Advanced Technology for Large Structural Systems Research Center Lehigh University Bethlehem, PA

Transcription

1 Advanced Technology for Large Structural Systems Research Center Lehigh University Bethlehem, PA MATERIAL TESTING ANALYSIS FOR THE SELF-CENTERING CONCENTRICALLY BRACED FRAME Prepared by: Javier Miranda (University of Puerto Rico-Mayagüez, July 31, 29 Project Team: Brent Chancellor (Graduate Student mentor, Lehigh University) David Roke (Graduate Student mentor, Lehigh University) James M. Ricles (Professor of Civil Engineering) Richard Sause (ATLSS Director and Professor of Civil Engineering) This work was supported by the National Science Foundation (NSF) and the Network for Earthquake Engineering Simulation (NEES).

2 Table of Contents ABSTRACT... 3 INTRODUCTION... 3 Overview... 3 PROCEDURE... 5 Testing Protocol for Material Testing on SC-CBF Project... 5 Procedure for Steel Coupons... 5 Procedure for Dywidag Bars... 7 RESULTS... 8 Data Analysis for Steel Coupons... 8 Data Analysis for Dywidag Bars CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

3 ABSTRACT Lehigh University is conducting extensive research on seismic bracing systems to increase building performance during earthquakes. The Network for Earthquake Engineering Simulation Research (NEESR) program at Lehigh University is currently investigating one type of braced frame: a steel concentrically braced frame (CBF). However, many CBF s often suffer damage during earthquakes due to their low lateral drift capacity after yielding. Thus, Lehigh is investigating a seismic building system that takes advantage of the economy and stiffness of a CBF while increasing the lateral drift capacity. This system is known as the self-centering concentrically braced frame (SC-CBF). The system uses rocking behavior and limited yielding in post-tensioning bars to increase the drift capacity. SC-CBF systems should provide stiffness, economy, and sustain little to no damage under seismic loads. Material testing was conducted to determine the mechanical properties of the post tensioning bars and the steel coupon sections taken from the frame. INTRODUCTION Overview Current code provisions require that structures be designed for life safety under a design basis earthquake (DBE). This means that after strong ground motion the structure will still be standing and will allow the tenants to exit the building safely, but it is no longer good for normal business. In many cases it may be desirable for the structure to be reusable immediately after a DBE. Thus, Lehigh University is conducting extensive research on seismic bracing systems to increase building performance during earthquakes. There are many types of bracing systems but, popular common system is the steel concentrically braced frame (CBF) due to its economy, strength and stiffness. However, CBFs can have a low lateral drift capacity thus producing damages during lateral loads. A new concept has been established during the last few years to take advantage of the stiffness and economy of the CBF. The new system has been called the self centering concentrically braced frame (SC-CBF). The self centering system is a concept that permits a rocking motion at its base to sustain less damage during an earthquake. Figures 1 & 2 shows a SC-CBF test structure in Lehigh s ATLSS Laboratory. The test structure is painted orange. 3

4 Fig1. The four story Self Centering Concentrically Braced Frame (SC-CBF) in the ATLSS Laboratory Fig2. The four story SC-CBF with post tension bars in place The testing program for the SC-CBF at Lehigh University includes both dynamic structural modeling and physical lab testing. In order to create a useful structural model the actual mechanical material properties of the constructed frame are desired. The objective of this report is to document the mechanical properties of the material used in the construction of the SC-CBF test. Six Dywidag steel post-tensioning bars and 38 steel coupons were tested to failure in tension. These samples are representativee of the material used in building the test frame. Furthermore, the collected data will be analyzed to obtain various steel mechanical properties and the results will be presented. 4

