Background
ClearCalcs calculates in real time for instant results. When designing a wood beam to ASD or LRFD, four conditions must be checked for adequate design: bending strength, shear strength, deflection and bearing capacity. This article will outline how to interpret your ClearCalcs results and determine ways to optimize results. This will allow for more efficient and cost-effective designs. If you’d prefer to follow along with the video, check out the video below.Summary
Moment Utilization
Moment utilization is the ratio of adjusted moment capacity to governing moment demand. Moment capacity represents the maximum bending that the beam can resist without excessively deflecting and failing.- Increase the depth of the member. Consider using a 2x10 member instead of a 2x8 if your beam is not passing in design. This impacts the net area and section modulus, therefore increasing the bending capacity.
- Add additional support to the beam. Reducing the effective span length will decrease the applied loads and lower the moment demand per span. For example, if you have a joist 14 ft long, but you add an additional support at the midspan, there will be a lower governing load demand for the same supporting capacity.
- Use a stronger wood species or grade. Using a hardwood, rather than a softwood, or a No. 1 compared to a No. 2 grade will have higher strength properties and will increase the moment capacity per member.
Shear Utilization
Similar to moment utilization, the shear utilization is the ratio of factored shear capacity to governing shear demand. Shear capacity represents the maximum shear force that can be supported before shear deformation or failure occurs.- Increase the member depth. Increasing the net area will result in a higher shear capacity. For example, if you have a 2x8 member close to failure, consider using a 2x10 member instead.
- Use a stronger wood species or grade. Strength values are based on species, grade, size and use and higher strength properties will increase the shear capacity per member
- Increase the number of plies. This will result in a higher cross sectional stiffness and will also increase the number of layers that can resist shear. For example, using a two-ply member instead of one ply with approximately double the capacity and halve the utilization
Bearing Utilization & Minimum Bearing Length (End Supports)
Bearing utilization is the ratio of bearing capacity at the most critical support divided by the governing bearing load. Bearing capacity represents the maximum allowable stress or pressure that the beam can support without deforming or failing. When the beam fails due to bearing stress, it can either crush itself or the support, potentially punching through the material. This is common in compression members.- Increase contact area between the wood beam and the support in which the bearing surface exists. For example, if your bearing length is 3 inches, consider changing your supporting post to have a larger area where your bearing length can be 5.5 inches. This would significantly decrease bearing stress, as you nearly doubled your bearing length and the bearing stress it is applied over a greater area.
Governing Live / Short-Term Deflection
Live / short-term deflection is the deflection that only considers short term serviceability loads such as live, snow, wind, earthquake or roof live loads.Governing Long-Term Deflection
Long-term deflection on the other hand considers long term loads including dead load compounded with the effects of creep.- Stronger beam stiffness, using a higher moment of inertia (I) or modulus of elasticity (E) will result in less deflection. This is because typical deflection equations are directly impacted by the material’s stiffness. For example, the deflection for a simply supported beam with a point load equals PL^2/48EI, where the deflection is impacted by load, length and stiffness.
- Shorter span lengths will increase stability and decrease the applied loads per span.