Improvised field testing of bolted bamboo connections for use in bracing of informal housing. 15 June 2012 Samuel G Stephens BE (Hons) Structural Engineer SAFE (NGO) Bangladesh
DISCLAIMER: The following report and all its recommendations are to be used for informative purposes only. The Author and SAFE (NGO) Bangladesh take no responsibility for damage to property or life resulting from this report being used for anything other than its intended purpose. The following does not purport to be a scientific or definitive document. Field testing was carried out in an improvised manner to provide at least some Engineering basis for the improvement of low-cost housing construction in Bangladesh.
Abstract Improvised field testing was carried out to ascertain a reasonable strength value for bolted bamboo connections used in the cross-bracing of informal housing. A single connection strength of 3.9kN was determined (which includes a 0.60 safety factor for variations in materials and construction) provided that the following requirements are met; • All cross-bracing members must terminate in a node (a 'node' is a naturally occurring, regular spaced thickening present in bamboo). • The end distance between the bolt and the end of the cross-bracing member must not be less than 2" (51mm) • The minimum wall thickness of bamboo to be used as a cross-bracing member is to be 3/4" (19mm) • The minimum outer diameter of bamboo to be used as a cross-bracing member is to be 2.5" (63mm) Further testing is required to confirm these results.
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Contents Abstract ................................................................................................................................................... 1 1.
Introduction .................................................................................................................................... 3
2.
Materials and methods ................................................................................................................... 3
3.
As-built cross-bracing survey .......................................................................................................... 5
4.
Results ............................................................................................................................................. 5
5.
Findings ........................................................................................................................................... 7 a.
Initial ........................................................................................................................................... 7
b.
Load Capacity ............................................................................................................................. 7
6.
Discussion........................................................................................................................................ 7 a.
Suitability of Load Capacity ......................................................................................................... 7
b.
Guidelines for construction......................................................................................................... 8
c.
Other possible failure mechanisms............................................................................................. 8 i.
Receiving member (bamboo beam or post) failure due to splitting. ..................................... 8
ii.
Foundations ............................................................................................................................ 9
iii. Tension failure of bamboo brace .............................................................................................. 9 7.
Knowledge dissemination ............................................................................................................... 9
8.
Literature cited................................................................................................................................ 9
Appendix A (As-built Survey) ............................................................................................................... 10 Appendix B (Test Data) ......................................................................................................................... 12
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1. Introduction SAFE (NGO) works in improvement low-cost housing in Bangladesh. Working primarily in Bamboo, SAFE strives to build stronger and longer lasting houses for low-income families. High-winds are one of the biggest hazards for housing in Bangladesh. SAFE has been providing bamboo cross-bracing to its bamboo framed houses to provide lateral stability. Testing of these bamboo bracing connections was undertaken, along with Structural Engineering Design Calculations, to provide an Engineering basis for future design and implementation.
2. Materials and methods Bracing bays are provided (as shown in figure 1) to every external wall, no more than 12' (3660mm) apart. Two sets of bracing are used (on either side of the posts) if the wall is shared between two houses or rooms.
Figure 1 Typical Bracing Bay
Half bamboo members are used to provide tension-only cross bracing. These half bamboo crossbraces are fixed to vertical bamboo posts or beams using M10 bolts and 32mm circular washers to construct bracing bays. By inspection, it was hypothesized that the weakest part of the bracing system would be this bolted connection between the cross brace and the post or beam (figure 2). The bamboo used in testing was of the same species, from the same cultivation area and had undergone the same treatment process as the bamboo used in SAFE construction.
