Structural Evaluation Summary 7 12 05

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Information about Structural Evaluation Summary 7 12 05
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Published on January 1, 2008

Author: Margot

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Performance of Steel Props at the UNR Fire Science Academy subjected to Repeated Fire:  Performance of Steel Props at the UNR Fire Science Academy subjected to Repeated Fire Project Summary Lyle Carden Ahmad Itani Patrick Laplace Department of Civil and Environmental Engineering Outline of Presentation:  Outline of Presentation Objectives from Proposal Deliverables from Proposal Background Literature – Response of Steel at different Temperatures FSA Props - Structural Drawings Recording of Temperatures in Props Structural Analysis of Props Conclusions Recommendations Objectives:  Objectives Ensure life safety for trainees at the facility by preventing structural collapse. Determine a maximum recommended temperature in the steel for critical regions of the structures. Understand the potential of progressive collapse failure mechanisms in the structures and determine the level of safety. Evaluate the effect of repeated fire loading on the long term performance of the structures. Provide recommendations for improving the immediate and long-term performance of the structures. Deliverables:  Deliverables A complete evaluation and current status report of the props from a life safety perspective. Immediate recommendations for any modifications to the structures. Suggested maintenance criteria for long term safety and asset preservation. Periodic testing can be performed using the instrumentation which will be implemented during this evaluation and site staff can be trained in the use of the instrumentation. The data acquisition system and instrumentation will remain the property of the Fire Science Academy. A set of “as-built” drawings of the structures will be provided which will be stamped by a professional engineer upon completion of any changes to the structures deemed necessary for safety. Best practices for design and connection details of future props will also be recommended. Steel in Fire:  Steel in Fire From Tide (1998) Steel in Fire – AISC Provisions:  Steel in Fire – AISC Provisions FSA Props - Structural Drawings:  FSA Props - Structural Drawings Refer to Appendix 1 Truck Loading Rack Tri-level Process Unit Walkway between Process Units Process Unit Pipeline Pump Row These are the structures with critical structural elements not previously engineered. FSA Props - Structural Drawings:  FSA Props - Structural Drawings Weld sizes not shown on drawings. Fillet welds are assumed to have the minimum weld size as specified in Table 2-1. Welds assume shielded metal arc welding procedure using E60XX or E70XX electrodes. FSA Props - Measurements:  FSA Props - Measurements Measurement of Temperatures on Tri-level Pump Row Coupon Tests on Steel Samples Temperature Recording:  Temperature Recording See Appendix 2 for RTD and Lead Wire Product Information Temperature Recording:  Temperature Recording Temperature Recording:  Temperature Recording See Appendix 3 for system operation instructions Temperature Recording:  Temperature Recording Software Temperature Recording:  Temperature Recording Software Temperature Recording:  Temperature Recording Calibration Recalibration for water Placement on Tri-level Placement on Pump Row Temperatures on Tri-level - Columns:  Temperatures on Tri-level - Columns Temperatures on Tri-level - Beams:  Temperatures on Tri-level - Beams Temperatures on Tri-level – Floor Grate:  Temperatures on Tri-level – Floor Grate Temperatures on Pump Row - Columns:  Temperatures on Pump Row - Columns Temperatures on Tri-level – Beams / Braces:  Temperatures on Tri-level – Beams / Braces Coupon Tests – RHSS Columns:  Coupon Tests – RHSS Columns Coupon Tests – Beams:  Coupon Tests – Beams Coupon Tests – Summary:  Coupon Tests – Summary Some variability between different samples. Variability cannot be attributed to fire. Yield strength above nominal yield strength in all cases. Structural Analysis – Tri-Level:  Structural Analysis – Tri-Level Analysis – Tri-Level:  Analysis – Tri-Level Considered seven different load combinations of dead load, live load and thermal loads Three cases of temperature loads T1 – Uniform change in temperature of 160 °F T2 – Differential temperature between top and bottom flange of beams of 50 °F T1 – Differential temperature in a single column of 100 °F Analysis – Tri-Level - Connections:  Analysis – Tri-Level - Connections Classify fillet welds in connections in three catagories: Critical – these should be checked regularly (weekly) Necessary – minimum weld should be maintained but not critical Redundant – can be removed, possibly improving the structure Analysis – Tri-Level - Connections:  Analysis – Tri-Level - Connections