56 Vidal et al. 2003; Cucchi et al. 2004; Peltola 2006), responses to wind or mechanical pulling (Petty and Swain 1985; Milne 1988; Peltola et al. 1993; Baker 1997; Saunderson et al. 1999), and wind tunnel experiments on canopies (Gardiner 1994; Wood 1995; Gardiner et al. 1997; Gardiner et al. 2005; Vollsinger et al. 2005). The approach to tree biomechanics by Mattheck and Breloer (1994) and their “axiom of uniform stress” is an example that has influenced arboricultural practice, but there was little, if any, dynamic analysis in these studies. Historically, while it was recognized that wind was not a static force and that trees responded to gusts of wind, to simplify the analysis, wind load- ing was oſten considered to be a static force with different values assigned to cope with varying wind forces. Later work approximated wind forces by pulling a tree with a rope at the equivalent force of an estimated wind force, and evaluating the stability of the tree (Brudi 2002). The forces applied to the trees depended on factors modelled, such as wind speed, upwind conditions, and tree characteristics, such as size, shape, and mass. The tree’s resistive forces depended on factors such as stem character- istics, wood strength, and root plate and soil inter- actions. The resistance to overturning and breakage is based on empirical relationships developed from tree pulling tests and timber strength tests. Static and Dynamic Loads on Tree Canopies Static pull tests were used to determine the mechan- ical resistance to overturning (Moore 2000; Cucchi et al. 2004), the strength parameters of a tree (in- cluding the strength of the trunk and the anchor- age strength of the root plate and soil combination) (Silins 2000), and to approximate the wind force acting on a tree and its responses. Other studies used the static pull test to assess tree strength and stability (Smith et al. 1987; Gardiner 1995; Hedden et al. 1995; Papesch et al. 1997; Flesch and Wilson 1999; Stokes 1999; Achim et al. 2003; Cucchi et al. 2004; Lundstrom et al. 2008), but trees failed or snapped at wind speeds considerably lower than those predicted by the tests on calm days (Fra- ser and Gardiner 1967; Oliver and Mayhead 1974; Gardiner 1995; Hassinen et al. 1998), probably because the static analyses failed to consider the dynamic forces affecting trees (Mayhead 1973). ©2014 International Society of Arboriculture Moore: Wind-Thrown Trees: Storms or Management? Dynamic loads can be defined simply as time- varying (Clough and Penzien 1993) and may vary with magnitude, direction, and/or posi- tion with time. The responses of tree structures also vary with time (Coutts and Grace 1995). Trees and their leafy canopies are flexible, and their surfaces realign themselves in high winds by reconfiguring their shape and reducing the total canopy area (Vogel 1989) as the whole canopy bends and changes shape, becoming more stream- lined, which reduces drag (Rudnicki et al. 2004). Mass Damping by Branches and Foliage Damping is a dynamic parameter that estimates how much energy is absorbed or transferred. Perhaps the best known examples of mass damping are the mass dampers that are placed in skyscraper buildings to reduce sway during earthquakes. Measuring the effect of mass damping in a tree is difficult because there are complex transfers of energy from the wind to the tree. The tree absorbs energy at its natural fre- quencies, with most energy absorbed at the tree’s first natural frequency (Holbo et al. 1980; Mayer 1987; Peltola 1996), which is the frequency of oscillation of a system under free vibration when no external force is applied (James 2010). Most modeling has consid- ered the tree as a single degree of freedom system, like a pole or a ship’s mast. However, trees are multi- degree of freedom systems due to their branches and foliage, so the natural frequency is the frequency of the first mode of vibration. Furthermore, in dealing with trees, energy from the wind may not be trans- ferred to the tree but returned back to the wind via small vortices at the scale of the leaves (James 2010). The work of James et al. (2003; 2006) highlights the mass damping capacity of foliage and branches during storm events. This raises questions about the validity of the view that mature and bigger trees are more likely to fail (Jacobs 1955; Mat- theck and Breloer 1994) simply because of their size and suggests that the failure of senescent trees may have more to do with root systems than sail area (Coder 2010). The capacity for mass damp- ing by smaller branches and foliage is an important arboricultural consideration, as a large tree with a full canopy and many branches not only has a big- ger canopy area, which may be exposed to strong wind gusts, but it also has the capacity to dissipate much of this force. Without data, it is not clear that
March 2014
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