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Figure 2 in-situ bracing connection
Figure 3 test specimen
It was expected that the connection would fail in a 'pull out' mode, which is to say that Bolt A would pull out the end of the bamboo cross-brace. This hypothesis was reached because of; • the small edge distance between the bolt and the end of the cross brace in the direction of loading. • the high strength of the bolt compared to the bamboo. • the much larger size of the full-bamboo post/beam when compared to the half-bamboo cross brace. From conventional timber Engineering knowledge (which, as another naturally occurring, cellular material can be considered a reasonable comparison) it was assumed that the wall thickness of the test specimen would have a greater effect on its resistance to bolt pull out (and therefore strength) than the outer diameter of the test specimen. To best test this connection, it was devised to suspend an idealised connection or test specimen (figure 3) from a frame (figure 4) with a known weight attached. Weight and therefore force on the test connection would then be gradually increased until the connection failed.
Figure 4 testing rig Picture 1 testing rig in use
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3. As-built cross-bracing survey A survey was undertaken of completed SAFE demonstration houses to ascertain the section properties of as-built cross-bracing. Summary of findings all values in mm min end distance brace diameter brace wall thickness
max 30 57 9
average 150 73.8 75 63.8 18 13.5
The results of the survey show large variations in the size and wall thickness of cross-bracing members as well as end distance. 27% of braces were found to not terminate at a node. Full survey results can be found in Appendix B.
4. Results Due to time constraints and the limitations of working in rural Bangladesh, the testing was beset by problems with the testing equipment. In all, 13 tests were undertaken though only three can be said to have been completed successfully. Succesful tests Wall (mm) Ult. Load (kN) Reason for invalidation Test B3 10 8.13 Test C 11 6.48 Test E 14 8.17 Invalid Tests Test Ai Test Bi Test Ci Test A Test A2 Test A3 Test B1 Test B2 Test D Test F
15 9 13 15 15 15 10 10 11 11
0.9 2.09 3.52 1.04 2.82 7.62 4.72 3.95 4.98 7.22
craddle reached ground additional weight required loading hook failed loading hook failed ropes slipped beam failed bolt rotation in beam beam rotation loading bolt b pullout beam failure
The sample size is too small to give a scientific indication of joint performance but further testing is not possible at this time.
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failure load (kN)
Bolted Bamboo Connection Testing Results 9.00 8.00 7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00
8.17
8.13 6.48
Succesful tests Invalid Tests
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test specimen wall thickness (mm)
Picture 2 test B3 under load
Picture 3 test B3 after failure
Picture 2 above shows a typical test specimen under load. Picture 3 shows the same specimen after it has failed due to Bolt A 'pulling out' through the end of the half bamboo test specimen. Nodes are visible at both ends of the test specimen. Full testing results can be found in Appendix B.
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5. Findings a. Initial Initial testing was fraught with equipment failure though some early assumptions were quickly confirmed. Namely; I.
End distance (the spacing between the bolt and the end of the bamboo cross-brace in the direction of the load) has a significant impact on its load carrying capacity. From initial testing, it was decided that a 2" (51mm) minimum end distance would give significant additional capacity whilst still being reasonable to construct (greater end distances are sometimes not feasible due to conflict with door openings and other framing members).
II.
Nodes. Bamboo has naturally occurring thickenings along its length from which shoots and leaves grow, known as nodes. These nodes are substantially thicker than the rest of the bamboo and also have a disrupted grain pattern. These physical characteristics provide an increased strength and resistance to bolt pull-out at the end of the bracing members. Therefore, all bracing members will be terminated at a node.
