Dear Rich: I got the following tips from Harold using upset rods and
felt to pass it on to you and SEAINT. He recommends the use of upset
rods , which i totally agree with him. For those who are not familiar
with upset rods, these are rods that the un-threaded part of the rod has
an area smaller than the area under the threads making yielding of the
gross area of the rod to govern over the fracture of net section. I
like to thank Harold for the reminder since quite honestly I did not
know that upset rods are still used so frequently.
In my experience I have observed that most important bridges until
1950's have used upset rods as anchor bolts, but, some how, in recent
years i have not seen many bridges using upset rods. However, after Loma
Prieta '89 and Northridge '94, investigating damage to steel bridges as
well as buildings, I observed more than a few cases where the regular
anchor bolts had fractured through the under-thread' reduced area in a
very brittle manner as expected.
On the other hand at least in one case that I was involved with as
consultant , the use of upset rods had saved a 120 feet tall "cracker"
tower in a refinery after a magnitude M>7 earthquake. The cracker which
is primarily a large steel pipe which apparently from little I know,
houses inside it various pipes and other machinery and processing
equipment which have high pressure and high temperature oil at various
stages of refining. You can imagine what would have happened if this
cracker was toppled.
In this case the large steel pipe structure of the cracker was anchored
to the foundation, if I remember it right now, by 32 one inch diameter
upset rods with a length of about 12-18 inches. What has happened during
the earthquake was that the cracker as a cantilever had started rocking
and due to elongation of the reduced part of the shank due to yielding,
the tower has experienced a rocking motion which generally is very long
period and as long as center of gravity stays within the footprint, the
tower would not topple. The consulting firm which was doing the
investigation conducted dynamic analysis and sure enough by considering
the rocking motion, the structure was stable where as assuming a rigid
base would attract enough inertia forces to brake the rods if they were
not upset rods. The solution that was recommended to client was not to
change anything other than to take out the elongated upset rods and
replace them with the new ones for future rock and roll of the cracker!
I have included this detail in my upcoming Steel TIPS hoping to post on
www.steeltips.org in June called: " Seismic Behavior and Design of Base
Plates in Steel Braced Frames", by A. Astaneh-Asl, June 2007. Well, I
guess I have done it again and have pitched in my own coming attraction! .
Best wishes as always.
Hassan
Abolhassan Astaneh-Asl, Ph.D., P.E., Professor, UC-Berkeley
Contact Info at www.ce.berkeley.edu/~astaneh
=================
Harold Sprague wrote:
> You may also want to consider pretensioned upset threaded rods. That
> is how water towers are constructed even in high seismic areas, and
> they perform very well.
>
> Regards,
> Harold Sprague
>
>
>
>
>
>> From: Abolhassan Astaneh-Asl <astaneh@ce.berkeley.edu>
>> Reply-To: <seaint@seaint.org>
>> To: seaint@seaint.org
>> CC: hassan@astaneh.net
>> Subject: Large Bay 'X' Bracing
>> Date: Thu, 31 May 2007 05:41:41 -0700
>>
>> Dear Rich:
>> I have put your question as a Problem below and offered my solution
>> to t. next semester I might give it as a HW or even a midterm!
>>
>> Problem: Design an X-bracing in an industrial building to resist
>> wind load. The height and width of the braced bay are 31' and 42'
>> respectively. The braces are to be designed as tension only members.
>> The service load in the diagonal is 21 kips. Use LRFD or ASD
>> according to AISC 2005 Specification (ANSI/AISC 360-05) and
>> ASCE-7-2005.
>>
>> Solution: Two options will be considered; Round HSS and threaded
>> rods with turnbuckle.
>> LRFD methods will be used. (ASD will result in identical solution
>> for tension members)
>>
>> Option A- Round HSS Shape:
>> Tu=Factored Load= 1.6x21 =34 kips
>> According to AISC Manual 13th Ed, Page 2-39, the preferred round HSS
>> material is A500-GrB(42/58 ksi). So, Fy=42 ksi and Fu=58 ksi.
>> Check yielding of gross area:
>> Ag=Tu/[(Fee-y)(Fy)= 34/(0.9x42)= 0.9 in2
>> Check fracture of net area:
>> An= Tu/(Fee-u)(Fu)(U)=34/(0.75x58x0.90)= 0.89 in2
>> Notice that a U factor of 0.90 is used for the net section at the
>> ends of the member. More precise U can be established by using
>> U=1-x/L and knowing L, the length of welds on the round HSS at its ends.
>>
>> If only strength is considered in design, a Round HSS2.375x0.218 with
>> Ag= 1.2 in2 will be sufficient to carry the service load of 21 kips.
>> With this section, probably a 3/8" gusset plate would be sufficient.
>> In that case the net are left at the ends of the member will be:
>> An=1.39 in2-2x(3/8)(0.218)=1.22 > 0.89 in2 O.K.
>> Notice that a lighter section such as Round HSS 2.375x0.154 will also
>> work resulting in An=0.893 > 0.89. But, I prefer to have some
>> thickness in the pipe near 1/4' so that welding at the ends can be
>> done properly. So, I choose Round HSS 2.375x0.218 for the braces for
>> strength considerations.