5 PROCEDURE All tensile tests and final results were done using the guidelines of ASTM E-8 (Standard Test Methods for Tension Testing of Metallic Materials) and the Guide to Stability Design Criteria for Metal Structures by Theodore V. Galambos. A test protocol was developed and is as follows: Procedure for Steel Coupons Testing Protocol for Material Testing on SC-CBF Project First, each of the thirty eight coupons should be marked in order to identify them later. A typical coupon is shown in Fig. 3. Each coupon should be two feet in length and about one and one-half inches wide. Then, punch mark an eight inch gage length at the center of the coupon using a hammer and a punch like those shown in Fig. 4. Afterwards, determine the thickness and width in three different locations inside the gage length (Top, Center, and Bottom). Write these values on a sheet of paper because it will be used to determine the cross sectional area of the specimen. Also, measure the gage length with a caliper and write it down as the original gage length. Once a coupon is properly identified it shall be gripped with an extensometer like the one shown in Fig.5. Then, the Partner Software Application has to be turned on in order to start the SATEC 6K tensile testing machine (see Fig.6). Before testing any specimen make sure that all pre-load is removed from the SATEC 6k. Once all loads are removed, clamp the two ends of the specimen in the SATEC 6k machine (see Fig. 6). Now, to start collecting data the PC 9 Software must be turned on. This program will save values like load, displacement, stress and strain. The SATEC 6k machine will be operated at three different speed zones for the test. The maximum allowable loading rate (crosshead separation) for a test is 16in/min/in of gage length so for an 8 in. gage length the maximum speed is ½ in/min. The first zone is a soft start at.1 in/min until a few hundred pounds of tensile force are on the test specimen. The loading rate then changes to.25 in/min until strain hardening (after yield) is reached. Once the material begins to strain harden the machine speed will be changed to the maximum allowed:.5 in/min. The SATEC machine is run at this speed until the test specimen fails in tensile fracture. One desired mechanical property from testing is the static yield stress. The static yield stress is independent of machine speed and is obtained by the following procedure. Once the stress vs. strain or load vs. crosshead displacement has reached a yield plateau or begins to demonstrate yielding, the testing machine will be stopped for 5 minutes or until the load stabilizes. The crosshead will be restarted at the same speed for a few seconds (or more if a speed less than the maximum is used) and then stopped again until the load stabilizes. This will be repeated three times. The testing of the coupon will then continue until failure. 5

6 After the coupons fails all data should be saved and the specimen needs to be taken out of SATEC Machine (see Fig. 7 and Fig. 8). Once the coupon is removed the two pieces of the separated specimen are clamped back to together in order to measure the change in the gage length with a caliper. Finally, all collected data should be saved (see Fig. 9) and presented in a technical report. Fig3. A. steel coupon, measuring tape and caliper Fig4. Hammer and Punch Marker Fig5. An extensometer and two LVDTs. Fig6. The SATEC 6k machine. Fig7. Coupon shortly after failing. Fig8. Taking coupon out of SATEC machine.. Fig9. Saving data from PC 9 Software. 6

7 Procedure for Dywidag Bars The procedure for testing the Dywidag post-tensioning bars is similar to that of the coupons except that the extensometer is replaced with strain gages due to the short bar length available for testing. Two strain gages should be placed on the center of both sides of the bar (Fig. 1) ). Each bar should be identified and marked with a specimen number. The gage length will be marked with both a two inch and an eight inch punch mark on every bar. Just like with the steel coupons the SATEC 6k machine, Partner application and PC 9 Software will be use. The strain gages cables will be connected to the data acquisition system. Then, the Dywidag bars are placed into the testing apparatus in the SAETEC machine. This is shown in Fig. 11. The load is then applied the same way as with the coupons until the bar fails (Fig. 12). Afterwards, save all data and measure the new gage length with a caliper. Fig1. A Dywidag steel bar with a strain gage and ready for testing Fig11. A Dywidag steel bar gripped to the SATEC machine. 7

8 Fig12. A Dywidag steel bar shortly after failing. RESULTS All of the collected data was analyzed to obtain various mechanical properties. The most important results are reported in this paper. The mechanical properties reported are static yield, ultimate strength, yield strength and percent elongation. Additionally, there will be stress vs. strain curves and load vs. strain curves. The final results are tabulated and shown. Data Analysis for Steel Coupons Stress (ksi) Stress vs. Strain Yield Strength Dynamic Yield Strength Static Yield Strength 2 1.E+ 5. E 3 1.E 2 1.5E 2 2.E 2 Strain (in/in) Strain Hardening 2.5E 2 3.E 2 Graph1. This is a partial Stress vs. Strain curve for a W8x67 Flange section. Graph 1 illustrates the elastic and plastic region of a stress vs. strain curve. The specimen identification is W8x67 Flange 3. The computer program did not calculate the exact stress due to the fact that it divided the load by an approximation of the original area. Thus, the stress had to be calculated dividing the load by an average area of the gage length. This steel coupon has a yielding point of ksi, a proportional limit of 46 ksi, dynamic yield strength of 5.55 ksi, 8