Most welds in tri-level are not critical (ie. necessary or redundant) as the main girders are bearing on top of the columns, therefore the welds are not carrying loads but merely providing stability. Critical welds in tri-level include: Girder/beam web to beam or column fillet welds where the beams are not bearing on top of the girders/beams or columns. Side wall of steps to connecting angles or plates and connecting angles or plates to the web of a beam. Redundant welds include: Girder/beam flange to beam or column connections where girders/beams are supported at both ends. Anchor bolts connecting steps to concrete slab can be removed. Analysis – Pump Row:  Analysis – Pump Row Analysis – Pump Row:  Analysis – Pump Row Considered five different load combinations of dead load, live load and thermal loads Two cases of temperature loads T1 – Uniform change in temperature of 630 °F in columns T2 – Differential temperature between two sides of a column of 500°F Analysis – Pump Row:  Analysis – Pump Row Differential temperature in the columns resulted in large bending moments. Columns should be cooled to prevent differential temperatures of more that 100 °F. Braces have large axial loads due to large temperatures. All but four braces at ends could be removed. Temperatures in braces should be limited to 300 °F. Most connections in Pump Row are not critical. Critical connections include: Flange and web connections for cantilever beams at east end of structure. Beams in tower at north-west corner of structure. Analysis – Walkway:  Analysis – Walkway Several critical connections: Anchor bolts connecting steps to concrete slab can be removed. Web connections for the beams to Process at the north end of the walkway have critical welds. The web connection at the top of the steps is also critical. The top flange of C8x11 channels should be connected with fillet welds to adjacent beam on the west end. The web connections at the east end of the C8x11 beams are critical. Analysis – Truck Rack:  Analysis – Truck Rack Loading investigated on trusses. Analysis – Truck Rack:  Analysis – Truck Rack Considered five different load combinations of dead load, live load and thermal loads Two cases of temperature loads T1 – Uniform increase in temperature of 835 °F T2 – Individual increase in temperature of 100 °F for each member Analysis – Truck Rack:  Analysis – Truck Rack Maximum increase in temperature should be 300 °F in critical members. Buckled members should be replaced. Support moved under the joint location. Angle supporting north end of the truss should be welded all round. Critical welds in other parts of the structure should be maintained. Other Props:  Other Props Minimum weld requirements should be maintained throughout. No structural issues foreseen. Fatigue:  Fatigue Fatigue life dictated by quality of welds and restraint provided by connections. Factors to maximize fatigue life: Removal of redundant welds will allow the structure to deform when heated without restraint forces reducing stresses in the member and increasing fatigue life. Where repeated fatigue of welded connections is observed removal of redundant welds should be considered. Good quality welds should be continuous along the full length of the connected parts, without the presence of notches and discontinuities. Members should be uniformly welded so there are no weak spots for concentration of stresses. Damaged members identified for replacement such as those in the Truck Rack should be promptly replaced to prevent consequential damage to other members due to improper load path. Conclusions – Temperature Effects:  Conclusions – Temperature Effects From the literature, the maximum temperature at which structural steel of the types used in the props will lose its strength and stiffness due to the onset of phase change in the steel is approximately 1200 °F. At a temperature of 900 °F the strength of the steel will be reduced to around 80% of its corresponding strength at ambient temperature. At this temperature steel is likely to creep resulting in permanent deformations in the structural members. This is considered the maximum allowable temperature. Resistance temperature detectors (RTDs) attached to a data acquisition system are able to measure and record temperatures in the steel structures, although some corrections have to be made to allow for shorting of the cables when wetted. Conclusions – Tri-level:  Conclusions – Tri-level The maximum temperature measured in the beam and columns of Tri-level is 225 °F and the maximum differential temperature in the beams and columns of Tri-level is around 100 °F in the region instrumented, well below the maximum allowable temperature. Differential temperature in the columns and beams of Tri-level result in deformations estimated at 2 times the elastic deformation, thus some localized plastic deformation is expected, although is within acceptable