Reliability-Based System Factor for Serviceability Design of Wood Floors

A structural analysis model for parallel-member wood joist floors is developed that includes the effect of component creep. Viscoelastic material models are calibrated using the data from a recently completed experimental program conducted as part of this overall study. Using this system model, deflection serviceability reliability analyses of parallel-member wood systems, including the effects of creep deformation, are conducted. Stochastic load models are used to simulate the time-varying nature of applied loads. Multiple limit state definitions for deflection serviceability of parallel-member wood floors are considered. Monte Carlo simulation is used to evaluate limit state probabilities. Reliability indices for current serviceability design provisions are also evaluated, and a serviceability system factor for Load and Resistance Factor Design (LRFD) is recommended

Governing Yield Modes for Common Bolted and Nailed Wood Connections

Connections in wood structures are important when designing for ductility. The 1997 Uniform Building Code has taken this into consideration when designating wind and earthquake load duration factors for connections. Factors of 1.6 or 1.33 may be applied to the connection strength, depending on the type of yield mode exhibited by the connection, which may be determined from the yield limit equations supplied in the National Design Specification for Wood Construction (NDS). The NDS provides the designer with multiple tables containing capacities for various common connections. Unfortunately, yield modes are not published along with tabulated capacities. Therefore, the designer must carry out potentially cumbersome calculations using the NDS yield limit equations simply to determine the governing yield mode before an appropriate Uniform Building Code load duration factor can be applied. In this paper, several NDS tables are extended to include capacity and yield mode, smaller side member thickness configurations are added to the existing nail/spike tables, and a useful toe-nail table is provided. The overall purpose of these tables is to accelerate the design process by eliminating time-consuming calculations

Thermal Effects on Load-Duration Behavior of Lumber. Part II: Effect of Cyclic Temperature

The effect of a cyclic thermal loading on the load-duration behavior of structural lumber in bending is presented. Select Structural and No. 2 grade Douglas-fir nominal 2 by 4 beams were tested under a constant bending load to determine time-to-failure. Two cyclic temperature environments were used in the investigation: 73 F to 100 F and 73 F to 130 F on a 24-hour cycle with a constant 50% relative humidity. An exponential damage accumulation model with a temperature factor was used to predict the observed times-to-failure. The damage model originally was fitted and calibrated using load-duration data from equivalent lumber samples subjected to constant temperature environments. The model predicted quite well the observed times-to-failure in the cyclic temperature environments. This is quantified using a standard errors analysis between the model predictions and the observed cyclic temperature data. These errors are comparable to those observed with the constant temperature data which were used to determine the model constants

Moisture Effects On Load-Duration Behavior of Lumber. Part II. Effect of Cyclic Relative Humidity

The effect of cyclic moisture conditions on the load-duration behavior of structural lumber is presented. Select Structural and No. 2. Douglas-fir nominal 2 by 4 specimens were tested in bending in two cyclic relative humidity (RH) environments: 35% to 95% RH on 24- and 96-hour cycles. A constant temperature of 73 F was maintained in both tests. Constant bending loads based on the 15th percentile of the static strength distributions for each grade at 73 F and 50% RH were used to load the beams. The load-duration behavior in the two cyclic RH environments is compared to previously reported results observed from three constant RH environments (35%, 50%, and 95% RH at 73 F). Analysis of test results indicated a trend toward shorter times-to-failure in cyclic RH conditions as compared to constant RH conditions. The effect, however, was no more evident in the No. 2 specimens than in the Select Structural specimens. To predict the load-duration behavior, an existing damage accumulation model was modified to account for the effect of changing moisture contents on the long-term strength of structural lumber. The developed model was found to predict the observed behavior quite well

Thermal Effects On Load-Duration Behavior Of Lumber. Part I. Effect Of Constant Temperature

The effect of constant thermal loadings on the load-duration relationships for structural lumber in bending is presented. Select Structural and No. 2 grade Douglas-fir nominal 2 by 4 (38.1 mm by 88.9 mm) beams were tested in bending under constant load. Constant temperature environments of 73 F, 100 F, and 130 F (22.8 C, 37.8 C, and 54.4 C) were used in the investigation. A constant 50% relative humidity (RH) was maintained for each temperature. The applied bending loads were based on the 15th percentile of the assumed static strength distributions for each grade at 73 F and 50% RH. An exponential damage accumulation model modified to account for temperature effects is used to define the load-duration response. The results indicate shorter times-to-failure with corresponding higher probabilities of failure for equal levels of mechanical stress as the temperature is increased

Efficacy of Interactive Internet-Based Education in Structural Timber Design

While traditional teaching methods (e.g., real-time, synchronous lectures) have proven effective for training future engineers, the Internet provides an avenue to reinforce the material and augment student learning, comprehension, and retention of material. This paper presents the integration and assessment of a library of interactive instructional modules specifically for a senior-level undergraduate elective course in civil engineering. An ongoing, comprehensive assessment process was implemented in the fall 1999 semester. The results of this quantitative assessment indicate that the use of well designed and pedagogically sound Internet-based supplemental modules provide students with a better understanding of course material. However, when Internet-based content does not promote critical thinking, little increase in the student performance and understanding of the material is realized. Interactive Web-based instruction should not be viewed as a “replacement” to traditional instruction, but rather a tool that provides a broader and more dynamic environment for students with a variety of learning styles