9 static yield strength of ksi, strain hardening point of ksi, and a Young modulus (E) of 29,457 ksi. The static yield strength was obtained with the average of the minimum values of the three peaks in the plastic region. It represents the average stress for a zero load rate and is independent of testing speeds. The slope of the linear or elastic region was calculated applying Hooke s Law. The slope represents the Young modulus of the material in which there was a 1.5% error compared to the standard value of 29, ksi. The average stress in the horizontal plateau of the plastic region at a constant rate gives us the Dynamic Yield strength. The percent elongation at failure was 28.38%. This means that the coupon was stretched to % of its original length before breaking. The yielding point elongation (YPE) is the difference in strain between the first zero slope or yielding point and the strain at the initiationn of strain hardening. This specimen had a YPE of 1.46%. YPE is defined in ASTM E8 as: the strain (expressed in percent) separating the stress-strain curve s first point of zero slope from the point of transitionn from discontinuous yielding to uniform strain hardening. Stress (ksi) E+ + 5.E 2 1.E 1 Stresss vs. Strain Ultimate Strength 1.5E 1 Strain (in/in) 2.E 1 Fracture 2.5E 1 3.E 1 Graph2. This is a complete Stress vs. Strain curve for a W8x67 Flange section. Graph 2 demonstrates the entire stress vs. strain curve of the W8x67 Flange 3. It clearly exemplifies a normal stress vs. strain curve except the end of it. This is because after it failed the LVDTs stretched a small amount increasing the strain just a little and since it failed the program stopped recording stress values. Thus, we can discardd the line at the end of the graph. The W8x67 Flange 3 specimen has a tensile strength of 69.4 ksi at a strain of in/in. 9

10 Load (kips) Load vs. Strain E+ 6.E-2 1.2E-1 1.8E-1 2.4E-1 3.E-1 3.6E-1 Strain (in/in) Graph3. This is a Load vs. Strain curve for a W8x67 Flange section. Graph 3 The load vs. strain curve shows a maximum load of 93 kips. The specimen yieldedd at a load of about 7 kips. After reaching the maximum load the specimen starts necking in the center and the area is lesss thus reduces the load necessary to pull it apart. The W8x67 Flange 3 specimen failed at a load of 65 ksi. Stress (ksi) E+ + 5.E 3 Stresss vs. Strain 1.E 2 Strain (in/in) 1.5E 2 2..E 2 Graph4. This is a partial Stress vs. Strain curve for a W8x67 Web section. 1

11 Graph 4 is a partial view of a stress vs. strain curve and it does not have a well defined yield point. In most cases you will have to use the.2% offset method to obtain the yield strength. This happened to most of the web coupons and is probably due to the way that W-shapes are rolled in the mill. 1 Stresss vs. Strain 8 Stress (ksi) E+ 2.E 2 4..E 2 6.E 2 8.E 2 1.E 1 1.2E 1 1.4E 1 1.6E 1 Strain (in/in) Graph5. This is a complete Stresss vs. Strain curve for a W8x67 Web section. Graph 5 is a complete view graph 4. This coupon had an ultimate strength of about 8 ksi and it is well above the minimum required (65 ksi). 11

12 Table 1. Tested Material Properties for the Steel Coupons Steel Average Static Average Ultimate Average Yield Coupon Yielding Strength Strength ID (ksi) (ksi) (ksi) W8x48Flange W8x48Web W8x58Flange W8x58Web W8x67Flange W8x67Web W12x5Flange W12x5Web W1x112Flange W1x112Web Plate t= 1 1/4" Plate t= 1" Plate t= 3/4" Plate t= 1/2" The above table gives the results for the W-shapes and plates used on the frame. Average static yielding, yield strength, and ultimate strength are reported. Four flange sections and two web sections were tested for most of the different W-shapes. A few coupons tests had significant experimental error and are not included in the results. In general the yielding for the webs was slightly higher than those from the flanges and plates. However, the flanges have higher ductility than webs sections. The W8x58 Flange and W8x67 Flange sections have static yield stress slightly under 5 ksi. 12

