TechNote#004 A GUIDE TO THE QUALITATIVE ASSESSMENT OF STRUCTURES

INTRODUCTION

This guide is written to assist you (manufacturers/suppliers) with the following:

  1. Understanding the limitations of your currently held pre-engineered solutions.
  2. Assist you with developing a qualitative understanding of structures so that you can identify problems before they eventuate and seek engineering assistance to resolve them.

THE GUIDE

Not having a qualitative understanding of the behaviour of structures results in a failure to understand the limitations of pre-engineered solutions, which then results in the following consequences:

  1. Delays occur.
  2. Cost estimates are based on member sizes that are too small.
  3. Buildings are constructed that are non-compliant with structural codes. They then have to be stabilised and strengthened to become compliant.
  4. Building Approval Applications are rejected.

The first step to avoiding these problems is to understand that most building approval applications are rejected because:

  1. Not enough information is submitted to specify intent. That is more drawings and written specifications are required to clearly indicate what is to be built.
  2. Not enough information is presented to demonstrate fitness-for-purpose. That is no documented evidence is submitted that “proves” compliance with the required codes of practice. This mostly involves the need for engineering calculations. The rejection is usually the result of submitting a pre-engineered solution that is inappropriate for the proposed building.

The following guidelines should assist in avoiding these problems.

  1. The starting point is to produce drawings and written specifications to clearly define the proposal. This process in itself will identify problems and difficulties and may take several attempts before a final concept is decided on.
  2. Modifying a building’s eaves height or width, that is changing the buildings height to span ratio (h/s) affects all the buildings components: portal columns, rafters, struts, bracing, end wall columns, girts, purlins, all connections, and footings. And this is without changing the loads.
  3. Modifying a building’s eaves height or width, modifies the behaviour of wind flowing around and over the building. Making a building smaller can actually increase the wind loads, for example making the buildings length smaller whilst keeping the buildings height and width constant.
  4. Changing the roof pitch modifies the behaviour of wind flowing around and over the building. The steeper the roof pitch, that is the more vertical it becomes, the more the roof surface starts to behave like a wall.
  5. The primary basis for all wind loading on buildings is a fully enclosed building with a rectangular floor plan. Start opening the building up by removing walls and roof panels, then the design starts becoming a matter of personal judgement and less science. Unless wind tunnel tests are carried out to determine the effect of the wind on the specific building. An engineering judgement accepted by one council maybe rejected by another. Similarly, moving away from a rectangular floor plan causes complexity, and introduces more art than science. Therefore note:
    • A fully enclosed shed with any of its walls removed is no longer a shed.
    • A carport or any other canopy with walls or doors attached is no longer a canopy
  6. If two standard buildings are placed side by side, then the interface between those two buildings needs to be given special consideration. If it is desired to remove one set of members, and share members between the two structures, then it is not just a simple matter of deleting one set off members. The shared member needs to have a structural capacity, at the minimum equal to the sum of the capacities of the two members replaced.
  7. Many structural properties are proportional to the areas supported by a structural member. Thus an understanding of the areas that are supported by a member; can assist in determining the need for engineering calculations, or the presence of possible economies. Simply put, if you double the area supported, then you double the structural capacity required, on the condition that the area wasn’t increased by changing the span; that is you only change the spacing between members. If you double the span then you quadruple the capacity required.
  8. If you double the spacing of a supporting member, then you have also doubled the span of the supported member. Thus one member has doubled in capacity and the other has quadrupled in capacity.
  9. A portal frame with a column removed and supported on a carry beam is no longer a portal frame. The propped portal frame supported by the carry beam requires designing, and the frames either side of the door that are used to support the carry beam also require designing. These frames are different than the typical portal frame in the building. The columns either side of the door now carry the area of roof that was previously carried by the columns that have been removed. The load on the door area is also now shared between fewer columns than would be provided to a wall.
  10. Many manufacturer/suppliers assume that because a report presenting a pre-engineered solution contains the size of c-section they desire to use, then the report is relevant to their current project. This is not the case.
  11. Attaching to an existing structure requires checking the structural adequacy of the existing structure and strengthening if required. Attaching carports and verandahs to houses usually requires strengthening the existing structure and providing additional tie-downs.
  12. Brick in a brick veneer house is little more than decorative cladding. Brick and mortar have no tensile capacity worth considering. Therefore a tie-down using the weight of such materials needs to be anchored near the base of such walls, not the top. So that the wind will attempt to lift the entire wall up, rather than pull the top course of bricks off the wall.
  13. Forces need to be carried from the point of application all the way down to the footings. This is no less true for Tie-downs. This means there is little point strengthening the connection between verandah and fascia if all the rest of the connections and members concerned are not strengthened as well. That is the rafter needs to be strengthened, the connection of rafter to top plate needs strengthening, the top plate to wall stud needs strengthening, the wall stud to bottom plate needs strengthening, and the bottom plate to footing/floor slab needs strengthening. All of this is generally inaccessible.
  14. Verandahs are usually located near doors and windows to create an outdoor/indoor living area. In consequence there is no brickwork available to provide additional tie-down to the new verandah. And it can only be used for holding the verandah down, if it wasn’t used for calculating the hold-down to the original house.
  15. When looking at attaching to an existing structure consider the following: one side of the verandah has concrete block footings, the weight of these concrete blocks acts equal and opposite to the wind uplift. If you would be unwilling to hang these concrete blocks from the eaves of your house, then do not consider attaching a verandah or carport to those same eaves. Also consider that the weight of the house footings is holding the house down, so where are you going to provide the extra hold-down weight for the verandah? In short attaching a verandah to a house increases the risk of structural damage to the house (eg. rip its roof off) and consequential damage to the contents of a house. {NB: the consequential storm damage to a wide screen digital TV may exceed the cost of the verandah.}
  16. It is preferable to avoid attaching a verandah to an existing house; and provide extra columns and footings instead. If columns adjacent to the house are considered unsightly, then the columns can be moved away from the house and the canopy cantilevered from the columns. Any connection between the house and canopy required for appearances can then be considered non-structural, as the canopy is self-supporting.

Hopefully this helps manufacturer/suppliers to specify required engineering services more clearly and obtain such services at the right time, and thus minimise delays and expense, and get greater value for money.


Revisions:

  1. [circa 2005  ] : First Published
  2. [08/10/2016] : First Published as Blog Post