b. Load Capacity Using the limited test data available, a bolted connection strength has been reached by the following rationale; • The lower-bound of successful tests was 6.48kN • A connection safety factor (∅) of 0.60 is deemed appropriate due to the uncontrolled nature of Bangladeshi construction, variability of materials and uncertainty due to the small testing sample size. = , Ă— ∅ = 6.48 Ă— 0.60
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= &. '()
6. Discussion a. Suitability of Load Capacity From 5. b. (above), a load capacity of 3.9kN has been ascertained for a single connection (including a 0.60 safety factor for connections). From structural analysis, it has been calculated that the likely (un-factored) force on this connection during wind loading is 2.32kN. 3.9 = 1.68 â&#x2030;Ľ 1.50 01234 5674 879:6;< â&#x2C6;´ ok 2.32
8 The value of 2.3kN has been determined from a design wind speed of 139km/h (35.5m/s). This design wind speed is based on the following conditions of IS 875:198722; 47m/s Basic Wind Speed 25 year design life terrain category three flat topography
b. Guidelines for construction From testing results and consideration of construction realities, the following parameters will be set for the construction of bamboo cross bracing; • All cross-bracing members must terminate in a node. • The end distance between the bolt and the end of the cross-bracing member must not be less than 2" (51mm) • The minimum wall thickness of bamboo to be used as a cross-bracing member is to be 3/4" (19mm) • The minimum outer diameter of bamboo to be used as a cross-bracing member is to be 2.5" (63mm) The minimum wall thickness prescribed above is significantly thicker than any used in testing. From experience, the use of thicker walled bracing will be achievable in practice and this will allow for some additional load capacity above that stated in this report.
c. Other possible failure mechanisms i. Receiving member (bamboo beam or post) failure due to splitting. During testing, failures were observed where Test Bolt A would rotate within the test beam resulting in the beam cracking longitudinally (picture 2). The testing involved the beam being loaded perpendicular to the grain (the weakest direction). In practice, the cross-brace will be placed at a 45⁰ angle and, therefore, the beam/post will be loaded at 45⁰ to the grain (as opposed to 90⁰ during the test). Studies have shown that bamboo's compressive strength is in the order of 4 times stronger in the direction of the grain (0⁰) than perpendicular to it (90⁰)1. Therefore, it follows that an increased strength magnitude of 2 could be reasonably assumed for loading at 45⁰ and applied to the testing results. From the two tests that failed due to bolt rotation and beam splitting (Test B1 and B2) the Picture 4 bolt rotation and cracking of test beam lowest strength ascertained was 3.95kN. However, this was using the same beam that had already split in test B1 where it achieved a load of 4.72kN and this would be a more accurate representation of the beams resistance to splitting. If a factor of 2 is applied to 4.72kN, a probable resistance of 9.44kN is found which is significantly higher than the lower-bound of 6.48kN used in the load capacity calculations pertaining to pull-out failure. Therefore this failure mechanism is assumed not to govern.
9 ii. Foundations After considering results of the bolted connection testing, it is now probable that the foundations (pre-cast concrete piles known as kaatlas) will govern the bracing design. However, design and testing of these kaatlas is outside the scope of this report.
iii. Tension failure of bamboo brace Tension failure of the half bamboo cross-brace itself is not expected to govern as this would have become apparent during tests. A short section of bamboo connected test bolt A from loading bolt B and would have served to test the bamboo tensile strength at the same time as testing bolt A.
7. Knowledge dissemination Findings from this testing will/have be actioned in three areas; • Structural drawings have been produced showing minimum dimensions and construction details of bracing for use in future SAFE projects. • This report will be made publicly available on the SAFE website. • A leaflet (in Bangla) has been produced and distributed to slum residents outlining construction requirements for cross-bracing. Refer to Figure 3 below for segment pertaining to dimensions of cross-bracing. The complete leaflet as well as an English draft can be found on the SAFE website (http://safebangladesh.wordpress.com)
Figure 3 section of Bangla cross-bracing leaflet
8. Literature cited 1. Mechanical and Physico-Chemical Properties of Bamboos carried out by Aerospace Engineering Department, Indian Institute of Technology – Bombay with Prof. NK Naik as Principal Investigator. Retrieved 16th June 2012 from http://www.bambootech.org/files/mechanicaltesting%20report.pdf 2. IS 875:19872. Indian Standard Code of practice for design loads (other than earthquake) For buildings and structures. Part 3 Wind Loads (Second Revision)
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Appendix A (As-built Survey)
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Appendix B (Test Data)
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