Capstone IIStructural Engineering Lecture 6 – Seismic Detailing

Outline 1. IBC Seismic Detailing Requirements 2. Shear Design 3. Torsion Design 3. Progress Reports 4. Work Tasks

IBC Seismic Detailing Requirements • How does having a structure in Seismic Design Category D affect the design of our building frame for gravity load? • What did we find out?

IBC Seismic Detailing Requirements • General concept: • In a moderate to severe EQ, inelastic behavior is expected • Yielding is allowed to take place • Fairly large displacements occur • We must have ductility for this to occur! • Ductility is provided by good detailing

IBC Seismic Detailing Requirements • Level of detailing required depends on the level of seismic risk: • Low seismic risk: SDC A, B • Medium seismic risk: SDC C • High seismic risk: SDC D, E, F

IBC Seismic Detailing – SDC D • IBC 1910.5 • For SDC D, all requirements of SDC C (and thus all requirements of SDC A & B) must be met • 1910.5.1 Seismic-force-resisting systems • 1910.5.2 Frame members not proportioned to resist EQ forces

IBC Seismic Detailing – 1910.5.2 • Since our frame is not designed to resist EQ forces (our masonry shear walls handle this load), we essentially are required to meet the provisions of Section 21.9 of ACI 318-99 • Note that we will have special detailing requirements for our masonry shear walls when we design them!

ACI 318-99 Section 21.9 • Principle: • Our frame is designed only for gravity loads • However, EQ forces will cause the structure to displace some laterally • This lateral displacement will cause some forces to develop in the frame elements

ACI 318-99 Section 21.9 • Principle: • Design the frame system such that the gravity load system maintains its vertical load capacity when subjected to the maximum lateral displacement expected for the design-basis earthquake

ACI 318-99 Section 21.9 • Example: • Two parking garages collapsed in the 1994 Northridge earthquake when columns that were designed only for gravity load failed • We are forced to detail the elements of our gravity load resisting system to behave in a ductile manner, even if we do not design them to resist EQ loads!

ACI 318-99 Section 21.9 • Two options: • 1) Perform analysis to determine moments and shears in frame system resulting from the design EQ (apply ACI 21.9.2) • You may do this analysis if you so desire (not easy!; use STAAD, consider cracking, etc.) • 2) Conservative, simple approach in which no analysis is required but more stringent detailing is necessary (apply ACI 21.9.3)

ACI 318-99 Section 21.9.3 • Requirements if no lateral analysis is conducted based on design displacement: • ACI 21.9.3.1: General requirements • ACI 21.9.3.2: Beam requirements • ACI 21.9.3.3: Column requirements

ACI 318-99 Section 21.9.3.1 • States four requirements by reference: • 21.2.4: Min. conc. strength = 3000 psi • 21.2.5: ASTM reinforcement • 21.2.6: Mechanical splices • 21.2.7.1: Welded splices

ACI 318-99 Section 21.9.3.2 • Three requirements for beams: • 21.3.2.1: Special minimum & maximum reinforcement limits • 21.3.4: Specifies a “capacity” design for shear • Max. stirrup spacing = d/2 for entire member

ACI 318-99 Section 21.9.3.3 • Three requirements for columns: • 21.4.4: Special requirements for transverse reinforcement (ties & spirals) • 21.4.5: Specifies a “capacity” design for shear • 21.5.2.1: Requires confining reinforcement in beam-column joints

Shear Design • Review your CEE 3422 notes and McCormac text Ch. 7 • Shear covered in ACI Chapter 11 • Seismic detailing requires that we use a “capacity design” approach • Reference: Ch. 15, Nawy (handed out)

Shear Design – “Capacity Design” • Which is more brittle: flexural failure or shear failure??? • Remember the pictures of columns that failed in shear? • Avoid brittle failures in general, but we really need to avoid them in seismic design!

Shear – “Capacity Design” Philosphy • Design for shear based on the highest possible shear that could occur in the structure, not the shear computed by loads • Assume that when flexural failure occurs, we still have enough shear capacity • Design assuming that plastic hinges (flexural failure) occur at each end of the beam

Shear Design – “Capacity Design” • Reverse cyclic loading that occurs during earthquakes will cause plastic hinges to form at beam ends – in positive moment on one end and negative on the other • This is a worst case scenario for shear! • Mpr = Probable flexural moment strength Mpr2 Mpr1

Shear Design – “Capacity Design” • Probable moment strength uses f = 1.0 and assumes steel is at 125% of fy to consider effect of strain hardening • We still must consider load along the beam! • Design for shear based on worst of two cases:

Shear Design – Final Comments • Conservative assumption: Vc = 0 • Considers that considers effects of repeated cracking and loss of concrete contribution near plastic hinges • Apply in accordance with ACI 21.3.4.2 • Note: You will probably wind up with a fairly tight stirrup/hoop spacing

Design for Torsion • Does torsion scare anybody? • We design for torsion together with shear • ACI Chapter 11 • See McCormac text, Chapter 14 • Text provides a good simple discussion • Remember, we will design for minimal torsion

Two Types of Torsion • Equilibrium torsion • Actually a torsional load acting directly on the structural member • Compatibility torsion • Torsion occurs in the member only because it is connected to another member that frames in • Indirect torsion, dependent on compatibility of deformations • Which type do we have?

ACI Torsion Design • ACI uses an approximation that all members are thin walled tubes to develop equations • If torsion is equilibrium torsion, we must design for the total torsional load • If torsion is compatibility torsion, we are only required to design for the torsional load that causes cracking • Torsion could never exceed this value!

Cracking Torque • Design for this value of torsion • See code and text for definitions of variables • After cracking, ACI assumes no strength contribution is supplied by concrete, so the entire torque must be resisted by closed hoops

ACI Torsion Design • Above equations are very similar to shear equations – they give the required torsion reinforcement for a given hoop spacing or vice versa.

ACI Torsion Design • Simply add the torsion reinforcement requirement to the shear reinforcement requirement • They are the same steel, not different bars! • An additional small amount of longitudinal steel will also be required • This steel must be distributed around the cross-section

Torsion Design – Additional Guidance • Text – see specifically Sections 14.7 and 14.9 (example problem) • This should not add a significant amount of steel to your beams, but will add some • Don’t be afraid to ask questions

Comments on Progress Report #1 • Agendas lack important details • Use the passive voice!!! • Good background information, some took a minimalist approach • Good wind and earthquake research, but 1 or 2 sources is not enough • Tables and Figures • Consistent format, sig figs, color, format, location in document, references

Comments on Progress Report #1 • References!!! • Appendices • Originator, checker, and dates for all drawings and calculations • Good meeting minutes • Good outcomes, but some of you need a little more support for your conclusions • Proof read everything after you put all of the parts together

Work Tasks • Determine final loads on the structure • Gravity loads (dead, live) • Lateral loads (seismic, wind) • Should probably be done by now! • Truss analysis on roof & design of roof members • Should be nearing completion • Detailing of roof-to-structure connection • Should be nearing completion • Hint: See IBC 1910.4.3

Work Tasks • Review IBC Seismic Detailing Requirements in detail • Work on design of one-way slab, beams, and girders • Framing plan should be finalized • Design for flexure, shear, and torsion • Determine column loadings • Column design discussed in CASE

CEE 4606 - Capstone II Structural Engineering