Creep and Creep-Recovery Models for Wood Under High Stress Levels

Forty small clear southern pine specimens were loaded under third-point bending to examine creep and creep-recovery behavior for wood under high stress levels. Stress levels of between 69% and 91% of the predicted static strength were applied for 23 h with 1 h allowed for recovery, and the resulting deflection vs. time behavior was studied. The experimental creep and creep-recovery behavior was modeled using modified power law functions. The results indicate that these functions provide the best fit to both primary and secondary experimental data. The empirical models can be used to simulate the viscoelastic behavior of wood under high stress levels. The simulation will provide a useful tool in future studies to examine duration-of-load (DOL) effect, which is one of the more important factors in wood structural design

Accreditation insights and the next body of knowledge

The American Society of Civil Engineers (ASCE) published the second edition of the Civil Engineering Body of Knowledge (BOK2) in 2008 expanding the knowledge, skills and attitudes required of future civil engineers. There were major changes to the BOK2 as the number of expected outcomes increased from 15 to 24 and the cognitive level of attainment was more precisely defined. A major implementation and enforcement mechanism for the BOK is the ABET accreditation criteria which includes both general criteria 3 and 5 and the discipline-specific program criteria. Of those, the program criteria are the easiest to change. In 2013, ASCE created the Civil Engineering Program Criteria Task Committee (CEPCTC) whose charge was to determine if the current CEPC should be changed to reflect an additional one or more of the 24 outcomes of BOK2. After two years of meetings, conference calls, draft criteria, constituency input, and associated revisions, a proposed change to the CEPC was approved by ASCE and submitted to ABET for approval. The CEPC was supplemented with an associated commentary. The proposed CEPC are currently going through the two-year ABET approval process and are expected to go into effect in September 2016. The results of the committee’s work were presented in papers at the 2014 and 2015 ASEE Annual Conferences in Indianapolis and Seattle.1,2 The Body of Knowledge is a living document that will continue to be updated and revised. ASCE has developed an eight year cycle of change that will make future iterations of the BOK and CEPC both systematic and predictable.3 As such, a Body of Knowledge Task Committee (BOKTC) is scheduled to be formed in October 2016. The BOKTC could recommend no revisions, minor revisions, or extensive revisions to BOK2. If substantive changes are recommended to BOK2, the master plan calls for the completion of the third edition of the Civil Engineering Body of Knowledge for the 21st Century (BOK3) before October 2018. Because the CEPC was created to be compatible with the BOK2 outcomes, the CEPCTC studied the BOK2 in depth. The BOK2 is an aspirational and visionary document which only partially accounts for the real-world constraints faced by engineering programs in terms of mandated maximum units in an undergraduate program and additional requirements imposed by a state government or a university. Conversely, the ABET accreditation criteria (general plus program) define the minimum requirements for a program to receive accreditation. There will naturally be a gap between those two standards. For the cycle of change to be successful, the insights and lessons learned from the development of the CEPC should be communicated with the BOKTC and vice versa. This paper attempts to do that. The paper will define the gap between (1) the BOK2 and (2) EAC/ABET accreditation criteria (general plus proposed CEPC) and make recommendations for closing the gap. During their work, the CEPCTC encountered issues with the BOK2 that suggest potential revisions for the BOK3. This paper is a mechanism for sharing CEPCTC insights, lessons learned, suggestions and recommendations with the rest of the academic and professional community

Predicting Strength of Matched Sets of Test Specimens

Five different methods for a priori estimating bending strength of wood and wood composite specimens are compared in this paper. They are: (1) edge-matching, (2) matching specimens by normal distribution, (3) matching specimens by log-normal distribution, (4) simple linear regression, and (5) multiple linear regression. It was found that the square root of mean square error (RMSE) of percent difference (PD) of predicted modulus of rupture (MOR) is the key measure in comparing the five methods. Multiple linear regression was found to be the best method to predict MOR of a specimen in an edge-matched set. Finally, how to create the prediction limits for mean MOR of a subgroup of specimens is discussed. The prediction limits for predicting MOR make it possible to quantitatively determine the effect of various treatments of wood materials

Application of Dynamic System Identification to Timber Beams - part I

In this first part of a two-part paper, development of a method of dynamic system identification for timber beams is presented with an analytical verification of the method using a finite-element model. A method of global nondestructive evaluation for identifying local damage and decay in timber beams is investigated in this paper. Experimental modal analysis is used in conjunction with a previously developed damage localization algorithm. The damage localization algorithm utilizes changes in modal strain energy between the mode shapes of a calibrated model, representing the undamaged state of the beam of interest, and the experimentally obtained mode shapes for a timber beam. Analytical evaluations were performed to demonstrate and verify the use of this method of global nondestructive evaluation for the localization of damage or decay in timber beams. In a companion paper, experimental laboratory tests are presented that verify the use of dynamic system identification to locate damage within timber beams