levels. The maximum temperature recorded in the floor grating of Tri-level is 700 °F in the region instrumented, which is below the maximum allowable temperature, but thermal expansion explains the buckling of the grating observed in this region. Conclusions – Pump Row & Truck Rack:  Conclusions – Pump Row & Truck Rack The maximum temperature recorded in the columns and braces of Pump Row is 700 °F and the maximum column differential temperature is estimated at 500 °F in the region instrumented. The maximum beam temperature is negligible. Large permanent curvatures of the columns are expected due to the differential temperatures in Pump Row explaining the deformation observed in the structure. Buckling is also expected in the braces between the beams and columns due to thermal expansion, as also observed. Uniform temperatures have little effect on the structures with the notable exception of the braces in Pump Row as the temperatures are below the maximum allowable temperature. The buckled braces in Truck Rack can be attributed to differential temperatures in excess of 300 °F. Bending in the bottom chord can be attributed to the location of the intermediate support resulting in significant bending stresses in the member. Conclusions – Connections & Material:  Conclusions – Connections & Material The welded connections in all of the structures are categorized into three groups, critical, necessary and redundant. The welded connections provide large restraint forces resulting in significant stresses, therefore fatigue of the connections is expected. The monotonic stress-strain properties of steel samples were not affected by a number of exposures to large temperatures. Recommendations:  Recommendations Temperatures should be maintained at below 900 °F or less where otherwise stated. Critical welds as identified on the drawings should be checked continuously (weekly) and maintained free of cracks at all times. The minimum weld thickness specified in Table 2-1 should be provided. SMAW procedure should be used with E60XX or E70XX electrodes. The floor grating in Tri-level and Process should span continuously between the main girders to provide a level of redundancy so that if some of the secondary beams were to become damaged catastrophic collapse will be prevented. The anchor bolts connecting the base of the steps to the concrete slab, in Tri-level, Process, Truck Rack and the walkway between Tri-level and Process, where not already removed, should be removed to allow the base of the steps to slide freely. Recommendations:  Recommendations Situations where large differential temperatures are observed between different sides of members, such as in the columns in pump row, should be eliminated by improved cooling and reorientation of the leaking flanges causing concentrated heating of a member. The differential temperature between two sides of a column in Pump Row should be limited to 300 °F. The columns in pump row should be straightened if the offset in the displacement of the top of the column exceeds 2.4 inch to prevent instabilities in the columns. All but four braces between the beams and columns in Pump Row where damaged can be removed without replacement or should be cooled more effectively limiting the maximum temperature to 300 °F . Four undamaged braces should be left for lateral stability of the structure. The damaged truss members supporting the walkway in the truck rack should be replaced. The buckled members indicate temperatures greater than the members can withstand. Therefore, after replacing the buckled members better cooling should be applied to these members to limit temperatures to 300 °F. As the members consist of relatively thin plates they are more susceptible to fire than other members with thicker plates. Recommendations:  Recommendations The intermediate support of the trusses should be moved to the north and located beneath the closest joint between horizontal, vertical and diagonal truss members. The angle supporting the north end of the trusses in Truck Rack should be welded all round to the adjacent column with the minimum size fillet weld for the members as indicated by Table 2-1. Redundant welds may be removed where there is weld cracking to reduce the stresses in the members and connections. Welds should be continuous, without notches, to minimize cracking due to fatigue. Recommendations:  Recommendations Further monitoring of the structures is recommended. In order to improve recorded measurements, the lead wires of the sensors may be coated with a waterproof product. Otherwise corrections to the data, upon wetting of sensor wires will be necessary. Further RTDs can be purchased where necessary and mechanical connections between the sensors and lead wires, encased in a high temperature cement, can be used to avoid welding issues. Slide45:  Questions, Comments and Discussion

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