>> Now I like to check the "preferred" slenderness ratio for tension
>> members. Notice that this is preferred and not "required' by the AISC
>> Spec. It is preferred that KL/r < or = 300.
>>
>> The value of K for members in X-brace can be taken as 0.75 (see S.C.
>> Goel et al paper in the Engineering Journal of the AISC at AISC.org
>> on this subject. Search under Goel). Gusset plates provide some
>> fixity for in plane direction and the compression brace connected to
>> the middle point provides fixity in out of plane direction. That is
>> why K is 0.75. L for the member is 52 feet.
>> KL/r < 300 results in:
>> r> 0.75x52x12/300 = 1.56
>>
>> If I want to satisfy this "preferred " criteria, the section to be
>> used will be Round HSS 4.5x0.237 (again preferring a section with a
>> wall thickness not too much smaller than the 1/4" for practicality of
>> fabrication and welding. Since this criteria is a "preferred' one
>> the choice between this heavier section and what we need to satisfy
>> strength is ours. I choose the heavier section to satisfy the KL/r <
>> 300 and possibly avoid complaints from the fabricators and erectors
>> on flimsiness of the braces!
>>
>> Option B- Threaded rods and turnbuckles:
>> Tu=Factored Load= 1.6x21 =34 kips
>> Let us use A36 threaded rods, with Fy=36 ksi and Fu=58 ksi.
>> Check tensile strength (see AISC Spec 2005 Table J3.2)
>> A=Tu/[(0.75)(Fu)]= 34/(0.75x58)= 0.89 in2
>> A 1-1/8" diameter A36 rod will be sufficient to carry the load. The
>> rods would need turnbuckles and post-tensioning to about say 10% of
>> the capacity to remove the sag which in that case there is no need to
>> check the KL/r< 300 preferred criteria since it is not meant for post
>> tensioned rods.Apparently when one goes back to bridge books, where
>> this criteria was first mentioned, one finds out that this was meant
>> only for rolled shapes and cold formed sections and for fabrication
>> and erection purposes. Also, bridge engineers were concerned with the
>> vibrations of very slender members in bridges under the impact of
>> trucks passing over. I observed this phenomenon a few years back in
>> Binicia-Martinez bridge near San Francisco when walking under the
>> deck on the catwalk, you could clearly see the vibration of single
>> angle cross braces. The vibration itself was not a major concern
>> since no one can see those vibrating braces, but, I got concerned
>> about fatigue issues since over the years such vibrations can cause
>> fatigue fracture. i did some back of envelop dynamic analysis and it
>> turned out that the vibration is because of dynamic resonance and the
>> fact that these braces are long and have relatively small stiffness
>> that their period of first mode is close to 1.0 Hz, the driving force
>> frequency. Then you look at KL/r < 300 and you really get fascinated
>> with the wisdom and knowledge of those bridge engineers of 19 and
>> early 20th century who has put all of this dynamics and fabrication
>> and erection concerns into very simple equation of KL/r < 300 and
>> avoided all the pitfalls of using very slender members. Notice that
>> KL/r has all the dynamic properties of geometry (L and A) and
>> stiffness (I) in it!
>> By the way, this was not the first time I was amazed how good those
>> old time engineers were. the first time was during my dissertation
>> work, I stumbled over the Whitmore's method for gusset plates which
>> was used in late 1800's by bridge engineers , almost hundred years
>> ahead of Whitmore's tests which showed this method works!
>>
>> Well, sorry for taking your time too long.With your permission I plan
>> to double check it and include in my upcoming textbook: "Behavior and
>> Design of Steel and Composite Structures" by Abolhassan Astaneh-Asl,
>> Volume One on Steel Structures to be released January 1, 2008). I
>> know you will not trust my calculations and if you use above numbers
>> in your actual design, you will double check them!
>>
>> Best wishes and thank you for bringing up an interesting question.
>> "Hassan"
>> Abolhassan Astaneh-Asl, Ph.D., P.E.,
>> (http://www.ce.berkeley.edu/~astaneh)
>> =============================
>>
>> From: "Rich Lewis" <seaint04@lewisengineering.com>
>> To: <seaint@seaint.org>
>> Subject: Large Bay 'X' Bracing
>>
>> This is a multi-part message in MIME format.
>>
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>> I have a warehouse condition of a 'X' braced bay that is 31 feet high
>> and 42
>> feet wide. The diagonal length is over 51 feet. I'm wondering if I
>> should
>> try to use the bracing as one large bay, or add a wind column in the
>> middle
>> and have two smaller bays. The bracing is tension only for wind
>> loads. The
>> load in the diagonal is about 21 kips. What makes me most uneasy is the
>> slenderness ratio of the brace. If I try to limit the L/r ratio of
>> the out
>> of plane axis to 300 then I get extremely large angles. If I ad a
>> column I
>> add almost 50% more bracing length, plus column and footing.
>>
>>
>>
>> Thanks for your insight.
>>
>>
>>
>> Rich
>>
>> a
>>
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