13 Data Analysis for Dywidag Bars Load vs. Avg. Strain Load (kips) Avg. Strain (ustrain) Graph6. This is a Load vs. Avg. Strain curve for a Dywidag Bar (PT-4). Graph 6 illustrates a Load vs. Average Strain curve for a Dywidag bar. The curve looks normal up to the static yield point. Afterwards it becomes worthless because the strain gage likely debondedd from the bar. But we could seee that the post tensioning bar is a high resistance material with high yield strength (approximately about 12 kips). The values predicted in the model were 12 kips yield strength and 127 kips for the ultimate strength. The graph clearly exemplifies greater values for yielding and ultimate strength. 13

14 Load vs. Crosshead Position Load (lbf) Crosshead Position (in) Graph7. This is a Load vs. Crosshead Position curve for a Dywidag Bar (PT-4). Graph 7 illustrates a Load vs. Crosshead position curve. The crosshead is the upper and lower part of the SATEC machine that separate to load the specimen. This data was taken from the Partner Software and this is how the Load vs. Ave, Stress of PT-4 independent of the strain gages so should have looked in the strain hardening region of the curve. This program is totally it was able to record data properly. Dywidag PT-Bars ID PT-Bar 1 PT-Bar 2 PT-Bar 3 PT-Bar 4 PT-Bar 5 PT-Bar 6 Average Static Yield Strength (kips) Average Ultimate Load (kips) Average Yield Strength (kips) Table 2. Tested Material Properties for the Dywidag Bars Table 2. gives the average results of every PT Bar. The model assumed a minimum yield load of 12 kips and an ultimate load of about 127 kips. Thus, the values obtain for the PT bars were higher than predicted. This means that the PT bars are a high resistance material and this may be problematic because of certain assumptions in the design of the SC-CBF frame. 14

15 CONCLUSIONS Mechanical material testing for Self Centering Concentrically Braced Frame (SC-CBF) project was conducted. Thirty eight steel coupons from five W-shapes and four plate thicknesses as well as six Dywidag post tensioning bars were tested and the material properties reported. In all cases the yield strength and the ultimate strength was higher than the minimum required (5 ksi and 65 ksi) for A992 steel. Most of the webs of W-shaped sections had a higher yielding than the flanges as expected. The Dywidag bars are a high resistance material with high yield strength, but it has a poor ductility compared to the flange and web specimens. Also, the yield strength for the material is higher of what was predicted by the structural model and manufacturer. This may turn out to be a problem. The model assumes that the PT Bars will yield first but since we obtain a higher yielding the frame may yield before the PT bars. So is better off to have the PT bars yield first due to economical reasons. It will be easier to fix the PT Bars instead of repairing the entire frame. ACKNOWLEDGEMENTS Thanks to the National Science Foundation (NSF) and the Network for Earthquake Simulation (NEES) for making this work possible and giving undergraduate student the opportunity to have on hand research experience. Also, special thanks to the students mentors Brent Chancellor and David Roke as well my project advisors, Dr.Richard Sause and Dr.James Ricles for guiding me trough the REU program. Likewise, Lehigh University and the ATLSS staff members, especially REU coordinator Gary Novak. 15

16 REFERENCES 1.) Gonner, Nathaniel. Design and Experimental Setup for Self-Centering Steel Concentrically Braced Frame Test Structure. May ) Self-Centering Damage-Free Seismic-Resistant Steel Frame Systems. NEES at Lehigh, Bethlehem, PA. Access June 1, 29 < 3.) Sause, Richard., et al. Design of Self-Centering Steel Concentrically Braced Frames Paper No.122, 4 th international Conference on Earthquake Engineering. OCT , ) ASTM International. Standard Test Methods for Tension Testing of Metallic Materials. (ATSM-E8). West Conshohocken, PA. (25) 5.) Galambos, Theodore V. Guide to Stability Design Criteria for Metal Structures, Pg John Wiley (June 1998) 16