Compatibility Forces in Floor Diaphragms of Steel Braced Multi-Story Buildings

Floors have a key role in the seismic behaviour of structures, especially in multi-story buildings. The in-plane behaviour of a floor system influences the seismic response of the structure significantly and affects the distribution of lateral forces between seismic resisting systems and over the height of the structure. In buildings where the seismic resisting systems are in the same location in plan on each floor over the height of the building, inertial and displacement compatibility shear forces are the principal shear forces generated at the interface between the floor system and the seismic-resisting system. These two are called interface diaphragm forces. These interface forces must be transferred into the appropriate lateral load resisting system and the interface must be well designed and detailed. Determination of the magnitude of the interface loads on concrete diaphragms are not well understood and still a matter of debate. There is no consensus of a design procedure for determining the diaphragm actions and distribution into the seismic resisting systems. In this paper, interface forces generated in floor diaphragms by asymmetrical actions of the braced framing system on each side of the building in the direction of analysis have been investigated. A numerical study using Numerical Integration Time History Analysis (NITH), has been undertaken to evaluate the interface forces of concrete floor diaphragms in a 12-story braced steel building. The results of nonlinear time history analyses using ground motion records from three different earthquakes are presented.

Measurement of Milwaukee Brace Pad Pressure in Adolescent Round Back Deformity Treatment

<sec><title>Study Design</title><p>In this prospective study, we measured the pad pressures of the Milwaukee brace in adolescent hyperkyphosis treatment.</p></sec><sec><title>Purpose</title><p>We evaluated the skin-brace interface forces exerted by the main pads of the Milwaukee brace.</p></sec><sec><title>Overview of Literature</title><p>A fundamental factor associated with brace effectiveness in spinal deformity is pad force adjustment. However, few studies have evaluated the in-brace force magnitude and its effect on curve correction.</p></sec><sec><title>Methods</title><p>Interface forces at four pads of the Milwaukee brace were measured in 73 patients withround back deformity (mean age, 14.04±1.97 years [range, 10–18]; mean initial Cobb angle,67.70°±9.23° [range, 50°–86°]). We used a modified aneroid sphygmomanometer to measure the shoulder and kyphosis pad pressures. Each patient underwent measurement in the standing and sitting positions during inhalation/exhalation.</p></sec><sec><title>Results</title><p>The mean pad pressures were significantly higher in the standing than in thesitting position, and significantly higher pressures were observed during inhalation compared toexhalation (<italic>p</italic>=0.001).There were no statistically significant differences between right and left shoulder pad pressures (<italic>p</italic>&gt;0.05); however, the pressure differences between the right and left kyphosis pads were statistically significant (<italic>p</italic>&lt;0.05). In a comparison of corrective forces with bracing for less or more than 6 months, corrective force was larger with bracing for less than 6 months (<italic>p</italic>=0.02). In the standing position, there were no statistically significant correlations between pad pressures and kyphosis curve correction.</p></sec><sec><title>Conclusions</title><p>In the sitting position, there was a trend toward lower forces at the skin-brace interface; therefore, brace adjustment in the standing position may be useful and more effective. There was no significant correlation between the magnitude of the pad pressures and the degree of in-brace curve correction.</p></sec>

Optimal Sensor Placement for Substructural Response Reconstruction

In an existing substructural dynamic response reconstruction method (Li, J., and Law, S.S., 2011. “Substructural Response Reconstruction in Wavelet Domain,” ASME J. Appl. Mech., 78(4), p. 041010) developed by Law, two sets of sensors are needed for the reconstruction of dynamic responses at selected degrees-of-freedom. A method to find the optimal sensor placement is presented in this paper for the substructural response reconstruction. It is based on the effective independence method but in the time domain. Unlike previous methods on sensor placement, two sets of optimal sensor placement are needed with the first set for estimating the interface forces between substructures, and the second set for reconstructing the responses. Sensors that capture the most information of the interface forces will be selected into the first set, and the subsequently estimated interface forces are used to reconstruct the responses at the second set of selected degrees-of-freedom. The selection of the second set of sensors is based on the least measurement noise effect in the response reconstruction process. A box-section bridge deck is adopted in the simulation studies. Numerical simulations with the forward and backward sequential sensor placement methods show that the proposed method could give reasonable predictions with smaller error in the reconstructed responses, and sensor locations along the major directions of the interface forces should be selected into the first or the second set of sensor configuration.

Synthesis and Control of Flexible Systems With Component-Level Uncertainties

An efficient and computationally robust method for synthesis of component dynamics is developed. The method defines the interface forces/moments as feasible vectors in transformed coordinates to ensure that connectivity requirements of the combined structure are met. The synthesized system is then defined in a transformed set of feasible coordinates. The simplicity of form is exploited to effectively deal with modeling parametric and nonparametric uncertainties at the substructure level. Uncertainty models of reasonable size and complexity are synthesized for the combined structure from those in the substructure models. In particular, we address frequency and damping uncertainties at the component level. The approach first considers the robustness of synthesized flexible systems. It is then extended to deal with nonsynthesized dynamic models with component-level uncertainties by projecting uncertainties to the system level. A numerical example is given to demonstrate the feasibility of the proposed approach.

Addressing Cam Wear and Follower Jump in Single-Dwell Cam-Follower Systems With an Adjustable Modified Trapezoidal Acceleration Cam Profile

Presented is a modified trapezoidal cam profile with an adjustable forward and backward acceleration. The profile is suitable for single-dwell cam and follower applications. The main benefit of the profile is that it allows cam designers to choose easily a value for the maximum forward or maximum backward acceleration to achieve design objectives. An additional benefit of the profile is that it has a continuous jerk curve. Follower acceleration is one of the primary factors affecting cam wear and follower jump, two main concerns of cam designers. Large forward acceleration against a load creates cam-follower interface forces that can cause excessive wear. Backward acceleration tends to reduce the cam-follower interface force, and if the backward acceleration is sufficiently large, separation between the cam and follower (“follower jump”) can occur. The cam profile presented in this paper gives cam designers an easy way to adjust the maximum forward or backward acceleration to prevent these problems.

Influence of Attachment Pressure and Kinematic Configuration on pHRI with Wearable Robots

The goal of this paper is to show the influence of exoskeleton attachment, such as the pressure on the fixation cuffs and alignment of the robot joint to the human joint, on subjective and objective performance metrics (i.e. comfort, mental load, interface forces, tracking error and available workspace) during a typical physical human-robot interaction (pHRI) experiment. A mathematical model of a single degree of freedom interaction between humans and a wearable robot is presented and used to explain the causes and characteristics of interface forces between the two. The pHRI model parameters (real joint offsets, attachment stiffness) are estimated from experimental interface force measurements acquired during tests with 14 subjects. Insights gained by the model allow optimisation of the exoskeleton kinematics. This paper shows that offsets of more than ±10 cm exist between human and robot axes of rotation, even if a well-designed exoskeleton is aligned properly before motion. Such offsets can create interface loads of up to 200 N and 1.5 Nm in the absence of actuation. The optimal attachment pressure is determined to be 20 mmHg and the attachment stiffness is about 300 N/m. Inclusion of passive compensation joints in the exoskeleton is shown to lower the interaction forces significantly, which enables a more ergonomic pHRI.